Patent Publication Number: US-11644669-B2

Title: Depth based foveated rendering for display systems

Description:
PRIORITY CLAIM 
     This application claims priority to U.S. Provisional Application No. 62/644,365 filed on Mar. 16, 2018; U.S. Provisional Application No. 62/475,012 filed on Mar. 22, 2017. U.S. Provisional Application No. 62/486,407 filed on Apr. 17, 2017, and U.S. Provisional Application No. 62/539,934 filed on Aug. 1, 2017. The above-recited patent applications are hereby incorporated by reference in their entirety for all purposes. 
     INCORPORATION BY REFERENCE 
     This application incorporates by reference the entirety of each of the following patent applications and publications: U.S. application Ser. No. 14/555,585 filed on Nov. 27, 2014, published on Jul. 23, 2015 as U.S. Publication No. 2015/0205126; U.S. application Ser. No. 14/690,401 filed on Apr. 18, 2015, published on Oct. 22, 2015 as U.S. Publication No. 2015/0302652; U.S. application Ser. No. 14/212,961 filed on Mar. 14, 2014, now U.S. Pat. No. 9,417,452 issued on Aug. 16, 2016; U.S. application Ser. No. 14/331,218 filed on Jul. 14, 2014, published on Oct. 29, 2015 as U.S. Publication No. 2015/0309263; U.S. application Ser. No. 15/902,927 filed on Feb. 22, 2018; U.S. Provisional Application No. 62/475,012 filed on Mar. 22, 2017; and U.S. Provisional Application No. 62/539,934 filed on Aug. 1, 2017. 
    
    
     BACKGROUND 
     Field 
     The present disclosure relates to display systems, including augmented reality imaging and visualization systems. 
     Description of the Related Art 
     Modern computing and display technologies have facilitated the development of systems for so called “virtual reality” or “augmented reality” experiences, in which digitally reproduced images or portions thereof are presented to a user in a manner wherein they seem to be, or may be perceived as, real. A virtual reality, or “VR”, scenario typically involves the presentation of digital or virtual image information without transparency to other actual real-world visual input; an augmented reality, or “AR”, scenario typically involves presentation of digital or virtual image information as an augmentation to visualization of the actual world around the user. A mixed reality, or “MR”, scenario is a type of AR scenario and typically involves virtual objects that are integrated into, and responsive to, the natural world. For example, an MR scenario may include AR image content that appears to be blocked by or is otherwise perceived to interact with objects in the real world. 
     Referring to  FIG.  1   , an augmented reality scene  10  is depicted. The user of an AR technology sees a real-world park-like setting  20  featuring people, trees, buildings in the background, and a concrete platform  30 . The user also perceives that he/she “sees” “virtual content” such as a robot statue  40  standing upon the real-world platform  30 , and a flying cartoon-like avatar character  50  which seems to be a personification of a bumble bee. These elements  50 ,  40  are “virtual” in that they do not exist in the real world. Because the human visual perception system is complex, it is challenging to produce AR technology that facilitates a comfortable, natural-feeling, rich presentation of virtual image elements amongst other virtual or real-world imagery elements. 
     Systems and methods disclosed herein address various challenges related to AR and VR technology. 
     SUMMARY 
     According to some embodiments, a system comprises one or more processors and one or more computer storage media storing instructions that when executed by the one or more processors, cause the one or more processors to perform operations. The operations comprise monitoring, based on information detected via one or more sensors, eye movements of a user. A fixation point at which the user&#39;s eyes are fixating is determined based on the eye movements, with the fixation point being a three-dimensional location in a field of view of the user. The operations include obtaining location information associated with one or more virtual objects to present to the user, the location information indicating three-dimensional positions of the virtual objects. The operations also include adjusting resolutions of at least one virtual object based, at least in part, on a proximity of the at least one virtual object to the fixation point. The operations also include causing a presentation to the user, via a display, of the virtual objects, with at least one virtual object being rendered according to the adjusted resolution. 
     According to some embodiments, a display system comprises a display device configured to present virtual content to a user, one or more processors, and one or more computer storage media storing instructions that when executed by the system, cause the system to perform operations. The operations comprise monitoring information associated with eye movements of the user. A fixation point within a display frustum of the display device is determined based on the monitored information, the fixation point indicating a three-dimensional location being fixated upon by eyes of the user. The operations also include presenting virtual content at three-dimensional locations within the display frustum based on the determined fixation point, with the virtual content being adjusted in resolution based on a proximity of the virtual content from the fixation point. 
     According to some other embodiments, a method comprises monitoring, based on information detected via one or more sensors, eye orientations of a user of a display device. A fixation point at which the user&#39;s eyes are fixating is determined based on the eye orientations, with the fixation point being a three-dimensional location in a field of view of the user. Location information associated with one or more virtual objects to present to the user is obtained, the location information indicating three-dimensional positions of the virtual objects. The resolution of at least one virtual object is adjusted based, at least in part, on a proximity of the at least one virtual object to the fixation point. The method also includes causing presentation to the user, via a display, of the virtual objects, with at least one virtual object being rendered according to the adjusted resolution. 
     According to some embodiments, a display system comprises a frame configured to mount on a head of a user, a light modulating system configured to output light to form images, and one or more waveguides attached to the frame and configured to receive the light from the light modulating system and to output the light across a surface of the one or more waveguides. The system also comprises one or more processors, and one or more computer storage media storing instructions that, when executed by the one or more processors, cause the one or more processors to perform various operations. The operations include determining an amount of light reaching a retina of an eye of the user; and adjusting resolution of virtual content to be presented to the user based on the amount of light reaching the retina. 
     According to some other embodiments, a display system comprises one or more processors; and one or more computer storage media storing instructions. When the instructions are executed by the one or more processors, they cause the one or more processors to perform various operations. The operations include determining an amount of light reaching a retina of an eye of a user of the display system; and adjusting resolution of virtual content to be presented to the user based on the amount of light reaching the retina. 
     According to some embodiments, a method is performed by a display system comprising one or more processors and a head-mountable display. The method comprises determining an amount of light reaching a retina of an eye of a user of the display system; and adjusting resolution of virtual content to be presented to the user based on the amount of light reaching the retina. 
     According to some other embodiments, a display system comprises a frame configured to mount on a head of a user; and light modulating system; one or more waveguides; one or more processors; and one or more computer storage media storing instructions. The light modulating system is configured to output light to form images. The one or more waveguides are attached to the frame and configured to receive the light from the light modulating system and to output the light across a surface of the one or more waveguides. The one or more computer storage media store instructions that, when executed by the one or more processors, cause the one or more processors to perform various operations. The operations comprise adjusting a resolution of component color images forming virtual content based on: a proximity of the virtual content from a user fixation point; and a color of the component color image. At least one of the component color images differs in resolution from component color images of another color. 
     According to yet other embodiments, a display system comprises one or more processors; and one or more computer storage media storing instructions. When the instructions are executed by the one or more processors, they cause the one or more processors to perform various operations. The operations include adjusting a resolution of component color images forming virtual content based on: a proximity of the virtual content from a user fixation point; and a color of the component color image, wherein at least one of the component color images differs in resolution from component color images of another color. 
     According to some other embodiments, a method is performed by a display system comprising one or more processors and a head-mountable display. The method comprises adjusting a resolution of component color images forming virtual content based on: a proximity of the virtual content from a user fixation point; and a color of the component color image, wherein at least one of the component color images differs in resolution from component color images of another color. 
     According to yet other embodiments, a display system comprises an image source comprising a spatial light modulator for providing a first image stream and a second image stream; a viewing assembly; one or more processors in communication with the image source; and one or more computer storage media storing instructions that, when executed by the one or more processors, cause the one or more processors to perform various operations. The viewing assembly comprises light guiding optics for receiving the first and second image streams from the image source and outputting the first and second image streams to a user. The various operations performed by the one or more processors comprise causing the image source to output the first image stream to the viewing assembly, wherein images formed by the first image stream have a first pixel density; and causing the image source to output the second image stream to the viewing assembly. The images formed by the second image stream have a second pixel density that is greater than the first pixel density, and correspond to portions of images provided by the first image stream. Images formed by the second image stream overlie corresponding portions of a field of view of provided by the first image stream. 
     According to some embodiments, a wearable display system may include an afocal magnifier with circular polarization handedness dependent magnification. The afocal magnifier may include a first fixed focal length lens element, a first geometric phase lens that exhibits a positive refractive power for a first handedness of incident circularly polarized light and exhibits a negative refractive power for a second handedness of incident circularly polarized light, and a second geometric phase lens. 
     According to some other embodiments, an optical subsystem for a wearable image projector may include a polarization selective reflector and a set of four lens elements positioned about the polarization selective reflector. 
     According to some other embodiments, a display system for projecting images to an eye of a user may include an eyepiece. The eyepiece may include a waveguide, and an in-coupling grating optically coupled to the waveguide. The display system may further include a first image source configured to project a first light beam associated with a first image stream. The first image stream may have a first field of view and may be incident on a first surface of the in-coupling grating. A portion of the first light beam may be coupled into the waveguide by the in-coupling grating for positioning the first image stream in a fixed position to the eye of the user. The display system may further include a second image source configured to project a second light beam associated with a second image stream. The second image stream may have a second field of view that is narrower than the first field of view. The display system may further include a scanning mirror configured to receive and reflect the second light beam such that the second light beam is incident on a second surface of the in-coupling grating opposite to the first surface thereof. A portion of the second light beam may be coupled into the waveguide by the in-coupling grating. The display system may further include an eye-gaze tracker configured to detect movement of the eye of the user, and control circuitry in communication with the eye gaze tracker and the scanning mirror. The control circuitry may be configured to position the scanning mirror such that a position of the second image stream is moved in accordance with the detected movement of the eye of the user. 
     According to some other embodiments, a display system for projecting images to an eye of a user may include an eyepiece. The eyepiece may include a waveguide, and an in-coupling grating optically coupled to the waveguide. The display system may further include an image source configured to project a first light beam associated with a first image stream in a first polarization, and a second light beam associated with a second image stream in a second polarization different from the first polarization. The first image stream may have a first field of view and the second image stream may have a second field of view that is narrower than the first field of view. The first light beam and the second light beam may be multiplexed. The display system may further include a polarization beam splitter configured to receive and reflect the first light beam along a first optical path, and receive and transmit the second light beam along a second optical path. The display system may further include a first optical reflector positioned along the first optical path and configured to receive and reflect the first light beam such that the first light beam is incident on a first surface of the in-coupling grating. A portion of the first light beam may be coupled into the waveguide by the in-coupling grating for positioning the first image stream in a fixed position to the eye of the user. The display system may further include a scanning mirror disposed along the second optical path and configured to receive and reflect the second light beam, and a second optical reflector positioned along the second optical path downstream from the scanning mirror. The second optical reflector may be configured to receive and reflect the second light beam such that the second light beam is incident on a second surface of the in-coupling grating opposite the first surface thereof. A portion of the second light beam may be coupled into the waveguide by the in-coupling grating. The display system may further include an eye-gaze tracker configured to detect movement of the eye of the user, and control circuitry in communication with the eye gaze tracker and the scanning mirror. The control circuitry may be configured to position the scanning mirror such that a position of the second image stream is moved in accordance with the detected movement of the eye of the user. 
     According to some other embodiments, a display system for projecting images to an eye of a user may include a waveguide, an image source configured to project a first light beam associated with a first image stream in a first polarization and a second light beam associated with a second image stream in a second polarization different from the first polarization. The first image stream may have a first field of view, and the second image stream having a second field of view that is narrower than the first field of view. The first light beam and the second light beam may be multiplexed. The display system may further include a polarization beam splitter configured to receive and reflect the first light beam along a first optical path, and to receive and transmit the second light beam along a second optical path. The display system may further include a first in-coupling prism positioned along the first optical path and adjacent a first surface of the waveguide. The first in-coupling prism may be configured to couple a portion of the first light beam into the waveguide for positioning the first image stream in a fixed position to the eye of the user. The display system may further include a scanning mirror disposed along the second optical path and configured to receive and reflect the second light beam. The display system may further include a second in-coupling prism positioned along the second optical path downstream from the scanning mirror and adjacent a second surface of the waveguide opposite to the first surface of the waveguide. The second in-coupling prism may be configured to couple a portion of the second light beam into the waveguide. The display system may further include an eye-gaze tracker configured to detect movement of the eye of the user, and control circuitry in communication with the eye gaze tracker and the scanning mirror. The control circuitry may be configured to position the scanning mirror such that a position of the second image stream is moved in accordance with the detected movement of the eye of the user 
     According to an embodiment, a display system for projecting images to an eye of a user includes an image source. The image source can be configured to project a first light beam associated with a first image stream in a first polarization, and a second light beam associated with a second image stream in a second polarization different from the first polarization. The first image stream can have a first field of view, and the second image stream can have a second field of view that is narrower than the first field of view. The first light beam and the second light beam can be multiplexed. The display system can further include a polarization beam splitter. The polarization beam splitter can be configured to receive and reflect the first light beam along a first optical path toward a viewing assembly for positioning the first image stream in a fixed position to the eye of the user, and receive and transmit the second light beam along a second optical path. The display system can further include a scanning mirror disposed along the second optical path and configured to receive and reflect the second light beam toward the viewing assembly. The display system can further include an eye-gaze tracker configured to detect movement of the eye of the user, and control circuitry in communication with the eye gaze tracker and the scanning mirror. The control circuitry can be configured to position the scanning mirror such that a position of the second image stream is moved in accordance with the detected movement of the eye of the user. 
     According to another embodiment, a display system for projecting images to an eye of a user include an image source. The image source can be configured to project a first light beam associated with a first image stream and a second light beam associated with a second image stream. The first image stream can have a first field of view, and the second image stream can have a second field of view that is narrower than the first field of view. The first light beam and the second light beam can be multiplexed. The display system can further include a scanning mirror configured to receive and reflect the first light beam and the second light beam toward a viewing assembly for projecting the first image stream and the second image stream. The display system can further include an eye-gaze tracker configured to detect movement of the eye of the user, and control circuitry in communication with the eye gaze tracker and the scanning mirror. The control circuitry can be configured to position the scanning mirror such that a position of the first image stream and a position of the second image stream are moved in accordance with the detected movement of the eye of the user. The display system can further include a switchable optical element disposed in an optical path of the first light beam and the second light beam. The switchable optical element can be configured to be switched to a first state for the first light beam such that the first light beam is angularly magnified by a first angular magnification, and be switched to a second state for the second light beam such that the second light beam is angularly amplified by a second angular magnification that is less than the first angular magnification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates a user&#39;s view of augmented reality (AR) through an AR device. 
         FIG.  2    illustrates a conventional display system for simulating three-dimensional imagery for a user. 
         FIGS.  3 A- 3 C  illustrate relationships between radius of curvature and focal radius. 
         FIG.  4 A  illustrates a representation of the accommodation-vergence response of the human visual system. 
         FIG.  4 B  illustrates examples of different accommodative states and vergence states of a pair of eyes of the user. 
         FIG.  4 C  illustrates an example of a representation of a top-down view of a user viewing content via a display system. 
         FIG.  4 D  illustrates another example of a representation of a top-down view of a user viewing content via a display system. 
         FIG.  5    illustrates aspects of an approach for simulating three-dimensional imagery by modifying wavefront divergence. 
         FIG.  6    illustrates an example of a waveguide stack for outputting image information to a user. 
         FIG.  7    illustrates an example of exit beams outputted by a waveguide. 
         FIG.  8    illustrates an example of a stacked waveguide assembly in which each depth plane includes images formed using multiple different component colors. 
         FIG.  9 A  illustrates a cross-sectional side view of an example of a set of stacked waveguides that each includes an incoupling optical element. 
         FIG.  9 B  illustrates a perspective view of an example of the plurality of stacked waveguides of  FIG.  9 A . 
         FIG.  9 C  illustrates a top-down plan view of an example of the plurality of stacked waveguides of  FIGS.  9 A and  9 B . 
         FIG.  9 D  illustrates an example of wearable display system. 
         FIG.  10 A  illustrates an example of a representation of a top-down view of a user viewing content via a display system. 
         FIG.  10 B  illustrates another example of a representation of a top-down view of a user viewing content via a display system. 
         FIG.  10 C  illustrates yet another example of a representation of a top-down view of a user viewing content via a display system. 
         FIG.  10 D  is a block diagram of an example display system. 
       FIG.  11 A 1  illustrates an example of a representation of a top-down view of adjustments in resolution in different resolution adjustment zones based on three-dimensional fixation point tracking. 
       FIG.  11 A 2  illustrates examples of representations of top-down views of resolution adjustment zones at different times as the sizes and numbers of the zones change. 
         FIG.  11 B  illustrates an example of a three-dimensional representation of a portion of the resolution adjustment zones of FIG.  11 A 1 . 
         FIG.  11 C  illustrates another example of a configuration for resolution adjustment zones. 
         FIG.  11 D  illustrates an example of a three-dimensional representation of the resolution adjustment zones of  FIG.  11 C . 
         FIG.  11 E  illustrates another example of a three-dimensional representation of the resolution adjustment zones of  FIG.  11 C . 
         FIGS.  12 A- 12 C  shown diagrams of examples of processes for adjusting resolutions of content according to proximity to a three-dimensional fixation point. 
         FIG.  13    illustrates an example of a representation of a user viewing multiple virtual objects aligned with the user&#39;s line of sight. 
         FIG.  14    is a diagram of an example of a process for adjusting virtual content based on angular proximity to a user&#39;s gaze. 
         FIG.  15    illustrates an example of a representation of the retina of an eye of a user. 
         FIG.  16    graphically illustrates an example of resolution, and rod and cone density, across the retina of  FIG.  15   . 
         FIG.  17    graphically illustrates an example of the relationship between pupil size and the amount of light incident on an eye of a user. 
         FIG.  18    is a diagram of an example of a process for adjusting virtual content based on the amount of light incident on an eye of a user. 
         FIG.  19    graphically illustrates an example of a change in resolution detectable by the eye of a user as the amount of light incident on the eye changes. 
         FIG.  20    graphically illustrates an example of differences in sensitivity of the eye to light of different colors at different levels of illumination. 
         FIG.  21    is a diagram of an example of a process for adjusting virtual content formed using multiple component color images, where the resolution adjustment is made based on the color of the component color image. 
         FIGS.  22 A- 22 C  illustrate examples of changing contrast sensitivity as the amount of light incident on the eye of the user decreases. 
         FIG.  23    illustrates an example of a representation of the optic nerve and peripheral blind spots of the eyes of a user. 
         FIG.  24    shows an exemplary monocular field of view for a human eye. 
         FIG.  25 A  shows an exemplary wearable display device configured to provide virtual content to a user. 
         FIG.  25 B  is a block diagram depicting an augmented reality system. 
         FIG.  25 C  illustrates schematically light paths in a viewing optics assembly (VOA) that may be used to present a digital or virtual image to a viewer. 
         FIGS.  26 A- 26 D  illustrate exemplary render perspectives to be used and light fields to be produced in an AR system for each of two exemplary eye orientations. 
         FIGS.  26 E- 26 F  illustrate schematically an exemplary configuration of images that can be presented to a user. 
         FIGS.  26 G- 26 H  illustrate schematically exemplary configurations of images that can be presented to a user. 
         FIG.  27    illustrates a field of view and a field of regard as shown in  FIG.  24   , overlaid upon one of the displays in the wearable display device as shown in  FIG.  25   . 
         FIGS.  28 A- 28 B  illustrate some of the principles described in  FIGS.  26 A- 26 D . 
         FIGS.  28 C- 28 D  illustrate some exemplary images that can be presented to a user. 
         FIG.  28 E  illustrates an exemplary high-FOV low-resolution image frame. 
         FIG.  28 F  illustrates an exemplary low-FOV high-resolution image frame. 
         FIG.  29 A  shows a simplified block diagram of a display system. 
         FIG.  29 B  illustrates schematically a cross-sectional view of an augmented reality (AR) system. 
         FIGS.  30 A- 30 B  illustrate schematically a display system for projecting image streams to an eye of a user. 
         FIG.  30 C  illustrates schematically a cross-sectional view of an augmented reality (AR) system. 
         FIG.  30 D  shows a simplified block diagram of a display system. 
         FIG.  31 A  illustrates schematically the operating principles of a first relay lens assembly in the display system illustrated in  FIGS.  30 A- 30 B . 
         FIG.  31 B  illustrates schematically the operating principles of a second relay lens assembly in the display system illustrated in  FIGS.  30 A- 30 B . 
         FIGS.  31 C- 31 D  illustrate schematically a display system. 
         FIGS.  32 A- 32 C  illustrate schematically a display system. 
         FIGS.  33 A- 33 B  illustrate schematically a display system. 
         FIGS.  34 A- 34 B  illustrate schematically a display system. 
         FIG.  35    illustrates schematically a display system. 
         FIG.  36 A  illustrates schematically an augmented reality near-eye display system. 
         FIG.  36 B  illustrates schematically another augmented reality near-eye display system. 
         FIG.  37 A  is a schematic illustration of a dual magnification afocal magnifier. 
         FIG.  37 B  is a schematic illustration of a dual focal magnification afocal magnifier. 
         FIGS.  38 A- 38 B  illustrates schematically an exemplary configuration of images that can be presented to a user. 
         FIGS.  39 A- 39 B  illustrate some exemplary images that can be presented to a user. 
         FIGS.  40 A- 40 D  illustrate schematically a display system for projecting image streams to an eye of a user. 
         FIGS.  41 A- 41 D  illustrate schematically a display system for projecting image streams to an eye of a user. 
         FIG.  42    illustrate an exemplary frame structure for a high-FOV low-resolution image stream and a low-FOV high-resolution image stream that are time-division multiplexed. 
         FIG.  43    illustrates schematically a display system for projecting image streams to an eye of a user. 
         FIG.  44    illustrates schematically a display system for projecting image streams to an eye of a user. 
         FIG.  45    illustrates schematically a display system for projecting image streams to an eye of a user. 
         FIG.  46    illustrates schematically a display system for projecting image streams to an eye of a user. 
         FIG.  47    illustrates schematically a display system for projecting image streams to an eye of a user. 
         FIG.  48    illustrates schematically a display system for projecting image streams to an eye of a user. 
         FIG.  49    illustrates schematically a display system for projecting image streams to an eye of a user. 
         FIG.  50    illustrates schematically a display system for projecting image streams to an eye of a user according to some embodiments. 
         FIG.  51    illustrates schematically a display system for projecting image streams to an eye of a user according to some embodiments. 
         FIGS.  52 A- 52 B  illustrate schematically a display system for projecting image streams to an eye of a user according to some embodiments. 
         FIGS.  53 A- 53 B  illustrate schematically a display system for projecting image streams to an eye of a user according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Rendering virtual content for augmented and virtual display systems is computationally intensive. Among other things, the computational intensity may undesirably use large amounts of memory, cause high latency, and/or may require the use of powerful processing units that may have high cost and/or high energy-consumption. 
     In some embodiments, methods and systems conserve computational resources, such as memory and processing time, by reducing the resolution of virtual content positioned at locations away from the fixation point of the user&#39;s eyes. For example, the system may render virtual content at a relative high (e.g., a highest) resolution at or proximate a fixation point of the user&#39;s eyes, while utilizing one or more lower resolutions for virtual content away from the fixation point. The virtual content is presented by a display system that can display virtual content on a plurality of different depths (e.g., a plurality of different depth planes, such as two or more depth planes), and the reduction in resolution preferably occurs along at least the z axis, where the z-axis is the depth axis (corresponding to distance away from the user). In some embodiments, the resolution reduction occurs along the z-axis and one or both of the x and y axes, where the x-axis is the lateral axis, and the y-axis is the vertical axis. 
     Determining the appropriate resolution of the virtual content may include determining the fixation point, in three-dimensional space, of a user&#39;s eyes. For example, the fixation point may be an x, y, z, coordinate in a field of view of the user, upon which the user&#39;s eyes are fixated. The display system may be configured to present virtual objects that have differences in resolution, with the resolution decreasing with decreasing proximity of a virtual object to the fixation point; stated another way, the resolution decreases with increasing distance from the fixation point. 
     As discussed herein, the display system may present virtual objects within a display frustum of the display system, with the virtual objects capable of being presented on different depth planes. In some embodiments, the display frustum is the field of view provided by the display system, over which the display system is configured to present virtual content to the user of the display system. The display system may be a head-mounted display system including one or more waveguides which may present virtual content (e.g., virtual objects, graphics, text, and so on), with the one or more waveguides configured to output light with different wavefront divergence and/or different binocular disparity corresponding to the different depth planes (e.g., corresponding to particular distances from the user). It will be appreciated that each eye may have an associated one or more waveguides. Using the different wavefront divergence and/or different binocular disparity, the display system may cause a first virtual object to appear to be located at a first depth in the user&#39;s field of view, while causing a second virtual object to appear to be located at a second depth in the user&#39;s field of view. In some embodiments, the depth plane of or a close depth plane to the fixation point may be determined and the resolution of content on other depth planes may be reduced based on distance of those depth planes to the depth plane on which the fixation point is disposed. It will be appreciated that references to the depth of virtual content herein (the distance of the virtual content from the user on the z-axis) refer to the apparent depth of the virtual content as intended to be seen to the user; in some embodiments, the depth of the virtual object may be understood to be the distance from the user of a real object having wavefront divergence and/or binocular disparity similar to that of the virtual object. 
     It will be appreciated that the proximity of a virtual object to the fixation point may be determined by various measures, non-limiting examples of which include determining the distance between the fixation point and the virtual object, determining the resolution adjustment zone occupied by the virtual object relative to a resolution adjustment zone occupied by the fixation point (in embodiments where the user&#39;s field of view is subdivided into resolution adjustment zones as described below), and determining the angular proximity of the virtual object to the fixation point of the user. The proximity may also be determined using a combination of the above-noted techniques. For example, the distance and/or angular proximity of a first zone (in which a virtual object is located) to a second zone (in which the fixation point is located) may be used to determine proximity. These various measures are further discussed below. 
     In some embodiments, determining the fixation point may include anticipating the fixation point of the user&#39;s eyes and utilizing the anticipated fixation point as the fixation point for determining the resolution of virtual content. For example, the display system may render particular content at a relatively high resolution with the expectation that the user&#39;s eyes will fixate on that content. As an example, it will be appreciated that the human visual system may be sensitive to sudden changes in a scene (e.g., sudden motion, changes in luminance, etc.). In some embodiments, the display system may determine that the virtual content is of a type (e.g., involving motion in a scene in which other virtual and real objects are still) that would cause the user&#39;s eyes to fixate on it, and then render that virtual content at high resolution with the expectation that the user&#39;s eyes will subsequently focus on that virtual content. 
     As noted above, in some embodiments, the distance from the determined fixation point to a virtual object may correspond to a distance extending in three-dimensions. As an example, a first virtual object located on a same depth from the user (e.g., at the same depth plane) as the determined fixation point, but located horizontally or longitudinally from the fixation point, may be similarly reduced in resolution as a second virtual object located at a further depth (e.g., a further depth plane) from the determined fixation point. Consequently, different resolutions may be associated with different distances from the fixation point. 
     In some embodiments, the environment around the user may be broken into volumes of space (herein also referred to as resolution adjustment zones) with the resolution of virtual objects in the same resolution adjustment zone being similar. The resolution adjustment zones may have arbitrary three-dimensional shapes, e.g., cubes, or other three-dimensional polygonal shapes, or curved three-dimensional shapes, as described herein. In some embodiments, all resolution adjustment zones have similar shapes, e.g., cuboid or spherical. In some other embodiments, different resolution adjustment zones may have different shapes or sizes (e.g., the shapes and/or sizes of the volumes may change with distance from the fixation point). 
     In some embodiments, the resolution adjustment zones are portions of the user&#39;s field of view. For instance, the field of view of the user may be separated into volumes of space forming the resolution adjustment zones. In some embodiments, each depth plane may be subdivided into one or more contiguous volumes of space, that is, one or more resolution adjustment zones. In some embodiments, each resolution adjustment zone can encompass a particular range of depths from the user (e.g., a depth plane value+/−a variance, wherein examples of variances include 0.66 dpt, 0.50 dpt, 0.33 dpt, or 0.25 dpt), and a particular lateral and a particular vertical distance. Virtual objects located within the same resolution adjustment zone as the determined fixation point may be presented (e.g., rendered) at a high (e.g., full) resolution, while virtual objects located in volumes of space outside of the fixation point&#39;s resolution adjustment zone may be rendered at lesser resolutions according to a distance of the volumes from the fixation point&#39;s volume of space. In some embodiments, each resolution adjustment zone may be assigned a particular resolution (e.g., a particular reduction in resolution relative to the full resolution) and virtual content falling within a given zone may be rendered at the associated resolution for that zone. In some embodiments, the distance between a volume and the volume occupied by the fixation point may be determined, and the resolution may be set based upon this distance. 
     Advantageously, the number and sizes of the resolution adjustment zones utilized to break up a user&#39;s field of view may be modified according to a confidence in the user&#39;s determined fixation point. For example, the size associated with each volume of space may be increased or decreased based on the confidence that the user&#39;s gaze is verging on a precise point in three-dimensional space. If a confidence in the fixation point is high, the display system may present only virtual objects within a compact resolution adjustment zone at a relative high resolution (the compact resolution adjustment zone including the fixation point), while reducing resolutions of other virtual objects, and thus conserving processing power. However, if the confidence is low, the display system may increase the size of each volume of space (e.g., reduce an overall number of the volumes), such that each volume of space encompasses a greater number of virtual objects in the fixation point&#39;s volume of space. It will be appreciated that the sizes and shapes of the volumes may be fixed during production of the display system, e.g., based upon expected tolerances in systems for determining the fixation point, and/or may be adjusted or set in the field depending upon a user&#39;s characteristics, the user&#39;s environment, and/or changes in software that change the tolerances for the systems for determining the fixation point. 
     It will be appreciated that the user&#39;s sensitivity to resolution may decrease with distance from the fixation point. Consequently, by ensuring that full resolution content is presented at the fixation point and by allowing a margin of error for where the fixation point is located, the perceptibility of reductions in resolution may be reduced or eliminated, thereby providing the perception of a high-resolution display without utilizing the computational resources typically required to present content for such a high resolution display. 
     In some embodiments, the proximity of a virtual object to the fixation point may be determined based on an angular proximity of the virtual object to a gaze of the user, and a resolution of the virtual object may decrease as the angular proximity decreases. In some embodiments, this may result in virtual objects located at different depths from the user being presented at a similar resolution. For example, a first virtual object at a location corresponding to a user&#39;s determined fixation point may be located in front (e.g., closer in depth to the user) of a second virtual object. Since the second virtual object will be along a gaze of the user, and thus similarly fall on the user&#39;s fovea, where the user&#39;s eye is most sensitive to changes in resolution, the second virtual object may optionally be presented at a similar (e.g. same) resolution as the first virtual object. Optionally, the second virtual object may be reduced in resolution, and further adjusted via a blurring process (e.g., a Gaussian blurring kernel may be convolved with the second virtual object), which may represent that the second virtual object is further (e.g., located on a farther depth plane) from the user. 
     The reductions in resolution may vary based upon how virtual content is presented by the display systems. In some embodiments, a first example display system referred to herein as a vari-focal display system may present virtual content on different depth planes, with all content (e.g., virtual objects) presented at a same depth plane (e.g., via a same waveguide) at a time, e.g. for each frame presented to the user. That is, the vari-focal display system may utilize a single depth plane (e.g., selected from multiple depth planes based on a fixation point of the user, or selected based on a depth of a particular presented virtual object) at a time to present content, and may change the depth plane in subsequent frames (e.g., select different depth planes). In some other embodiments, a second example display system referred to herein as a multi-focal display system may present virtual content on different depth planes, with content simultaneously displayed on multiple depth planes. As will be further described herein, the vari-focal display system may optionally utilize a single frame buffer, and with respect to the example above regarding blurring a second virtual object, the second virtual object may be blurred prior to presentation to the user from the single frame buffer. In contrast, the multi-focal display system may present the second virtual object on a further depth (e.g., on a further depth plane) from the first virtual object optionally at a reduced resolution, and the second virtual object may appear to the user as being blurred (e.g., the second virtual object will be blurred based on the natural physics of the user&#39;s eyes, without further processing). 
     As disclosed herein, the display system may present virtual objects at relatively high (e.g. full) resolution at or near the determined fixation point, and may present virtual objects at reduced resolutions farther from the fixation point. Preferably, the relatively high resolution is the highest resolution for presentation of virtual objects in the user&#39;s field of view. The relatively high resolution may be a maximum resolution of the display system, a user-selectable resolution, a resolution based on specific computing hardware presenting the virtual objects, and so on. 
     It will be appreciated that adjusting resolution of a virtual object may include any modification to the virtual object to alter a quality of presentation of the virtual object. Such modifications may include one or more of adjusting a polygon count of the virtual object, adjusting primitives utilized to generate the virtual object (e.g., adjusting a shape of the primitives, for example adjusting primitives from triangle mesh to quadrilateral mesh, and so on), adjusting operations performed on the virtual object (e.g., shader operations), adjusting texture information, adjusting color resolution or depth, adjusting a number of rendering cycles or a frame rate, and so on, including adjusting quality at one or more points within a graphics pipeline of graphics processing units (GPUs). 
     In some embodiments, on the x and y-axes, changes in the resolution of virtual content away from the fixation point may generally track changes in the distribution of photoreceptors in the retina of an eye of the user. For example, it will be appreciated that a view of the world and of virtual content may be imaged on the retina, such that different parts of the retina may be mapped to different parts of the user&#39;s field of view. Advantageously, the resolution of virtual content across the user&#39;s field of view may generally track the density of corresponding photoreceptors (rods or cones) across the retina. In some embodiments, the resolution reduction away from the fixation point may generally track the reduction in density of cones across the retina. In some other embodiments, the resolution reduction away from the fixation point may generally track the reduction in density of rods across the retina. In some embodiments, the trend of the resolution reduction away from the fixation point may be within ±50%, ±30%, ±20%, or ±10% of the trend in the reduction in the density of rods and/or cones across the retina. 
     The rods and cones are active at different levels of incident light. For example, cones are active under relatively bright conditions, while rods are active under relatively low light conditions. Consequently, in some embodiments where the reduction in resolution generally tracks the densities of rods or cones across the retina, the display system may be configured to determine the amount of light incident on the retina. Based on this amount of light, the appropriate adjustment in resolution may be made. For example, the reduction in resolution may generally track the changes in the density of rods across the retina in low light conditions, while the reduction in resolution may generally track the changes in the density of cones in bright conditions. Consequently, in some embodiments, the display system may be configured to change the profile of the reduction in image resolution based upon the amount of light incident on the retina. 
     It will be appreciated that the ability of the human eye to resolve fine details may not be directly proportional to the densities of rods or cones in the retina. In some embodiments, changes in the resolution of virtual content across the user&#39;s field of view generally track changes in the ability of the eye to resolve fine details. As noted above, the progression of the changes in resolution of the virtual content may vary with the amount of light reaching the retina. 
     In some embodiments, the amount of light reaching the retina may be determined by detecting the amount of ambient light incident on a sensor mounted on the display device. In some embodiments, determining the amount of light reaching the retina may also include determining the amount of light outputted by the display device to the user. In yet other embodiments, the amount of light reaching the retina may be determined by imaging the eye of the user to determine pupil size. Because pupil size is related to the amount of light reaching the retina, determining pupil size allows the amount of light reaching the retina to be extrapolated. 
     It will be appreciated that full color virtual content may be formed by a plurality of component color images, which, in the aggregate, provide the perception of full color. The human eye may have different sensitivities to different wavelengths, or colors, of light. In some embodiments, in addition to changing based on proximity to a fixation point, the changes in resolution of the virtual content may vary based upon the color of the component color image that is presented by the display system. For example, were the component color images comprise red, green, and blue images, the green component color images may have a higher resolution than the red component color images, which may have a higher resolution than the blue component color images. In some embodiments, to account for changes in the sensitivities of the eye to different colors at different levels of incident light, the amount of light reaching the retina may be determined, and the resolution adjustment for a given component color image may also vary based upon the determination of the amount of light reaching the retina. 
     It will be appreciated that the contrast sensitivity of the eye may also vary based on the amount of light incident on the retina. In some embodiments, the size or total number of gradations in contrast in the virtual content may vary based upon the amount of light reaching the retina. In some embodiments, the contrast ratio of images forming the virtual content may vary based upon the amount of light incident on the retina, with the contrast ratio decreasing with decreasing amounts of light. 
     In some embodiments, certain parts of the user&#39;s field of view may not be provided with any virtual content. For example, the display system may be configured to not provide virtual content in a blind spot caused by the optic nerve and/or a peripheral blind spot of a given eye. 
     As discussed herein, the display system may be configured to display high resolution content in one part of the user&#39;s field of view and lower resolution content in another part of the user&#39;s field of view. It will be appreciated that the high resolution content may have a higher pixel density than the lower resolution content. In some environments, the display system may be configured to provide such high and low resolution content by effectively superimposing high-resolution and low resolution images. For example, the system may display a low resolution image that spans the entire field of view, and then display a high resolution image spanning a small portion of the field of view, with the high-resolution image being located at the same location as a corresponding portion of the low resolution image. The high and low resolution images may be routed through different optics, which output light at appropriate angles to determine how much of the field of view those images occupy. 
     In some embodiments, a single spatial light modulator (SLM) may be used to encode light with image information, and a beam splitter or optical switch may be used to split a single light stream from the SLM into two streams, one stream to propagate through optics for the low-resolution images and a second stream to propagate through optics for the high-resolution images. In some other embodiments, the polarization of the light encoded with image information may be selectively switched and passed through optics that effectively provide different angular magnifications for light of different polarizations, thereby providing the high and low resolution images. 
     Advantageously, various embodiments disclosed herein reduce requirements for processing power for providing content on display systems. Since a larger share of processing power may be devoted to virtual objects that are proximate to a user&#39;s three-dimensional fixation point, while processing power for virtual objects further away may be reduced, the overall required processing power for the display system may be reduced, thus reducing one or more of the size of processing components, the heat generated by the processing components, and the energy requirements for the display system (e.g., the display system may optionally be battery powered, require lower capacity batteries, and/or operate for a longer duration with a given battery). Therefore, embodiments described herein address technological problems arising out of augmented or virtual reality display systems. Additionally, the described techniques manipulate graphical content such that upon presentation to the user, the graphical content is presented fundamentally differently (e.g., resolutions are modified), while the graphical content may appear to the user as being the same. Thus, the display system transforms graphical content while preserving visual fidelity, and conserving processing power, as the user looks around their ambient environment. 
     It will be appreciated that the display system may be part of an augmented reality display system, or a virtual reality display system. As one example, the display of the display system may be transmissive and may allow the user a view of the real world, while providing virtual content in the form of images, video, interactivity, and so on, to the user. As another example, the display system may block the user&#39;s view of the real world, and virtual reality images, video, interactivity, and so on, may be presented to the user. 
     Reference will now be made to the drawings, in which like reference numerals refer to like parts throughout. 
       FIG.  2    illustrates a conventional display system for simulating three-dimensional imagery for a user. It will be appreciated that a user&#39;s eyes are spaced apart and that, when looking at a real object in space, each eye will have a slightly different view of the object and may form an image of the object at different locations on the retina of each eye. This may be referred to as binocular disparity and may be utilized by the human visual system to provide a perception of depth. Conventional display systems simulate binocular disparity by presenting two distinct images  190 ,  200  with slightly different views of the same virtual object—one for each eye  210 ,  220 —corresponding to the views of the virtual object that would be seen by each eye were the virtual object a real object at a desired depth. These images provide binocular cues that the user&#39;s visual system may interpret to derive a perception of depth. 
     With continued reference to  FIG.  2   , the images  190 ,  200  are spaced from the eyes  210 ,  220  by a distance  230  on a z-axis. The z-axis is parallel to the optical axis of the viewer with their eyes fixated on an object at optical infinity directly ahead of the viewer. The images  190 ,  200  are flat and at a fixed distance from the eyes  210 ,  220 . Based on the slightly different views of a virtual object in the images presented to the eyes  210 ,  220 , respectively, the eyes may naturally rotate such that an image of the object falls on corresponding points on the retinas of each of the eyes, to maintain single binocular vision. This rotation may cause the lines of sight of each of the eyes  210 ,  220  to converge onto a point in space at which the virtual object is perceived to be present. As a result, providing three-dimensional imagery conventionally involves providing binocular cues that may manipulate the vergence of the user&#39;s eyes  210 ,  220 , and that the human visual system interprets to provide a perception of depth. 
     Generating a realistic and comfortable perception of depth is challenging, however. It will be appreciated that light from objects at different distances from the eyes have wavefronts with different amounts of divergence.  FIGS.  3 A- 3 C  illustrate relationships between distance and the divergence of light rays. The distance between the object and the eye  210  is represented by, in order of decreasing distance, R 1 , R 2 , and R 3 . As shown in  FIGS.  3 A- 3 C , the light rays become more divergent as distance to the object decreases. Conversely, as distance increases, the light rays become more collimated. Stated another way, it may be said that the light field produced by a point (the object or a part of the object) has a spherical wavefront curvature, which is a function of how far away the point is from the eye of the user. The curvature increases with decreasing distance between the object and the eye  210 . While only a single eye  210  is illustrated for clarity of illustration in  FIGS.  3 A- 3 C  and other figures herein, the discussions regarding eye  210  may be applied to both eyes  210  and  220  of a viewer. 
     With continued reference to  FIGS.  3 A- 3 C , light from an object that the viewer&#39;s eyes are fixated on may have different degrees of wavefront divergence. Due to the different amounts of wavefront divergence, the light may be focused differently by the lens of the eye, which in turn may require the lens to assume different shapes to form a focused image on the retina of the eye. Where a focused image is not formed on the retina, the resulting retinal blur acts as a cue to accommodation that causes a change in the shape of the lens of the eye until a focused image is formed on the retina. For example, the cue to accommodation may trigger the ciliary muscles surrounding the lens of the eye to relax or contract, thereby modulating the force applied to the suspensory ligaments holding the lens, thus causing the shape of the lens of the eye to change until retinal blur of an object of fixation is eliminated or minimized, thereby forming a focused image of the object of fixation on the retina (e.g., fovea) of the eye. The process by which the lens of the eye changes shape may be referred to as accommodation, and the shape of the lens of the eye required to form a focused image of the object of fixation on the retina (e.g., fovea) of the eye may be referred to as an accommodative state. 
     With reference now to  FIG.  4 A , a representation of the accommodation-vergence response of the human visual system is illustrated. The movement of the eyes to fixate on an object causes the eyes to receive light from the object, with the light forming an image on each of the retinas of the eyes. The presence of retinal blur in the image formed on the retina may provide a cue to accommodation, and the relative locations of the image on the retinas may provide a cue to vergence. The cue to accommodation causes accommodation to occur, resulting in the lenses of the eyes each assuming a particular accommodative state that forms a focused image of the object on the retina (e.g., fovea) of the eye. On the other hand, the cue to vergence causes vergence movements (rotation of the eyes) to occur such that the images formed on each retina of each eye are at corresponding retinal points that maintain single binocular vision. In these positions, the eyes may be said to have assumed a particular vergence state. With continued reference to  FIG.  4 A , accommodation may be understood to be the process by which the eye achieves a particular accommodative state, and vergence may be understood to be the process by which the eye achieves a particular vergence state. As indicated in  FIG.  4 A , the accommodative and vergence states of the eyes may change if the user fixates on another object. For example, the accommodated state may change if the user fixates on a new object at a different depth on the z-axis. 
     Without being limited by theory, it is believed that viewers of an object may perceive the object as being “three-dimensional” due to a combination of vergence and accommodation. As noted above, vergence movements (e.g., rotation of the eyes so that the pupils move toward or away from each other to converge the lines of sight of the eyes to fixate upon an object) of the two eyes relative to each other are closely associated with accommodation of the lenses of the eyes. Under normal conditions, changing the shapes of the lenses of the eyes to change focus from one object to another object at a different distance will automatically cause a matching change in vergence to the same distance, under a relationship known as the “accommodation-vergence reflex.” Likewise, a change in vergence will trigger a matching change in lens shape under normal conditions. 
     With reference now to  FIG.  4 B , examples of different accommodative and vergence states of the eyes are illustrated. The pair of eyes  222   a  are fixated on an object at optical infinity, while the pair eyes  222   b  are fixated on an object  221  at less than optical infinity. Notably, the vergence states of each pair of eyes is different, with the pair of eyes  222   a  directed straight ahead, while the pair of eyes  222  converge on the object  221 . The accommodative states of the eyes forming each pair of eyes  222   a  and  222   b  are also different, as represented by the different shapes of the lenses  210   a ,  220   a.    
     Undesirably, many users of conventional “3-D” display systems find such conventional systems to be uncomfortable or may not perceive a sense of depth at all due to a mismatch between accommodative and vergence states in these displays. As noted above, many stereoscopic or “3-D” display systems display a scene by providing slightly different images to each eye. Such systems are uncomfortable for many viewers, since they, among other things, simply provide different presentations of a scene and cause changes in the vergence states of the eyes, but without a corresponding change in the accommodative states of those eyes. Rather, the images are shown by a display at a fixed distance from the eyes, such that the eyes view all the image information at a single accommodative state. Such an arrangement works against the “accommodation-vergence reflex” by causing changes in the vergence state without a matching change in the accommodative state. This mismatch is believed to cause viewer discomfort. Display systems that provide a better match between accommodation and vergence may form more realistic and comfortable simulations of three-dimensional imagery. 
     Without being limited by theory, it is believed that the human eye typically may interpret a finite number of depth planes to provide depth perception. Consequently, a highly believable simulation of perceived depth may be achieved by providing, to the eye, different presentations of an image corresponding to each of these limited numbers of depth planes. In some embodiments, the different presentations may provide both cues to vergence and matching cues to accommodation, thereby providing physiologically correct accommodation-vergence matching. 
     With continued reference to  FIG.  4 B , two depth planes  240 , corresponding to different distances in space from the eyes  210 ,  220 , are illustrated. For a given depth plane  240 , vergence cues may be provided by the displaying of images of appropriately different perspectives for each eye  210 ,  220 . In addition, for a given depth plane  240 , light forming the images provided to each eye  210 ,  220  may have a wavefront divergence corresponding to a light field produced by a point at the distance of that depth plane  240 . 
     In the illustrated embodiment, the distance, along the z-axis, of the depth plane  240  containing the point  221  is 1 m. As used herein, distances or depths along the z-axis may be measured with a zero point located at the exit pupils of the user&#39;s eyes. Thus, a depth plane  240  located at a depth of 1 m corresponds to a distance of 1 m away from the exit pupils of the user&#39;s eyes, on the optical axis of those eyes. As an approximation, the depth or distance along the z-axis may be measured from the display in front of the user&#39;s eyes (e.g., from the surface of a waveguide), plus a value for the distance between the device and the exit pupils of the user&#39;s eyes, with the eyes directed towards optical infinity. That value may be called the eye relief and corresponds to the distance between the exit pupil of the user&#39;s eye and the display worn by the user in front of the eye. In practice, the value for the eye relief may be a normalized value used generally for all viewers. For example, the eye relief may be assumed to be 20 mm and a depth plane that is at a depth of 1 m may be at a distance of 980 mm in front of the display. 
     With reference now to  FIGS.  4 C and  4 D , examples of matched accommodation-vergence distances and mismatched accommodation-vergence distances are illustrated, respectively. As illustrated in  FIG.  4 C , the display system may provide images of a virtual object to each eye  210 ,  220 . The images may cause the eyes  210 ,  220  to assume a vergence state in which the eyes converge on a point  15  on a depth plane  240 . In addition, the images may be formed by light having a wavefront curvature corresponding to real objects at that depth plane  240 . As a result, the eyes  210 ,  220  assume an accommodative state in which the images are in focus on the retinas of those eyes. Thus, the user may perceive the virtual object as being at the point  15  on the depth plane  240 . 
     It will be appreciated that each of the accommodative and vergence states of the eyes  210 ,  220  are associated with a particular distance on the z-axis. For example, an object at a particular distance from the eyes  210 ,  220  causes those eyes to assume particular accommodative states based upon the distances of the object. The distance associated with a particular accommodative state may be referred to as the accommodation distance, A d . Similarly, there are particular vergence distances, V d , associated with the eyes in particular vergence states, or positions relative to one another. Where the accommodation distance and the vergence distance match, the relationship between accommodation and vergence may be said to be physiologically correct. This is considered to be the most comfortable scenario for a viewer. 
     In stereoscopic displays, however, the accommodation distance and the vergence distance may not always match. For example, as illustrated in  FIG.  4 D , images displayed to the eyes  210 ,  220  may be displayed with wavefront divergence corresponding to depth plane  240 , and the eyes  210 ,  220  may assume a particular accommodative state in which the points  15   a ,  15   b  on that depth plane are in focus. However, the images displayed to the eyes  210 ,  220  may provide cues for vergence that cause the eyes  210 ,  220  to converge on a point  15  that is not located on the depth plane  240 . As a result, the accommodation distance corresponds to the distance from a particular reference point of the user (e.g., the exit pupils of the eyes  210 ,  220 ) to the depth plane  240 , while the vergence distance corresponds to the larger distance from that reference point to the point  15 , in some embodiments. Thus, the accommodation distance is different from the vergence distance and there is an accommodation-vergence mismatch. Such a mismatch is considered undesirable and may cause discomfort in the user. It will be appreciated that the mismatch corresponds to distance (e.g., V d −A d ) and may be characterized using diopters (units of reciprocal length, 1/m). For example, a V d  of 1.75 diopter and an A d  of 1.25 diopter, or a V d  of 1.25 diopter and an A d  of 1.75 diopter, would provide an accommodation-vergence mismatch of 0.5 diopter. 
     In some embodiments, it will be appreciated that a reference point other than exit pupils of the eyes  210 ,  220  may be utilized for determining distance for determining accommodation-vergence mismatch, so long as the same reference point is utilized for the accommodation distance and the vergence distance. For example, the distances could be measured from the cornea to the depth plane, from the retina to the depth plane, from the eyepiece (e.g., a waveguide of the display device) to the depth plane, and so on. 
     Without being limited by theory, it is believed that users may still perceive accommodation-vergence mismatches of up to about 0.25 diopter, up to about 0.33 diopter, and up to about 0.5 diopter as being physiologically correct, without the mismatch itself causing significant discomfort. In some embodiments, display systems disclosed herein (e.g., the display system  250 ,  FIG.  6   ) present images to the viewer having accommodation-vergence mismatch of about 0.5 diopter or less. In some other embodiments, the accommodation-vergence mismatch of the images provided by the display system is about 0.33 diopter or less. In yet other embodiments, the accommodation-vergence mismatch of the images provided by the display system is about 0.25 diopter or less, including about 0.1 diopter or less. 
       FIG.  5    illustrates aspects of an approach for simulating three-dimensional imagery by modifying wavefront divergence. The display system includes a waveguide  270  that is configured to receive light  770  that is encoded with image information, and to output that light to the user&#39;s eye  210 . The waveguide  270  may output the light  650  with a defined amount of wavefront divergence corresponding to the wavefront divergence of a light field produced by a point on a desired depth plane  240 . In some embodiments, the same amount of wavefront divergence is provided for all objects presented on that depth plane. In addition, it will be illustrated that the other eye of the user may be provided with image information from a similar waveguide. 
     In some embodiments, a single waveguide may be configured to output light with a set amount of wavefront divergence corresponding to a single or limited number of depth planes and/or the waveguide may be configured to output light of a limited range of wavelengths. Consequently, in some embodiments, a plurality or stack of waveguides may be utilized to provide different amounts of wavefront divergence for different depth planes and/or to output light of different ranges of wavelengths. As used herein, it will be appreciated at a depth plane may follow the contours of a flat or a curved surface. In some embodiments, for simplicity, the depth planes may follow the contours of flat surfaces. 
       FIG.  6    illustrates an example of a waveguide stack for outputting image information to a user. A display system  250  includes a stack of waveguides, or stacked waveguide assembly,  260  that may be utilized to provide three-dimensional perception to the eye/brain using a plurality of waveguides  270 ,  280 ,  290 ,  300 ,  310 . It will be appreciated that the display system  250  may be considered a light field display in some embodiments. In addition, the waveguide assembly  260  may also be referred to as an eyepiece. 
     In some embodiments, the display system  250  may be configured to provide substantially continuous cues to vergence and multiple discrete cues to accommodation. The cues to vergence may be provided by displaying different images to each of the eyes of the user, and the cues to accommodation may be provided by outputting the light that forms the images with selectable discrete amounts of wavefront divergence. Stated another way, the display system  250  may be configured to output light with variable levels of wavefront divergence. In some embodiments, each discrete level of wavefront divergence corresponds to a particular depth plane and may be provided by a particular one of the waveguides  270 ,  280 ,  290 ,  300 ,  310 . 
     With continued reference to  FIG.  6   , the waveguide assembly  260  may also include a plurality of features  320 ,  330 ,  340 ,  350  between the waveguides. In some embodiments, the features  320 ,  330 ,  340 ,  350  may be one or more lenses. The waveguides  270 ,  280 ,  290 ,  300 ,  310  and/or the plurality of lenses  320 ,  330 ,  340 ,  350  may be configured to send image information to the eye with various levels of wavefront curvature or light ray divergence. Each waveguide level may be associated with a particular depth plane and may be configured to output image information corresponding to that depth plane. Image injection devices  360 ,  370 ,  380 ,  390 ,  400  may function as a source of light for the waveguides and may be utilized to inject image information into the waveguides  270 ,  280 ,  290 ,  300 ,  310 , each of which may be configured, as described herein, to distribute incoming light across each respective waveguide, for output toward the eye  210 . Light exits an output surface  410 ,  420 ,  430 ,  440 ,  450  of the image injection devices  360 ,  370 ,  380 ,  390 ,  400  and is injected into a corresponding input surface  460 ,  470 ,  480 ,  490 ,  500  of the waveguides  270 ,  280 ,  290 ,  300 ,  310 . In some embodiments, each of the input surfaces  460 ,  470 ,  480 ,  490 ,  500  may be an edge of a corresponding waveguide, or may be part of a major surface of the corresponding waveguide (that is, one of the waveguide surfaces directly facing the world  510  or the viewer&#39;s eye  210 ). In some embodiments, a single beam of light (e.g. a collimated beam) may be injected into each waveguide to output an entire field of cloned collimated beams that are directed toward the eye  210  at particular angles (and amounts of divergence) corresponding to the depth plane associated with a particular waveguide. In some embodiments, a single one of the image injection devices  360 ,  370 ,  380 ,  390 ,  400  may be associated with and inject light into a plurality (e.g., three) of the waveguides  270 ,  280 ,  290 ,  300 ,  310 . 
     In some embodiments, the image injection devices  360 ,  370 ,  380 ,  390 ,  400  are discrete displays that each produce image information for injection into a corresponding waveguide  270 ,  280 ,  290 ,  300 ,  310 , respectively. In some other embodiments, the image injection devices  360 ,  370 ,  380 ,  390 ,  400  are the output ends of a single multiplexed display which may, e.g., pipe image information via one or more optical conduits (such as fiber optic cables) to each of the image injection devices  360 ,  370 ,  380 ,  390 ,  400 . It will be appreciated that the image information provided by the image injection devices  360 ,  370 ,  380 ,  390 ,  400  may include light of different wavelengths, or colors (e.g., different component colors, as discussed herein). 
     In some embodiments, the light injected into the waveguides  270 ,  280 ,  290 ,  300 ,  310  is provided by a light projector system  520 , which comprises a light module  530 , which may include a light emitter, such as a light emitting diode (LED). The light from the light module  530  may be directed to and modified by a light modulator  540 , e.g., a spatial light modulator, via a beam splitter  550 . The light modulator  540  may be configured to change the perceived intensity of the light injected into the waveguides  270 ,  280 ,  290 ,  300 ,  310  to encode the light with image information. Examples of spatial light modulators include liquid crystal displays (LCD) including a liquid crystal on silicon (LCOS) displays. It will be appreciated that the image injection devices  360 ,  370 ,  380 ,  390 ,  400  are illustrated schematically and, in some embodiments, these image injection devices may represent different light paths and locations in a common projection system configured to output light into associated ones of the waveguides  270 ,  280 ,  290 ,  300 ,  310 . In some embodiments, the waveguides of the waveguide assembly  260  may function as ideal lens while relaying light injected into the waveguides out to the user&#39;s eyes. In this conception, the object may be the spatial light modulator  540  and the image may be the image on the depth plane. 
     In some embodiments, the display system  250  may be a scanning fiber display comprising one or more scanning fibers configured to project light in various patterns (e.g., raster scan, spiral scan, Lissajous patterns, etc.) into one or more waveguides  270 ,  280 ,  290 ,  300 ,  310  and ultimately to the eye  210  of the viewer. In some embodiments, the illustrated image injection devices  360 ,  370 ,  380 ,  390 ,  400  may schematically represent a single scanning fiber or a bundle of scanning fibers configured to inject light into one or a plurality of the waveguides  270 ,  280 ,  290 ,  300 ,  310 . In some other embodiments, the illustrated image injection devices  360 ,  370 ,  380 ,  390 ,  400  may schematically represent a plurality of scanning fibers or a plurality of bundles of scanning fibers, each of which are configured to inject light into an associated one of the waveguides  270 ,  280 ,  290 ,  300 ,  310 . It will be appreciated that one or more optical fibers may be configured to transmit light from the light module  530  to the one or more waveguides  270 ,  280 ,  290 ,  300 ,  310 . It will be appreciated that one or more intervening optical structures may be provided between the scanning fiber, or fibers, and the one or more waveguides  270 ,  280 ,  290 ,  300 ,  310  to, e.g., redirect light exiting the scanning fiber into the one or more waveguides  270 ,  280 ,  290 ,  300 ,  310 . 
     A controller  560  controls the operation of one or more of the stacked waveguide assembly  260 , including operation of the image injection devices  360 ,  370 ,  380 ,  390 ,  400 , the light source  530 , and the light modulator  540 . In some embodiments, the controller  560  is part of the local data processing module  140 . The controller  560  includes programming (e.g., instructions in a non-transitory medium) that regulates the timing and provision of image information to the waveguides  270 ,  280 ,  290 ,  300 ,  310  according to, e.g., any of the various schemes disclosed herein. In some embodiments, the controller may be a single integral device, or a distributed system connected by wired or wireless communication channels. The controller  560  may be part of the processing modules  140  or  150  ( FIG.  9 D ) in some embodiments. 
     With continued reference to  FIG.  6   , the waveguides  270 ,  280 ,  290 ,  300 ,  310  may be configured to propagate light within each respective waveguide by total internal reflection (TIR). The waveguides  270 ,  280 ,  290 ,  300 ,  310  may each be planar or have another shape (e.g., curved), with major top and bottom surfaces and edges extending between those major top and bottom surfaces. In the illustrated configuration, the waveguides  270 ,  280 ,  290 ,  300 ,  310  may each include out-coupling optical elements  570 ,  580 ,  590 ,  600 ,  610  that are configured to extract light out of a waveguide by redirecting the light, propagating within each respective waveguide, out of the waveguide to output image information to the eye  210 . Extracted light may also be referred to as out-coupled light and the out-coupling optical elements light may also be referred to light extracting optical elements. An extracted beam of light may be outputted by the waveguide at locations at which the light propagating in the waveguide strikes a light extracting optical element. The out-coupling optical elements  570 ,  580 ,  590 ,  600 ,  610  may, for example, be gratings, including diffractive optical features, as discussed further herein. While illustrated disposed at the bottom major surfaces of the waveguides  270 ,  280 ,  290 ,  300 ,  310 , for ease of description and drawing clarity, in some embodiments, the out-coupling optical elements  570 ,  580 ,  590 ,  600 ,  610  may be disposed at the top and/or bottom major surfaces, and/or may be disposed directly in the volume of the waveguides  270 ,  280 ,  290 ,  300 ,  310 , as discussed further herein. In some embodiments, the out-coupling optical elements  570 ,  580 ,  590 ,  600 ,  610  may be formed in a layer of material that is attached to a transparent substrate to form the waveguides  270 ,  280 ,  290 ,  300 ,  310 . In some other embodiments, the waveguides  270 ,  280 ,  290 ,  300 ,  310  may be a monolithic piece of material and the out-coupling optical elements  570 ,  580 ,  590 ,  600 ,  610  may be formed on a surface and/or in the interior of that piece of material. 
     With continued reference to  FIG.  6   , as discussed herein, each waveguide  270 ,  280 ,  290 ,  300 ,  310  is configured to output light to form an image corresponding to a particular depth plane. For example, the waveguide  270  nearest the eye may be configured to deliver collimated light (which was injected into such waveguide  270 ), to the eye  210 . The collimated light may be representative of the optical infinity focal plane. The next waveguide up  280  may be configured to send out collimated light which passes through the first lens  350  (e.g., a negative lens) before it may reach the eye  210 ; such first lens  350  may be configured to create a slight convex wavefront curvature so that the eye/brain interprets light coming from that next waveguide up  280  as coming from a first focal plane closer inward toward the eye  210  from optical infinity. Similarly, the third up waveguide  290  passes its output light through both the first  350  and second  340  lenses before reaching the eye  210 ; the combined optical power of the first  350  and second  340  lenses may be configured to create another incremental amount of wavefront curvature so that the eye/brain interprets light coming from the third waveguide  290  as coming from a second focal plane that is even closer inward toward the person from optical infinity than was light from the next waveguide up  280 . 
     The other waveguide layers  300 ,  310  and lenses  330 ,  320  are similarly configured, with the highest waveguide  310  in the stack sending its output through all of the lenses between it and the eye for an aggregate focal power representative of the closest focal plane to the person. To compensate for the stack of lenses  320 ,  330 ,  340 ,  350  when viewing/interpreting light coming from the world  510  on the other side of the stacked waveguide assembly  260 , a compensating lens layer  620  may be disposed at the top of the stack to compensate for the aggregate power of the lens stack  320 ,  330 ,  340 ,  350  below. Such a configuration provides as many perceived focal planes as there are available waveguide/lens pairings. Both the out-coupling optical elements of the waveguides and the focusing aspects of the lenses may be static (i.e., not dynamic or electro-active). In some alternative embodiments, either or both may be dynamic using electro-active features. 
     In some embodiments, two or more of the waveguides  270 ,  280 ,  290 ,  300 ,  310  may have the same associated depth plane. For example, multiple waveguides  270 ,  280 ,  290 ,  300 ,  310  may be configured to output images set to the same depth plane, or multiple subsets of the waveguides  270 ,  280 ,  290 ,  300 ,  310  may be configured to output images set to the same plurality of depth planes, with one set for each depth plane. This may provide advantages for forming a tiled image to provide an expanded field of view at those depth planes. 
     With continued reference to  FIG.  6   , the out-coupling optical elements  570 ,  580 ,  590 ,  600 ,  610  may be configured to both redirect light out of their respective waveguides and to output this light with the appropriate amount of divergence or collimation for a particular depth plane associated with the waveguide. As a result, waveguides having different associated depth planes may have different configurations of out-coupling optical elements  570 ,  580 ,  590 ,  600 ,  610 , which output light with a different amount of divergence depending on the associated depth plane. In some embodiments, the light extracting optical elements  570 ,  580 ,  590 ,  600 ,  610  may be volumetric or surface features, which may be configured to output light at specific angles. For example, the light extracting optical elements  570 ,  580 ,  590 ,  600 ,  610  may be volume holograms, surface holograms, and/or diffraction gratings. In some embodiments, the features  320 ,  330 ,  340 ,  350  may not be lenses; rather, they may simply be spacers (e.g., cladding layers and/or structures for forming air gaps). 
     In some embodiments, the out-coupling optical elements  570 ,  580 ,  590 ,  600 ,  610  are diffractive features that form a diffraction pattern, or “diffractive optical element” (also referred to herein as a “DOE”). Preferably, the DOE&#39;s have a sufficiently low diffraction efficiency so that only a portion of the light of the beam is deflected away toward the eye  210  with each intersection of the DOE, while the rest continues to move through a waveguide via TIR. The light carrying the image information is thus divided into a number of related exit beams that exit the waveguide at a multiplicity of locations and the result is a fairly uniform pattern of exit emission toward the eye  210  for this particular collimated beam bouncing around within a waveguide. 
     In some embodiments, one or more DOEs may be switchable between “on” states in which they actively diffract, and “off” states in which they do not significantly diffract. For instance, a switchable DOE may comprise a layer of polymer dispersed liquid crystal, in which microdroplets comprise a diffraction pattern in a host medium, and the refractive index of the microdroplets may be switched to substantially match the refractive index of the host material (in which case the pattern does not appreciably diffract incident light) or the microdroplet may be switched to an index that does not match that of the host medium (in which case the pattern actively diffracts incident light). 
     In some embodiments, a camera assembly  630  (e.g., a digital camera, including visible light and infrared light cameras) may be provided to capture images of the eye  210  and/or tissue around the eye  210  to, e.g., detect user inputs and/or to monitor the physiological state of the user. As used herein, a camera may be any image capture device. In some embodiments, the camera assembly  630  may include an image capture device and a light source to project light (e.g., infrared light) to the eye, which may then be reflected by the eye and detected by the image capture device. In some embodiments, the camera assembly  630  may be attached to the frame  80  ( FIG.  9 D ) and may be in electrical communication with the processing modules  140  and/or  150 , which may process image information from the camera assembly  630 . In some embodiments, one camera assembly  630  may be utilized for each eye, to separately monitor each eye. 
     With reference now to  FIG.  7   , an example of exit beams outputted by a waveguide is shown. One waveguide is illustrated, but it will be appreciated that other waveguides in the waveguide assembly  260  ( FIG.  6   ) may function similarly, where the waveguide assembly  260  includes multiple waveguides. Light  640  is injected into the waveguide  270  at the input surface  460  of the waveguide  270  and propagates within the waveguide  270  by TIR. At points where the light  640  impinges on the DOE  570 , a portion of the light exits the waveguide as exit beams  650 . The exit beams  650  are illustrated as substantially parallel but, as discussed herein, they may also be redirected to propagate to the eye  210  at an angle (e.g., forming divergent exit beams), depending on the depth plane associated with the waveguide  270 . It will be appreciated that substantially parallel exit beams may be indicative of a waveguide with out-coupling optical elements that out-couple light to form images that appear to be set on a depth plane at a large distance (e.g., optical infinity) from the eye  210 . Other waveguides or other sets of out-coupling optical elements may output an exit beam pattern that is more divergent, which would require the eye  210  to accommodate to a closer distance to bring it into focus on the retina and would be interpreted by the brain as light from a distance closer to the eye  210  than optical infinity. 
     In some embodiments, a full color image may be formed at each depth plane by overlaying images in each of the component colors, e.g., three or more component colors.  FIG.  8    illustrates an example of a stacked waveguide assembly in which each depth plane includes images formed using multiple different component colors. The illustrated embodiment shows depth planes  240   a - 240   f , although more or fewer depths are also contemplated. Each depth plane may have three or more component color images associated with it, including: a first image of a first color, G; a second image of a second color, R; and a third image of a third color, B. Different depth planes are indicated in the figure by different numbers for diopters (dpt) following the letters G, R, and B. Just as examples, the numbers following each of these letters indicate diopters (1/m), or inverse distance of the depth plane from a viewer, and each box in the figures represents an individual component color image. In some embodiments, to account for differences in the eye&#39;s focusing of light of different wavelengths, the exact placement of the depth planes for different component colors may vary. For example, different component color images for a given depth plane may be placed on depth planes corresponding to different distances from the user. Such an arrangement may increase visual acuity and user comfort and/or may decrease chromatic aberrations. 
     In some embodiments, light of each component color may be outputted by a single dedicated waveguide and, consequently, each depth plane may have multiple waveguides associated with it. In such embodiments, each box in the figures including the letters G, R, or B may be understood to represent an individual waveguide, and three waveguides may be provided per depth plane where three component color images are provided per depth plane. While the waveguides associated with each depth plane are shown adjacent to one another in this drawing for ease of description, it will be appreciated that, in a physical device, the waveguides may all be arranged in a stack with one waveguide per level. In some other embodiments, multiple component colors may be outputted by the same waveguide, such that, e.g., only a single waveguide may be provided per depth plane. 
     With continued reference to  FIG.  8   , in some embodiments, G is the color green, R is the color red, and B is the color blue. In some other embodiments, other colors associated with other wavelengths of light, including magenta and cyan, may be used in addition to or may replace one or more of red, green, or blue. 
     It will be appreciated that references to a given color of light throughout this disclosure will be understood to encompass light of one or more wavelengths within a range of wavelengths of light that are perceived by a viewer as being of that given color. For example, red light may include light of one or more wavelengths in the range of about 620-780 nm, green light may include light of one or more wavelengths in the range of about 492-577 nm, and blue light may include light of one or more wavelengths in the range of about 435-493 nm. 
     In some embodiments, the light source  530  ( FIG.  6   ) may be configured to emit light of one or more wavelengths outside the visual perception range of the viewer, for example, infrared and/or ultraviolet wavelengths. In addition, the in-coupling, out-coupling, and other light redirecting structures of the waveguides of the display  250  may be configured to direct and emit this light out of the display towards the user&#39;s eye  210 , e.g., for imaging and/or user stimulation applications. 
     With reference now to  FIG.  9 A , in some embodiments, light impinging on a waveguide may need to be redirected to in-couple that light into the waveguide. An in-coupling optical element may be used to redirect and in-couple the light into its corresponding waveguide.  FIG.  9 A  illustrates a cross-sectional side view of an example of a plurality or set  660  of stacked waveguides that each includes an in-coupling optical element. The waveguides may each be configured to output light of one or more different wavelengths, or one or more different ranges of wavelengths. It will be appreciated that the stack  660  may correspond to the stack  260  ( FIG.  6   ) and the illustrated waveguides of the stack  660  may correspond to part of the plurality of waveguides  270 ,  280 ,  290 ,  300 ,  310 , except that light from one or more of the image injection devices  360 ,  370 ,  380 ,  390 ,  400  is injected into the waveguides from a position that requires light to be redirected for in-coupling. 
     The illustrated set  660  of stacked waveguides includes waveguides  670 ,  680 , and  690 . Each waveguide includes an associated in-coupling optical element (which may also be referred to as a light input area on the waveguide), with, e.g., in-coupling optical element  700  disposed on a major surface (e.g., an upper major surface) of waveguide  670 , in-coupling optical element  710  disposed on a major surface (e.g., an upper major surface) of waveguide  680 , and in-coupling optical element  720  disposed on a major surface (e.g., an upper major surface) of waveguide  690 . In some embodiments, one or more of the in-coupling optical elements  700 ,  710 ,  720  may be disposed on the bottom major surface of the respective waveguide  670 ,  680 ,  690  (particularly where the one or more in-coupling optical elements are reflective, deflecting optical elements). As illustrated, the in-coupling optical elements  700 ,  710 ,  720  may be disposed on the upper major surface of their respective waveguide  670 ,  680 ,  690  (or the top of the next lower waveguide), particularly where those in-coupling optical elements are transmissive, deflecting optical elements. In some embodiments, the in-coupling optical elements  700 ,  710 ,  720  may be disposed in the body of the respective waveguide  670 ,  680 ,  690 . In some embodiments, as discussed herein, the in-coupling optical elements  700 ,  710 ,  720  are wavelength selective, such that they selectively redirect one or more wavelengths of light, while transmitting other wavelengths of light. While illustrated on one side or corner of their respective waveguide  670 ,  680 ,  690 , it will be appreciated that the in-coupling optical elements  700 ,  710 ,  720  may be disposed in other areas of their respective waveguide  670 ,  680 ,  690  in some embodiments. 
     As illustrated, the in-coupling optical elements  700 ,  710 ,  720  may be laterally offset from one another. In some embodiments, each in-coupling optical element may be offset such that it receives light without that light passing through another in-coupling optical element. For example, each in-coupling optical element  700 ,  710 ,  720  may be configured to receive light from a different image injection device  360 ,  370 ,  380 ,  390 , and  400  as shown in  FIG.  6   , and may be separated (e.g., laterally spaced apart) from other in-coupling optical elements  700 ,  710 ,  720  such that it substantially does not receive light from the other ones of the in-coupling optical elements  700 ,  710 ,  720 . 
     Each waveguide also includes associated light distributing elements, with, e.g., light distributing elements  730  disposed on a major surface (e.g., a top major surface) of waveguide  670 , light distributing elements  740  disposed on a major surface (e.g., a top major surface) of waveguide  680 , and light distributing elements  750  disposed on a major surface (e.g., a top major surface) of waveguide  690 . In some other embodiments, the light distributing elements  730 ,  740 ,  750 , may be disposed on a bottom major surface of associated waveguides  670 ,  680 ,  690 , respectively. In some other embodiments, the light distributing elements  730 ,  740 ,  750 , may be disposed on both top and bottom major surface of associated waveguides  670 ,  680 ,  690 , respectively; or the light distributing elements  730 ,  740 ,  750 , may be disposed on different ones of the top and bottom major surfaces in different associated waveguides  670 ,  680 ,  690 , respectively. 
     The waveguides  670 ,  680 ,  690  may be spaced apart and separated by, e.g., gas, liquid, and/or solid layers of material. For example, as illustrated, layer  760   a  may separate waveguides  670  and  680 ; and layer  760   b  may separate waveguides  680  and  690 . In some embodiments, the layers  760   a  and  760   b  are formed of low refractive index materials (that is, materials having a lower refractive index than the material forming the immediately adjacent one of waveguides  670 ,  680 ,  690 ). Preferably, the refractive index of the material forming the layers  760   a ,  760   b  is 0.05 or more, or 0.10 or less than the refractive index of the material forming the waveguides  670 ,  680 ,  690 . Advantageously, the lower refractive index layers  760   a ,  760   b  may function as cladding layers that facilitate total internal reflection (TIR) of light through the waveguides  670 ,  680 ,  690  (e.g., TIR between the top and bottom major surfaces of each waveguide). In some embodiments, the layers  760   a ,  760   b  are formed of air. While not illustrated, it will be appreciated that the top and bottom of the illustrated set  660  of waveguides may include immediately neighboring cladding layers. 
     Preferably, for ease of manufacturing and other considerations, the material forming the waveguides  670 ,  680 ,  690  are similar or the same, and the material forming the layers  760   a ,  760   b  are similar or the same. In some embodiments, the material forming the waveguides  670 ,  680 ,  690  may be different between one or more waveguides, and/or the material forming the layers  760   a ,  760   b  may be different, while still holding to the various refractive index relationships noted above. 
     With continued reference to  FIG.  9 A , light rays  770 ,  780 ,  790  are incident on the set  660  of waveguides. It will be appreciated that the light rays  770 ,  780 ,  790  may be injected into the waveguides  670 ,  680 ,  690  by one or more image injection devices  360 ,  370 ,  380 ,  390 ,  400  ( FIG.  6   ). 
     In some embodiments, the light rays  770 ,  780 ,  790  have different properties, e.g., different wavelengths or different ranges of wavelengths, which may correspond to different colors. The in-coupling optical elements  700 ,  710 ,  720  each deflect the incident light such that the light propagates through a respective one of the waveguides  670 ,  680 ,  690  by TIR. In some embodiments, the incoupling optical elements  700 ,  710 ,  720  each selectively deflect one or more particular wavelengths of light, while transmitting other wavelengths to an underlying waveguide and associated incoupling optical element. 
     For example, in-coupling optical element  700  may be configured to deflect ray  770 , which has a first wavelength or range of wavelengths, while transmitting rays  780  and  790 , which have different second and third wavelengths or ranges of wavelengths, respectively. The transmitted ray  780  impinges on and is deflected by the in-coupling optical element  710 , which is configured to deflect light of a second wavelength or range of wavelengths. The ray  790  is deflected by the in-coupling optical element  720 , which is configured to selectively deflect light of third wavelength or range of wavelengths. 
     With continued reference to  FIG.  9 A , the deflected light rays  770 ,  780 ,  790  are deflected so that they propagate through a corresponding waveguide  670 ,  680 ,  690 ; that is, the in-coupling optical elements  700 ,  710 ,  720  of each waveguide deflects light into that corresponding waveguide  670 ,  680 ,  690  to in-couple light into that corresponding waveguide. The light rays  770 ,  780 ,  790  are deflected at angles that cause the light to propagate through the respective waveguide  670 ,  680 ,  690  by TIR. The light rays  770 ,  780 ,  790  propagate through the respective waveguide  670 ,  680 ,  690  by TIR until impinging on the waveguide&#39;s corresponding light distributing elements  730 ,  740 ,  750 . 
     With reference now to  FIG.  9 B , a perspective view of an example of the plurality of stacked waveguides of  FIG.  9 A  is illustrated. As noted above, the in-coupled light rays  770 ,  780 ,  790 , are deflected by the in-coupling optical elements  700 ,  710 ,  720 , respectively, and then propagate by TIR within the waveguides  670 ,  680 ,  690 , respectively. The light rays  770 ,  780 ,  790  then impinge on the light distributing elements  730 ,  740 ,  750 , respectively. The light distributing elements  730 ,  740 ,  750  deflect the light rays  770 ,  780 ,  790  so that they propagate towards the out-coupling optical elements  800 ,  810 ,  820 , respectively. 
     In some embodiments, the light distributing elements  730 ,  740 ,  750  are orthogonal pupil expanders (OPE&#39;s). In some embodiments, the OPE&#39;s deflect or distribute light to the out-coupling optical elements  800 ,  810 ,  820  and, in some embodiments, may also increase the beam or spot size of this light as it propagates to the out-coupling optical elements. In some embodiments, the light distributing elements  730 ,  740 ,  750  may be omitted and the in-coupling optical elements  700 ,  710 ,  720  may be configured to deflect light directly to the out-coupling optical elements  800 ,  810 ,  820 . For example, with reference to  FIG.  9 A , the light distributing elements  730 ,  740 ,  750  may be replaced with out-coupling optical elements  800 ,  810 ,  820 , respectively. In some embodiments, the out-coupling optical elements  800 ,  810 ,  820  are exit pupils (EP&#39;s) or exit pupil expanders (EPE&#39;s) that direct light in a viewer&#39;s eye  210  ( FIG.  7   ). It will be appreciated that the OPE&#39;s may be configured to increase the dimensions of the eye box in at least one axis and the EPE&#39;s may be to increase the eye box in an axis crossing, e.g., orthogonal to, the axis of the OPEs. For example, each OPE may be configured to redirect a portion of the light striking the OPE to an EPE of the same waveguide, while allowing the remaining portion of the light to continue to propagate down the waveguide. Upon impinging on the OPE again, another portion of the remaining light is redirected to the EPE, and the remaining portion of that portion continues to propagate further down the waveguide, and so on. Similarly, upon striking the EPE, a portion of the impinging light is directed out of the waveguide towards the user, and a remaining portion of that light continues to propagate through the waveguide until it strikes the EP again, at which time another portion of the impinging light is directed out of the waveguide, and so on. Consequently, a single beam of incoupled light may be “replicated” each time a portion of that light is redirected by an OPE or EPE, thereby forming a field of cloned beams of light, as shown in  FIG.  6   . In some embodiments, the OPE and/or EPE may be configured to modify a size of the beams of light. 
     Accordingly, with reference to  FIGS.  9 A and  9 B , in some embodiments, the set  660  of waveguides includes waveguides  670 ,  680 ,  690 ; in-coupling optical elements  700 ,  710 ,  720 ; light distributing elements (e.g., OPE&#39;s)  730 ,  740 ,  750 ; and out-coupling optical elements (e.g., EP&#39;s)  800 ,  810 ,  820  for each component color. The waveguides  670 ,  680 ,  690  may be stacked with an air gap/cladding layer between each one. The in-coupling optical elements  700 ,  710 ,  720  redirect or deflect incident light (with different in-coupling optical elements receiving light of different wavelengths) into its waveguide. The light then propagates at an angle which will result in TIR within the respective waveguide  670 ,  680 ,  690 . In the example shown, light ray  770  (e.g., blue light) is deflected by the first in-coupling optical element  700 , and then continues to bounce down the waveguide, interacting with the light distributing element (e.g., OPE&#39;s)  730  and then the out-coupling optical element (e.g., EPs)  800 , in a manner described earlier. The light rays  780  and  790  (e.g., green and red light, respectively) will pass through the waveguide  670 , with light ray  780  impinging on and being deflected by in-coupling optical element  710 . The light ray  780  then bounces down the waveguide  680  via TIR, proceeding on to its light distributing element (e.g., OPEs)  740  and then the out-coupling optical element (e.g., EP&#39;s)  810 . Finally, light ray  790  (e.g., red light) passes through the waveguide  690  to impinge on the light in-coupling optical elements  720  of the waveguide  690 . The light in-coupling optical elements  720  deflect the light ray  790  such that the light ray propagates to light distributing element (e.g., OPEs)  750  by TIR, and then to the out-coupling optical element (e.g., EPs)  820  by TIR. The out-coupling optical element  820  then finally out-couples the light ray  790  to the viewer, who also receives the out-coupled light from the other waveguides  670 ,  680 . 
       FIG.  9 C  illustrates a top-down plan view of an example of the plurality of stacked waveguides of  FIGS.  9 A and  9 B . As illustrated, the waveguides  670 ,  680 ,  690 , along with each waveguide&#39;s associated light distributing element  730 ,  740 ,  750  and associated out-coupling optical element  800 ,  810 ,  820 , may be vertically aligned. However, as discussed herein, the in-coupling optical elements  700 ,  710 ,  720  are not vertically aligned; rather, the in-coupling optical elements are preferably non-overlapping (e.g., laterally spaced apart as seen in the top-down view). As discussed further herein, this nonoverlapping spatial arrangement facilitates the injection of light from different resources into different waveguides on a one-to-one basis, thereby allowing a specific light source to be uniquely coupled to a specific waveguide. In some embodiments, arrangements including nonoverlapping spatially-separated in-coupling optical elements may be referred to as a shifted pupil system, and the in-coupling optical elements within these arrangements may correspond to sub pupils. 
       FIG.  9 D  illustrates an example of wearable display system  60  into which the various waveguides and related systems disclosed herein may be integrated. In some embodiments, the display system  60  is the system  250  of  FIG.  6   , with  FIG.  6    schematically showing some parts of that system  60  in greater detail. For example, the waveguide assembly  260  of  FIG.  6    may be part of the display  70 . 
     With continued reference to  FIG.  9 D , the display system  60  includes a display  70 , and various mechanical and electronic modules and systems to support the functioning of that display  70 . The display  70  may be coupled to a frame  80 , which is wearable by a display system user or viewer  90  and which is configured to position the display  70  in front of the eyes of the user  90 . The display  70  may be considered eyewear in some embodiments. In some embodiments, a speaker  100  is coupled to the frame  80  and configured to be positioned adjacent the ear canal of the user  90  (in some embodiments, another speaker, not shown, may optionally be positioned adjacent the other ear canal of the user to provide stereo/shapeable sound control). The display system  60  may also include one or more microphones  110  or other devices to detect sound. In some embodiments, the microphone is configured to allow the user to provide inputs or commands to the system  60  (e.g., the selection of voice menu commands, natural language questions, etc.), and/or may allow audio communication with other persons (e.g., with other users of similar display systems. The microphone may further be configured as a peripheral sensor to collect audio data (e.g., sounds from the user and/or environment). In some embodiments, the display system  60  may further include one or more outwardly-directed environmental sensors  112  configured to detect light, objects, stimuli, people, animals, locations, or other aspects of the world around the user. For example, environmental sensors  112  may include one or more cameras, which may be located, for example, facing outward so as to capture images similar to at least a portion of an ordinary field of view of the user  90 . In some embodiments, the display system may also include a peripheral sensor  120   a , which may be separate from the frame  80  and attached to the body of the user  90  (e.g., on the head, torso, an extremity, etc. of the user  90 ). The peripheral sensor  120   a  may be configured to acquire data characterizing a physiological state of the user  90  in some embodiments. For example, the sensor  120   a  may be an electrode. 
     With continued reference to  FIG.  9 D , the display  70  is operatively coupled by communications link  130 , such as by a wired lead or wireless connectivity, to a local data processing module  140  which may be mounted in a variety of configurations, such as fixedly attached to the frame  80 , fixedly attached to a helmet or hat worn by the user, embedded in headphones, or otherwise removably attached to the user  90  (e.g., in a backpack-style configuration, in a belt-coupling style configuration). Similarly, the sensor  120   a  may be operatively coupled by communications link  120   b , e.g., a wired lead or wireless connectivity, to the local processor and data module  140 . The local processing and data module  140  may comprise a hardware processor, as well as digital memory, such as non-volatile memory (e.g., flash memory or hard disk drives), both of which may be utilized to assist in the processing, caching, and storage of data. Optionally, the local processor and data module  140  may include one or more central processing units (CPUs), graphics processing units (GPUs), dedicated processing hardware, and so on. The data may include data a) captured from sensors (which may be, e.g., operatively coupled to the frame  80  or otherwise attached to the user  90 ), such as image capture devices (such as cameras), microphones, inertial measurement units, accelerometers, compasses, GPS units, radio devices, gyros, and/or other sensors disclosed herein; and/or b) acquired and/or processed using remote processing module  150  and/or remote data repository  160  (including data relating to virtual content), possibly for passage to the display  70  after such processing or retrieval. The local processing and data module  140  may be operatively coupled by communication links  170 ,  180 , such as via a wired or wireless communication links, to the remote processing module  150  and remote data repository  160  such that these remote modules  150 ,  160  are operatively coupled to each other and available as resources to the local processing and data module  140 . In some embodiments, the local processing and data module  140  may include one or more of the image capture devices, microphones, inertial measurement units, accelerometers, compasses, GPS units, radio devices, and/or gyros. In some other embodiments, one or more of these sensors may be attached to the frame  80 , or may be standalone structures that communicate with the local processing and data module  140  by wired or wireless communication pathways. 
     With continued reference to  FIG.  9 D , in some embodiments, the remote processing module  150  may comprise one or more processors configured to analyze and process data and/or image information, for instance including one or more central processing units (CPUs), graphics processing units (GPUs), dedicated processing hardware, and so on. In some embodiments, the remote data repository  160  may comprise a digital data storage facility, which may be available through the internet or other networking configuration in a “cloud” resource configuration. In some embodiments, the remote data repository  160  may include one or more remote servers, which provide information, e.g., information for generating augmented reality content, to the local processing and data module  140  and/or the remote processing module  150 . In some embodiments, all data is stored and all computations are performed in the local processing and data module, allowing fully autonomous use from a remote module. Optionally, an outside system (e.g., a system of one or more processors, one or more computers) that includes CPUs, GPUs, and so on, may perform at least a portion of processing (e.g., generating image information, processing data) and provide information to, and receive information from, modules  140 ,  150 ,  160 , for instance via wireless or wired connections. 
     I. Adjusting Quality Based on Depth Information 
     As described herein, display systems (e.g., augmented reality display systems such as the display system  60 ,  FIG.  9 D ) according to various embodiments may determine a three-dimensional fixation point of the user, e.g., by monitoring a user&#39;s eyes. The fixation point may indicate the location of the point in space along (1) an x-axis (e.g., a lateral axis), (2) a y-axis (e.g., a vertical axis), and (3) a z-axis (e.g., a depth of the point, for example a depth from the user). In some embodiments, the display system may utilize cameras, sensors, and so on, to monitor the user&#39;s eyes (e.g., a pupil, cornea, and so on, of each eye), to determine a gaze of each eye. The gaze of each eye may be understood to be a vector extending from generally a center of the retina of that eye through the lens of the eye. For example, the vector may extend generally from the center of the macula (e.g., the fovea) through the lens of the eye. The display system may be configured to determine where the vectors associated with the eyes intersect, and this intersection point may be understood to be the fixation point of the eyes. Stated another way, the fixation point may be location in three-dimensional space on which the user&#39;s eyes are verging. In some embodiments, the display system may filter small movements of the user&#39;s eyes for example during rapid movements (e.g., saccades, microsaccades), and may update the fixation point upon determining that the eyes are fixating on a location in three-dimensional space. For example, the display system may be configured to ignore movements of the eye that fixate on a point for less than a threshold duration. 
     The resolution of content presented by the display system, such as virtual objects or content, may be adjusted based on proximity to the fixation point as discussed herein. It will be appreciated that the display system may have stored within it, or may have access to, information regarding the locations, in three-dimensional space, of virtual objects. Based on the known locations of the virtual objects, the proximity of a given virtual object to the fixation point may be determined. For example, the proximity of the virtual object to the fixation point may be determined by determining one or more of the (1) three-dimensional distance of a virtual object from the fixation point of the user; (2) the resolution adjustment zone in which the virtual object is located, relative to the resolution adjustment zone in which the fixation point is located, in cases where the display system&#39;s display frustum is divided into resolution adjustment zones; and (3) the angular separation between the virtual object and a gaze of the user. Virtual content that is closer in proximity to the fixation point may be presented at a greater resolution than content farther from the fixation point. In some embodiments, the resolution of virtual content changes depending upon the proximity of the depth plane on which that virtual content is disposed to the fixation point or the depth plane on which the fixation point is disposed. In some embodiments, adjustments to the resolution may be made by a rendering engine, such as rendering engines included in one or more graphics processing units, for instance in one or more of modules  140 ,  150  ( FIG.  9 D ). 
       FIG.  10 A  illustrates an example of a representation of a top-down view of a user viewing content (e.g., content included in a display frustum  1004 ) presented by a display system (e.g., the display system  60 ,  FIG.  9 D ). The representation includes the user&#39;s eyes  210 ,  220 , and a determination of a fixation point  1006  of the eyes  210 ,  220 . As illustrated, the gaze of each eye is represented as a vector (e.g., vectors  1003 A,  1003 B) and the display system has detected the fixation point  1006  by, e.g., determining where those vectors converge in front of the eyes  210 ,  22 . In the illustrated example, the fixation point  1006  coincides with the location of a first virtual object  1008 A presented by the display system. Examples of systems and methods for eye-tracking may be found in U.S. application Ser. No. 14/690,401 filed on Apr. 18, 2015, which is incorporated by reference for all of purposes; and in the attached Appendix. For example, eye-tracking systems and methods are described in, at least, FIGS. 25-27 of the Appendix, and can be utilized, at least in part, for eye-tracking and/or to determine fixation points as described herein. 
     With continued reference to  FIG.  10 A , a second virtual object  1008 B is also presented by the display system in the display frustum  1004 . The view of these virtual objects  1008 A,  1008 B, as seen by the viewer, is shown in a rendered frame  1010 . The rendered frame  1010  may include the first virtual object  1008 A rendered at a first resolution, while the second virtual object  1008 B, located away from the fixation point  1006 , is rendered at a second, lesser resolution. Specifically, the second virtual object  1008 B may be determined to be located at a greater depth than, and towards the side of, the first virtual object  1008 A. For example, the display system may determine the depth of the second virtual object  1008 B, as discussed herein, or optionally a content provider associated with the virtual content may indicate depths of virtual objects which the display system may utilize for rendering that virtual object. Therefore, the fixation point  1006 , as described above, describes a three-dimensional location in space at which the user is looking, and the second virtual object  1008 B may be determined to be located further in depth from the user along with being laterally displaced from the fixation point  1006 . 
     Without being limited by theory, it is believed that, with the user&#39;s eyes  210 ,  220  looking at the first virtual object  1008 A, an image of the first virtual object  1008 A may fall on the user&#39;s fovea, while an image of the second virtual object  1008 B does not fall on the fovea. As a result, the second virtual object  1008 B may be reduced in resolution without significant impact to the perceived image quality of the display system, due to a lower sensitivity of the human visual system to that second virtual object  1008 B. In addition, the lower resolution advantageously reduces the computational load required to provide the images. As discussed herein, the resolution at which the second virtual object  1008 B is rendered may be based on a proximity to the fixation point  1006 , and the reduction in resolution (e.g., with respect to the resolution of the first virtual object  1008 A) may increase with decreasing proximity (or increasing distance) between the fixation point  1006  and the virtual object  1008 A. In some embodiments, the rate of decrease of the resolution may be in conformance with a rate of reduction of the density of cones in the human eye, or with a visual acuity drop-off away from the fovea. 
     It will be appreciated that the resolutions of the various virtual objects presented by the display system may vary dynamically as the fixation point changes location. For example,  FIG.  10 B  illustrates another example of a representation of a top-down view of a user viewing content presented by the display system. As illustrated in  FIG.  10 B , the user is now focusing on the second virtual object  1008 B, as compared to  FIG.  10 A , in which the user was focusing on the first virtual object  1008 A. By monitoring the gaze  1003 A,  1003 B of the user, the display system determines that the eyes  210 ,  220  are verging on the second virtual object  1008 B, and sets that location as the new fixation point  1006 . 
     Upon detecting this change in the location of the fixation point  1006 , the display system now renders second virtual object  1008 B at a greater resolution than the first virtual object  1008 A, as shown in the rendered frame  1010 . Preferably, the display system monitors the user&#39;s gaze  1003 A,  1003 B at a sufficiently high frequency, and changes the resolution of virtual objects sufficiently quickly, that the transition in resolution of the first virtual object  1008 A and second virtual object  1008 B is substantially imperceptible to the user. 
       FIG.  10 C  illustrates another example of a representation of a top-down view of a user viewing content via a display system (e.g., the display system  60 ,  FIG.  9 D ). In the example, the user&#39;s field of view  1004  is illustrated along with a fixation point  1006 . Three virtual objects are illustrated, with a first virtual object  1012 A being closer in proximity to the fixation point  1006  than a second virtual object  1012 B or a third virtual object  1012 C. Similarly, the second virtual object  1012 B is illustrated as being closer in proximity to the fixation point  1006  than the third virtual object  1012 C. Therefore, when the virtual objects  1012 A- 1012 C are presented to the user, the display system may allocate resources such that rendering the first virtual object  1012 A is accorded a greater resource allocation (e.g., the object  1012 A is rendered at a greater resolution) than the second virtual object  1012 B, and the second virtual object  1012 B receives a greater resource allocation than the third virtual object  1012 C. The third virtual object  1012 C may optionally not be rendered at all, as it is outside of the field of view  1004 . 
     Resolution adjustment zones are illustrated in the example of  FIG.  10 C , with the zones being ellipses (e.g., circles) described along depth and lateral axes. As illustrated, the fixation point  1006  is inside a center zone  1014 A, with the first virtual object  1012 A extending between zones  1014 B,  1014 C and within the user&#39;s cone  1004   a  of foveal vision. The first virtual object  1012 A may therefore be presented to the user at a resolution associated with zone  1014 B or  1014 C, or optionally a portion of the object  1012 A within zone  1014 B may be presented according to the resolution of zone  1014 B and remaining portion within zone  1014 C may be presented according to the resolution of zone  1014 C. For example, in an embodiment in which the zones are assigned resolutions reduced from a maximum (e.g., highest) resolution, the first virtual object  1012 A may be presented at the assigned resolutions. Optionally, the first virtual object  1012 A may be presented at either of the resolutions (e.g., the display system may be programmed to display at the highest revolution associated with any zones across which the first virtual object  1012 A extends), or a measure of central tendency of the resolutions (e.g., the measure can be weighted according to an extent to which the object  1012 A is located within the zones  1014 B,  1014 C). With continued reference to  FIG.  10 C , it will be appreciated that the resolution adjustment zones at different distances from the fixation point  1006  may have different shapes. For example, the zone  1014 C may have a different shape from the zones  1014 A- 1014 C, and conform to the contours of the field of view  1004 . In some other embodiments, one or more of the zones  1014 A- 1014 C may have different shapes from one or more others of the zones  1014 A- 1014 C. 
       FIG.  10 D  is a block diagram of an example display system. The example display system (e.g., the display system  60 .  FIG.  9 D ) may be an augmented reality display system and/or a mixed reality display system, which can adjust usage of rendering hardware resources according to a user&#39;s fixation point as described herein. For example, as described above with respect to  FIG.  10 C , rendering hardware resources  1021  can be adjusted according to the user&#39;s fixation point. A resource arbiter  1020  may be implemented to regulate usage of such resources  1021 , for example the arbiter  1020  can allocate the resources  1021  to particular application processes  1022  associated with presenting virtual objects to the user. The resource arbiter  1020  and/or rendering hardware resources  1021  may optionally be included in the local processing &amp; data module  140  (e.g., as illustrated in  FIG.  9 D ), and/or the remote processing module  150 , of the display system  60 . For example, the rendering hardware resources  1021  may comprise graphics processing units (GPUs), which may be included in module  140  and/or module  150  as described above with respect to  FIG.  9 D . 
     As an example of adjusting resources  1021 , and with respect to  FIG.  10 C , a first virtual object  1012 A associated with a first application process can be allocated a greater share of resources  1021  than a second virtual object  1012 B associated with a second application process. Virtual objects associated with the application processes  1022  can be rendered based on the allocated resources  1021 , and included in frame buffers  1024  to be composited (e.g., by compositor  1026 ) into a final frame buffer  1028 . The final frame buffer  1028  can then be presented by display hardware  1030 , for example the display  70  illustrated in  FIG.  9 D , with the rendered virtual objects adjusted in resolution. 
     As disclosed herein, the resolution of a virtual object may be determined based upon the proximity of the virtual object to the fixation point. In some embodiments, the resolution may be modified as a function of the distance between the virtual object and the fixation point. In some embodiments, the modifications may occur in discrete steps; that is, a similar modification may be applied to all virtual objects disposed in a particular volume or zone. FIG.  11 A 1  illustrates an example of a representation of a top-down view of adjustments in resolution in different resolution adjustment zones based on three-dimensional fixation point tracking. The display system may divide the display frustum into multiple volumes or resolution adjustment zones, and modify resolution in discrete steps corresponding to these zones. Thus, in some embodiments, to determine an adjustment in the resolution of virtual content, the display system may utilize information describing volumes of space (referred hereinafter as resolution adjustment zones), and assignments of resolution adjustments to each volume of space. As illustrated, a field of view provided by the display system (e.g., the display frustum of the display) is separated into a plurality of different zones each encompassing a range of depths from a user (e.g., depth ranges  1102 A- 1102 E). In some embodiments, each depth range  1102 A- 1102 E has a single associated depth plane that may be presented by the display system. With continued reference to FIG.  11 A 1 , five zones encompass each identified range of depths from the user and are contiguous along a lateral direction. In the illustrated example top-down view, the field of view is divided into a grid  1100  of 25 zones. Each zone represents a volume of real-world space in which virtual content may be placed for a user. 
     It will be appreciated that the zones may also extend in a vertical direction (e.g., along the y-axis, not shown), such that the illustrated grid  1100  may be understood to represent one cross-section along this vertical direction. In some embodiments, multiple zones are also provided in the vertical direction. For example, there may be 5 vertical zones per depth range, for a total of 125 resolution adjustment zones. An example of such zones extending in three dimensions is illustrated in  FIG.  11 B , and described below. 
     With continued reference to FIG.  11 A 1 , a user&#39;s eyes  210 ,  220  fixate on a particular fixation point  1006  within the grid  1100 . The display system may determine the location of the fixation point  1006 , and the zone in which the fixation point  1006  is located. The display system may adjust resolutions of content based on the proximity of virtual content to the fixation point  1006 , which may include determining the proximity of the virtual content to the zone in which the fixation point  1006  is located. As an example, for content included in a zone in which the fixation point  1006  is located, the resolution may be set at a particular polygon count, which in the example is 10,000 polygons. Based on a distance from the fixation point  1006 , content included in the remaining zones may be adjusted accordingly. For example, content included in an adjacent zone to a zone that includes the fixation point  1006  may be rendered at a lower resolution (e.g., 1,000 polygons). While the example of FIG.  11 A 1  illustrates adjusting a polygon count as an example, as described herein, adjusting resolution may encompass making other modifications to the resolution of presented content. For example, the adjustment in resolution may include one or more of: adjusting the polygon count, adjusting primitives utilized to generate the virtual object (e.g., adjusting a shape of the primitives, for example adjusting primitives from triangle mesh to quadrilateral mesh, and so on), adjusting operations performed on the virtual object (e.g., shader operations), adjusting texture information, adjusting color resolution or depth, adjusting a number of rendering cycles or a frame rate, and adjusting quality at one or more points within a graphics pipeline of graphics processing units (GPUs)). 
     In addition, while the example of FIG.  11 A 1  provides particular examples of differences in polygon count in different resolution adjustment zones, other absolute numbers of polygons and other rates of change in resolution with distance from the fixation point  1006  are contemplated. For example, while a drop-off of resolution from the fixation point  1006  may be based on a drop-off rate symmetric about depth and lateral distance from the fixation point  1006 , other drop-off relationships may also be utilized. For instance, a lateral distance from the fixation point  1006  may be associated with a greater drop-off in resolution relative to a depth distance from the fixation point  1006 . Furthermore, the size of each zone (e.g., size of a volume of space of the zone) included in the grid may optionally be different (e.g., the zones may vary radially from a foveal axis). In some embodiments, the drop-off may be continuous from the fixation point  1006 , such that discrete zones having assigned resolutions or resolution relationships with the zone containing the fixation point  1006  are not utilized. For instance, a drop-off from the fixation point  1006  to a particular zone  1108  (e.g., a zone in which content is rendered at a resolution of 100 polygons) may be modified to be a continuous drop-off from the fixation point  1006  to an edge of the grid (e.g., edge of the particular zone  1108 ). It will be appreciated that each of the considerations above also apply to zones extending in the vertical direction. 
     In some embodiments, the number and sizes of zones included in the grid may be based on a confidence associated with a determination of the user&#39;s fixation point  1006 . For instance, the confidence may be based on an amount of time that the user&#39;s eyes have been fixed on the fixation point  1006 , with a lesser amount of time being associated with a lesser confidence. For example, the display system may monitor the user&#39;s eye at a particular sampling rate (e.g., 30 Hz, 60 Hz, 120 Hz, 1 kHz), and may increase a confidence in the fixation point  1006  as successive samples indicate the user is generally maintaining the fixation point  1006 . Optionally, particular thresholds of fixation may be utilized, for instance a fixation for a particular duration (e.g., 100-300 milliseconds) on a same, or similar, fixation point may be associated with a high confidence, while less than the particular duration may be associated with a lesser confidence. Similarly, fluctuations in the eyes, such as pupil dilation, and so on, which may affect a determination of the user&#39;s fixation point, may cause the display system to reduce the confidence. It will be appreciated that the display system may monitor the eye with sensors, such as camera imaging devices (e.g., camera assembly  630 ,  FIG.  6   ). Optionally, the display system may utilize a combination of the sensors to determine an eye gaze of the user (e.g., different eye gaze determination processes may be utilized, such as an infrared sensor utilized to detect infrared reflections from the eye and to identify a pupil, a visible light imaging device utilized to detect an iris of the eye, and so on). The display system may increase a confidence when multiple eye gaze determination processes are in conformance, and may decrease the confidence level if they disagree. Similarly, for display systems which conduct only one of the eye gaze determination processes, each eye gaze determination process may be associated with a particular confidence level (e.g., one determination process may be considered more accurate than others) and the sizes of the resolution adjustment zones may be selected, at least in part, on the process being implemented. 
     In some embodiments, the display system may increase, or decrease, a number of zones for each updating of the fixation point  1006 . For example, more zones may be utilized as the confidence associated with the fixation point  1006  increases and fewer zones may be utilized as confidence decreases. FIG.  11 A 2  illustrates examples of representations of top-down views of resolution adjustment zones at different times as the sizes and numbers of the zones change. At time t=1, as seen in a top down view, the user&#39;s field of view may be divided into an initial set of zones. At time t=2, confidence in the location of the fixation point  1006  increases and the display system may also decrease the size of the zone that is occupied by the fixation point  1006  and that is render at high resolution. Optionally, as illustrated, the sizes of the other zones may also decrease. At time t=3, confidence in the location of the fixation point  1006  decreases and the display system may also increase the size of the zone that is occupied by the fixation point  1006  and that is render at high resolution. Optionally, as illustrated, the sizes of the other zones may also increase. It will be appreciated that a plurality of zones may also extend in the y-axis and that similar increase or decreases in the sizes and numbers of zones may also be instituted on that axis. For example, the sizes of the zones extending vertically on the y-axis may decrease with increasing confidence, while the sizes may increase with decreasing confidence. Optionally, the display system may determine a confidence of the fixation point  1006  for each frame presented by the display system to the user and t=1, t=2, and t=3 may represent different frames. Since assigning more zones may require an increase in computational power (e.g., the display system may have to adjust resolutions of more content, identify which zones content are included in, and so on), the display system may balance the increase in required computational power afforded by the increase in the number zones with the savings in computation power afforded by the potential decrease in the resolution of content. 
     With reference again to FIG.  11 A 1 , the grid may change dynamically in the sense that the fixation point  1006  may be set as being located at a center (e.g., centroid) of the grid. Therefore, the display system may avoid edge cases in which the fixation point  1006  is determined to be located on vertices of the grid. For example, as the user&#39;s eyes rotate and then fixate on different three-dimensional locations in space, the grid may be similarly moved with the user&#39;s gaze. 
       FIGS.  11 B- 11 E  illustrate examples of various resolution adjustment zone configurations. Additional shapes and configures of resolution adjustment zones that are not illustrated may be utilized, and the examples should not be considered exhaustive. In addition, in some drawings, the user&#39;s eyes  210 ,  220  may be illustrated spaced apart from the various resolution adjustment zones for ease and clarity of illustration. For all these drawings, it will be appreciated that the eyes  210 ,  220  may be disposed at the boundary of, or in, the zone (see, e.g., FIG.  11 A 1 ). 
       FIG.  11 B  illustrates an example of a three-dimensional representation of a portion of the resolution adjustment zones of FIG.  11 A 1 . It will be appreciated that FIG.  11 A 1  may be understood to illustrate a cross-sectional view taken along the plane  11 A 1 - 11 A 1  of the three-dimensional representation of  FIG.  11 B , with  FIG.  11 B  omitting some of the resolution adjustment zones of FIG.  11 A 1  for clarity of illustration. With continued reference to FIG.  11 A 1 , a field of view provided by a display system is separated into 27 zones. That is, the field of view is separated into 3 depth ranges  1102 B- 1102 D, and at each depth range a 3×3 grid of zones is included that extends laterally and vertically at the depth range. 
     A determined fixation point  1006  is illustrated as being within a zone located in the center of the field of view. Virtual objects located within zones outside of a zone that includes the fixation point  1006  may be reduced in resolution according to a distance from the fixation point&#39;s  1006  zone, as discussed herein. Since the zones extend laterally as well as vertically, reduction in resolution can occur based on distance on lateral, vertical, and depth axes (x, y, and z-axes respectively) from the resolution adjustment zone of the fixation point. For example, in some embodiments, virtual objects located in zone  1108  can be reduced in resolution according to lateral distance as shown in FIG.  11 A 1  (e.g., zone  1108  includes a same vertical portion of the user&#39;s field of view as the zone that includes the fixation point  1006 , and may be on the same depth plane). 
     Similar to the above, and similar to the zones described in  FIGS.  11 C- 11 E  below, the user&#39;s fixation point can optionally maintained located at the center (e.g., centroid) of the zones, or the zones can be fixed with respect to the user&#39;s field of view and the user&#39;s fixation point can be located within any of the zones. 
       FIG.  11 C  illustrates another example of a configuration for resolution adjustment zones. In the example, a field of view provided by a display system is illustrated as being separated into zones of ellipses that each encompass a particular three-dimensional volume of space. Similar to FIG.  11 A 1 , each zone (e.g., zone  1112 A- 112 D) extends along lateral and depth dimensions. In some embodiments, each zone also extends to encompass at least a portion of the user&#39;s vertical field of view. A fixation point  1006  is illustrated as being at a center of the zones (e.g., within zone  1112 A). Virtual objects located within zones outside of zone  1112 A may be reduced in resolution according to a distance from zone  1112 A, for instance according to the techniques described herein. For example, each zone outside of zone  1112 A can be assigned a particular resolution, or a drop-off can be utilized, to determine a reduction in resolution. Zone  1112 D is illustrated as being a furthest zone from zone  1110 A, and the reduction in resolution can be the greatest in zone  1112 D. 
       FIG.  11 D  illustrates an example of a three-dimensional representation of the resolution adjustment zones of  FIG.  11 C , with  FIG.  11 C  showing a cross-sectional view taken along the plane  11 C- 11 C. In this example, the field of view provided by the display system is illustrated as being separated into zones of ellipsoids that each encompass a three-dimensional volume of space. The user&#39;s fixation point  1006  is illustrated at a centroid of the user&#39;s field of view, and located within zone  1112 A. Optionally,  FIG.  11 D  can represent each ellipse of  FIG.  11 C  being converted into an ellipsoid. In some embodiments, the size of  FIG.  11 C &#39;s zone  1112 A along depth and lateral directions can define the size of the principal axes of  FIG.  11 D &#39;s zone  1112 A along the X and Z axes. The various zones may form concentric spheres or ellipsoids. 
       FIG.  11 E  illustrates another example of a three-dimensional representation of the resolution adjustment zones of  FIG.  11 C , with  FIG.  11 C  showing a cross-sectional view taken along the plane  11 C- 11 C. The field of view provided by the display system is illustrated as being separated into stacked levels of similar concentric zones. For example,  FIG.  11 E  may represent the ellipses of  FIG.  11 C  being extended along a vertical direction to create cylinders. The cylinders may then be separated in the vertical direction, such that each cylinder encompasses a portion of the user&#39;s vertical field of view. Therefore,  FIG.  11 E  illustrates 9 zones of cylinders. Each zone additionally excludes any interior zones (e.g., ellipsoid  1112 B would encompass a volume of space that excludes a volume of space encompassed by ellipsoid  1112 A). In the example, the fixation point  1006  is illustrated as being within a center zone  1110 A, and virtual objects located outside of the center zone  1110 A can be reduced in resolution according to the techniques described herein. 
       FIG.  12 A  illustrates a flowchart of an example process  1200  for adjusting resolutions of content according to proximity to a three-dimensional fixation point. For convenience, the process  1200  may be described as being performed by a display system (e.g., the wearable display system  60 , which may include processing hardware and software, and optionally may provide information to an outside system of one or more computers or other processing, for instance to offload processing to the outside system, and receive information from the outside system). 
     At block  1202 , the display system determines a three-dimensional fixation point of a user. As described above, the display system may include sensors to monitor information associated with the user&#39;s eyes (e.g., the orientation of the eyes). A non-exhaustive list of sensors includes infrared sensors, ultraviolet sensors, visible wavelength light sensors. The sensors may optionally output infrared, ultraviolet, and/or visible light onto the user&#39;s eyes, and determine reflections of the outputted light from the user&#39;s eyes. As an example, infrared light may be output by an infrared light emitter, and an infrared light sensor. It will be appreciated that the sensor, which may include a light emitter, may correspond to the imaging device  630  of  FIG.  6   . 
     The display system may utilize the sensors to determine a gaze associated with each eye (e.g., a vector extending from the user&#39;s eye, such as extending from the fovea through the lens of the eye), and an intersection of the gazes of each eye. For example, the display system may output infrared light on the user&#39;s eyes, and reflections from the eye (e.g., corneal reflections) may be monitored. A vector between a pupil center of an eye (e.g., the display system may determine a centroid of the pupil, for instance through infrared imaging) and the reflections from the eye may be used to determine the gaze of the eye. The intersection of the gazes may be determined and assigned as the three-dimensional fixation point. The fixation point may therefore indicate a location at which content is to be rendered at a full or maximum resolution. For example, based on the determined gazes the display system may triangulate a three-dimensional location in space at which the user is fixating. Optionally, the display system may utilize orientation information associated with the display system (e.g., information describing an orientation of the display system in three-dimensional space) when determining the fixation point. 
     At block  1204 , the display system obtains location information associated with content being, or that is to be, presented by the display system to the user. Prior to rendering content for presentation to the user (e.g., via outputs of waveguides, as described above), the display system may obtain location information associated with content that is to be presented to the user. For instance, as described above, the virtual content may be presented to the user such that the content appears to be located in the real-world (e.g., the content may be located at different depths within the user&#39;s field of view). It will be appreciated that the display system include or may have access to a three-dimensional map of the ambient environment, which can inform locations of any virtual content in this ambient environment. With reference to this map, the display system may access and provide information specifying three-dimensional locations of virtual content within the user&#39;s field of view (e.g., locations within a display frustum, as illustrated in  FIGS.  10 A- 10 B ). 
     At block  1206 , the display system adjusts resolution of virtual content to be displayed to the user. The display system adjusts the resolution of content based on its proximity to the three-dimensional fixation point. For instance, a rendering engine, such as a rendering engine implemented by processing devices (e.g., central processing units, graphics processing units) which renders content for presentation to the user, may adjust resources invested in rendering the content (e.g., the rendering engine may adjust a resolution of the content). 
     The display system may determine a distance in three-dimensional space between content to be presented to the user and the user&#39;s fixation point, and may reduce a resolution of the content based on the determined distance. The reduction may be determined according to a drop-off rate, for instance a continuous function that correlates distance to the resolution of content, and the display system may obtain the resolution to render the content based on the continuous function. Optionally, the display system may determine the distance from a centroid of the content to the fixation point, and may render the content at a resolution based on the distance. Optionally, the display system may render portions of a same content at different resolutions according to the distance of various portions to the fixation point (e.g., the display system may separate the content into portions, and may render further portions at reduced resolutions as compared to closer portions). 
     In some embodiments, the display system may access information usable to separate a field of view of the user (e.g., corresponding to the display frustum) into zones, with each zone representing a volume of space in which content may be included. The accessed information, for example the grid illustrated in FIG.  11 A 1 , may indicate a particular resolution to utilize when rendering content that is to be included in each zone, with the three-dimensional fixation point being set at a center of the grid. Additionally, the grid may indicate drop-offs in resolution to utilize when rendering content. For content that is included in multiple zones (e.g., content located in three-dimensional space claimed by two zones), the display system may optionally adjust a resolution of the content to correspond to a single zone, or optionally adjust portions of the content according to corresponding zones in which the portions are located. 
     When setting the resolution of content, the display system renders content located at the fixation point (e.g., in a same zone as the fixation point) at a full or maximum resolution. The maximum resolution may be based on a maximum value that hardware and/or software of the display system is capable of rendering, while ensuring that content is presented to the user at greater than a threshold refresh rate (e.g., 60 Hz, 120 Hz) and optionally ensuring that the content is updated at speeds greater than vergence rates (e.g., greater than 60 ms) and greater than accommodation times (e.g., 20 ms to 100 ms) to reduce the perceptibility of changes in resolution. The display system may dynamically modify the maximum resolution, for instance prior to the display system rendering each frame, based on available resources of the display system. For example, as more content is to be presented to the user, a maximum resolution of content may be decreased, ensuring that the display system may present frames of rendered content at above threshold rates desired for reducing the perceptibility of changes in resolution. The display system may optionally monitor the frames per second at which content is being presented, and may adjust the maximum resolution, and/or adjust resolution drop-off rates based on distance from the fixation point, to ensure the presented frames per second does not drop below the threshold rate. As an example, the display system may render content, such as a first virtual object, located in the fixation point&#39;s zone at a maximum resolution. Instead of reducing the maximum resolution of the first virtual object, to ensure the frames per second remains above a particular threshold, the display system may dynamically increase drop-off rates of resolution based on distance. In this way, the display system may adjust resolutions assigned to each zone outside of the fixation point&#39;s zone. Optionally, the display system may set a minimum resolution that may be used in each zone outside of the fixation point&#39;s zone, and may adjust the maximum resolution if the minimum resolution would be exceeded (e.g., if the display system needs to reduce resolution of content below the minimum to maintain the threshold rate, the display system may reduce the maximum resolution). Similarly, the display system may reduce the maximum resolution while not reducing resolutions of content in zones outside of the fixation point&#39;s zone. Optionally, a user of the display system may indicate whether he/she prefers that content located proximate to the fixation point is to be given preference over other content. 
     In some embodiments, and as will be described in more detail below with respect to  FIGS.  13 - 14   , the display system may optionally utilize an angular proximity of content to a gaze of the user to adjust resolution of the content. For example, if particular content is located outside of a zone in which the fixation point is located, but is within a threshold proximity of a gaze of the user such that the particular content will fall on a fovea of the user&#39;s eye, the display system may cause the particular content to be rendered at a greater resolution (e.g., the maximum resolution, or at a resolution greater than indicated in the grid illustrated in FIG.  11 A 1 ). Optionally, the display system may reduce a resolution of the particular content, and apply a blurring process (e.g., Gaussian blur) to the particular content. In this way, the particular content may be rendered at a lesser resolution, while being blurred to represent that the particular content is, for instance, further away from the user than the fixation point. In addition, the blurring may reduce the perceptibility of the lower resolution (e.g., the blurring may reduce the perceptibility of increases in pixel size due to the lower resolution). 
     Example operations associated with presenting virtual content are illustrated in  FIGS.  12 B- 12 C  (e.g., a rendering pipeline). In the example of  FIG.  12 B , a three-dimensional scene is presented to a user, without adjustments to resolution made as described herein. In  FIG.  12 C , adjustments to resolution are performed according to fixation point information as described herein. For example, one or more of the following adjustments can be performed: reducing vertex operation complexity, reducing tessellation level of detail, reducing geometry generation, reducing pixel operation complexity/aggregation of multiple pixels, and so on. The adjustments, as illustrated, can advantageously be performed at different steps within a pipeline to present virtual content, and can be optimized according to particular software and/or hardware utilized to present the virtual content. It will be appreciated that the fidelity zones noted in  FIG.  12 C  are resolution adjustment zones. 
     With reference again to  FIG.  12 A , the display system presents adjusted content to the user at block  1208 . As described above, the display system has adjusted the resolutions of content based on proximity to the three-dimensional fixation point. Subsequently, the display system presents rendered content at associated locations to the user. In some embodiments, the display system may perform process  1200  for each frame of content to be rendered, or may adjust resolutions of content as the user adjusts his/her fixation point. 
     As noted above, in some embodiments, virtual objects may be within a user&#39;s line of sight while also being presented at different depths.  FIG.  13    illustrates an example of a representation of a user viewing multiple virtual objects aligned with the user&#39;s line of sight. The example representation includes a user&#39;s field of view (e.g., display frustum  1004  of the display system), along with a gaze  1003 A,  1003 B of the user&#39;s eyes  210 ,  220 , which are fixated at a fixation point on a first virtual object  1008 A. 
     As illustrated, a second virtual object  1008 B is within an angular proximity of a gaze of the user (e.g., one or both of gaze vectors  1003 A,  1003 B) such that the second virtual object  1008 B will fall on the user&#39;s fovea (e.g., fall on at least one fovea of either eye). For example, upon rendering frame  1110 , the second virtual object  1008 B is located behind (e.g., at a greater perceived depth from) the first virtual object  1008 A. It will be appreciated that the fovea is the portion of the retina having the highest visual acuity. Since the second virtual object  1008 B will fall on the user&#39;s fovea, if a resolution of the second virtual object  1008 B is reduced (e.g., reduced as described above, with respect to, at least, FIG.  11 A 1 ) the user may perceive the reduction in resolution. To avoid a perceptible reduction in resolution, the display system may (1) cause the second virtual object  1008 B to be rendered at a same resolution as the first virtual object  1008 A, or within a threshold resolution of the first virtual object  1008 A, and/or (2) cause the second virtual object  1008 B to be rendered at a reduced resolution (e.g., as indicated in FIG.  11 A 1 ) and apply a blur to the second virtual object prior to presentation to the user. Without being limited by theory, the blur may mask the reduction in resolution while providing a depth cue. 
       FIG.  14    is a flowchart of an example of a process  1400  for adjusting virtual content based on angular distance from a user&#39;s gaze. For convenience, the process  1400  will be described as being performed by a display system (e.g., the wearable display system  60 , which may include processing hardware and software, and optionally may provide information to an outside system of one or more computers or other processing units, for instance to offload processing to the outside system, and receive information from the outside system). In the example process  1400 , the display system is a vari-focal display system, in which each frame is presented on the same depth plane, and optionally having all content to be presented collapsed into a single frame buffer; that is, the vari-focal display system presents virtual content on one depth plane at a time. 
     The display system determines a three-dimensional fixation point of a user (block  1402 ) and obtains location information associated with presented content (block  1404 ). The blocks  1402  and  1404  may correspond to the blocks  1202  and  1204 , respectively, of  FIG.  12 A . As described above with reference to  FIG.  12 A , the display system monitors eye movements (e.g., eye orientations) of the user and determines fixation points of the user. The display system may obtain location information of content to be presented (e.g., in a next frame), and may subsequently adjust resolutions of the content. 
     With continued reference to  FIG.  14   , the display system determines content to be reduced in resolution and that is located within a threshold angular distance from the user&#39;s gaze (block  1406 ). The display system identifies content that is to be reduced in resolution due to the proximity of the content from the fixation point (e.g., the content is located at a greater depth than the fixation point), but that will fall on the user&#39;s fovea (e.g., fall within a threshold angle from the user&#39;s gaze). Since the content will fall on the user&#39;s fovea, the user may be able to perceive the reduction in resolution, as by the three-dimensional fixation point foveated rendering described herein. It will be appreciated that content block  1406  may comprise performing the blocks illustrated in  FIG.  12 C , particularly the blocks identified in the section “GPU”. 
     Consequently, at block  1408 , the display system may optionally cause the determined content to be rendered at a greater resolution. The display system may adjust the resolution of the determined content to be at full resolution (e.g., at the same resolution as content located at the fixation point, or within a same zone, or volume of space, as the fixation point), or to be at greater than the reduced resolution that would otherwise be assigned to the content (e.g., as described in block  1406 ). 
     At block  1410 , display system may optionally reduce the resolution of the content, and may blur the content prior to presentation to the user. As described above, a vari-focal display system may utilize a single display buffer to present content to the user. Since the vari-focal display system is presenting all content at the same depth plane, the vari-focal display system may utilize the same display buffer to output the content, for instance, from a rendering engine. 
     Optionally, the display system may utilize initial depth buffers, with each depth buffer assigned one or more depth planes, and may combine the initial depth buffers to obtain the display buffer. With reference to the illustration of  FIG.  13   , a first depth buffer may include the first virtual object  1306 , while a second depth buffer may include the second virtual object  1308 . The display system may then apply a blurring process to the second depth buffer, or to particular content included in the second depth buffer (e.g., the display system may apply the blurring process to the second virtual content  1308 , but not to other content located on a same depth plane but at a further angular distance from the user&#39;s gaze). After performing the blurring process, the display system may combine the first depth buffer and second depth buffer (e.g., the display system may add occlusions, for instance removing a portion of the second virtual object  1308  not visible due to occlusion by the first virtual object  1306 ), to obtain the display buffer. 
     An example blurring process may include the display system performing a convolution of a kernel associated with blurring (e.g., a Gaussian kernel, circular kernel such as to reproduce a bokeh effect, box blur, and so on) to the content. In this way, the reduction in resolution may be masked, while the processing savings from reducing the resolution may be maintained. Optionally, a strength associated with the blurring process (e.g., a degree to which the content is blurred) may be based on a difference in depth between the user&#39;s fixation point and the content, and/or an angular proximity of the content to the user&#39;s gaze. For example, the degree of blurring may increase with increasing proximity to the user&#39;s gaze. 
     In some embodiments, the display system may utilize the features of block  1408  or  1410  according to hardware and/or software of the display system. For example, particular hardware (e.g., graphics processing units) may be able to perform the blurring process in hardware without a threshold hit to performance of the hardware. For this particular hardware, the display system may be configured to reduce resolution of content and then blur the content. However, other hardware may be slow to perform the blurring process, and rendering content at greater resolutions might enable greater performance. For this other hardware, the display system may be configured to render content at greater resolutions. Furthermore, the decision between whether to render content at a greater resolution, or at a lower resolution with blurring may depend on the type of content to be displayed. For instance, the display system may be configured to render text at a greater resolution, while rendering shapes at a lower resolution and blurring. 
     With continued reference to  FIG.  14   , at block  1412  the display system presents content to the user. The display system may present the adjusted content to the user, for instance from a same display buffer as described above. 
     II. Adjusting Resolution Based on Ambient Illumination Levels 
     In addition to or as an alternative to reductions in resolution along the z-axis, various other schemes for presenting virtual content with reductions in resolution may be implemented in some embodiments. Advantageously, as noted herein, some aspects of the virtual content may be presented at relatively high resolution and some other aspects may be presented in relatively low resolution, which may reduce the use of computational and energy resources by the display system, while preferably having low impact on the perceived image quality of the virtual content. 
     With reference now to  FIG.  15   , an example is illustrated of a representation of the retina of an eye of a user. The illustrated view shows a retina  1500  as seen when viewed head-on along the visual axis of that retina. The retina  1500  includes a fovea  1510  surrounded by a peripheral area  1530 . Within the fovea  1510  is the foveola  1520 , which intersects the visual axis. 
     It will be appreciated that the retina includes two types of photoreceptors: rods and cones. In addition, the distributions of these photoreceptors across the retina varies, providing different rod and cone densities across the retina. 
     With reference now to  FIG.  16   , an example of resolution, and rod and cone density, across the retina  1500  of  FIG.  15    is graphically illustrated. The x-axis indicates degrees of eccentricity relative to a point at which the visual axis intersects the retina. The rightward direction on the page is the nasal direction and the leftward direction on the page is the temporal direction. As illustrated, the resolution of the human eye roughly correlates with the densities of photoreceptors (rods and cones) in the retina. Consequently, in some embodiments, the reduction or taper in the resolution (e.g., spatial resolution) of virtual content on the x and y-axes (e.g., on a given depth plane) may substantially follow the reductions across the retina of cone density, rod density, or an aggregate of rod and cone density. For example, the trend of the resolution reduction away from the fixation point across the user&#39;s field of view may be within ±50%, ±30%, ±20%, or ±10% of the trend in the changes in the photoreceptor density (e.g., cone density, rod density, or an aggregate of rod and cone density) over corresponding portions of the retina. In some embodiments, the reduction in resolution away from the fixation point is gradual and substantially follows the density changes. In some other embodiments, the reduction in resolution may occur in steps (e.g., one step, two steps, etc.). For example, there may be two steps: a highest resolution region of the field of view correlated with the foveola, a medium resolution region correlated with the fovea, and a lower resolution region correlated with the peripheral area. 
     With continued reference to  FIG.  16   , it will be appreciated that different photoreceptors have different levels of activity under different light conditions, e.g., at different ambient illumination levels. As a result, it is possible that, while reductions in resolution that follow the densities of photoreceptors may not be consciously perceptible to the user at some illumination levels, they may be perceptible at other illumination levels. Consequently, in some embodiments, reductions in the resolution of virtual content, along the x, y, or z-axes, maybe set with reference to external light conditions. 
     For example, the vision behavior of the eye may be divided into three modes, based on the light conditions. The three modes are photopic vision, mesotopic vision, and scotopic vision. Photopic vision typically occurs in bright conditions, e.g., ambient light or illumination levels of about 3 cd/m 2  or more, including about 10 to 10 8  cd/m 2 . In photopic vision, cones are primarily active. In scotopic vision, rods are primarily active. In mesotopic vision, both rods and cones may be active. As used herein, ambient light conditions or illumination levels refer to the amount of light that the eye of the user and his/her retina are exposed to. 
     Mesotopic vision typically occurs under lower light conditions, e.g., illumination levels of about 10 −3  to 10 0.5  cd/m 2 . Both cones and rods are active in at least some illumination levels within mesotopic vision, with the dominance of the rods or cones changing over time depending upon whether ambient illumination levels are increasing or decreasing. As the eye adapts to a brighter environment, more cones become activated in comparison to rods; on the other hand, as the eyes adapt to a dark environment, more rods are activated in comparison to cones. 
     Scotopic vision typically occurs in light conditions in which the illumination levels are less than the illumination levels for photopic vision. For example, scotopic vision may occur at illumination levels of about 10 −2  cd/m 2  or less, or about 10 −3  cd/m 2  or less, including about 10 −3  to 10 −6  cd/m 2 . Rods are primarily active in scotopic vision. It will be appreciated that the illumination levels noted herein for photopic, mesotopic, and scotopic vision are examples. In some embodiments, the illumination levels associated with each of type of vision may be assigned arbitrarily, based on user preferences, and/or customization for a group to which the user belongs (e.g., based on gender, age, ethnicity, the presence of visual abnormalities, etc.). 
     In some embodiments, the type of vision (photopic, mesotopic, or scotopic) active in the user may be determined based on measurements of ambient illumination levels. For example, the display system may be configured to measure ambient illumination levels using a light sensor, such as the outwardly-facing camera  112  ( FIG.  9 D ). In some embodiments, the display system may be in communication with another sensor or device which provides information regarding the ambient illumination levels. 
     It will be appreciated that head-mounted display systems may block or attenuate some of the ambient light, such that an outwardly-facing camera may not give luminance levels that accurately reflect the amount of light impinging on the eye. In addition, the display system, in projecting light to the eye to provide virtual content, is also a source of light that may alter the illumination levels to which the eye is exposed. In some other embodiments, an inwardly-facing camera may be utilized to determine luminance levels. For example, luminance levels are roughly correlated with the size of the pupil.  FIG.  17    graphically illustrates an example of the relationship between pupil size and the amount of light incident on an eye of a user. The x-axis shows values for luminance and the y-axis shows values for pupil area. Consequently, the display system may be configured to determine the pupil area of the user and then extrapolate luminance based on this pupil area. For example, the display system may be configured to use the inwardly-facing camera  500  ( FIG.  6   ) to capture an image of the eye  210  of the user and then analyze the image to determine the pupil area or other metric indicative of pupil area (e.g., pupil diameter or width). For example, the area occupied by the pupil of the eye  210  in the image captured by the camera may be determined and then corrected for any scaling factor caused by the optics of the camera. Advantageously, using pupil area to determine luminance levels may effectively take into account both reductions in ambient luminance levels caused by the display blocking some ambient light and also contributions to the luminance levels by the light output of the display itself. 
     With continued reference to  FIG.  17   , the display system may be configured to determine whether the user&#39;s eyes are in a photopic, mesotopic, or scotopic vision mode based upon the determined pupil area. For example, the display system may have resident in memory a table or other stored information specifying the vision mode expected for particular pupil area. As examples, in line with the graph shown in  FIG.  17   , the display system may categorize pupil areas of about 3 mm 2  or less as being indicative of photopic vision, pupil areas of 3 mm 2  or more up to about 38 mm 2  as being indicative of mesotopic vision, and pupil areas of more than 38 mm 2  as being indicative of scotopic vision. It will be appreciated that these luminance values and associated vision modes are examples and that other values may be substituted. For example, different values may be applied to different users in response to input from the users, or different values may be applied based on the particular category in which the user may fall (e.g., gender, age, ethnicity, the presence of visual abnormalities, etc.). In addition, it will be appreciated that the display system does not necessarily identify a specific vision mode. Rather, the display system may be configured to simply associate particular measured pupil areas with particular resolution levels or adjustments. 
     In some embodiments, inputs from both the inwardly-facing camera  510  ( FIG.  6   ) and the outwardly-facing camera  112  ( FIG.  9 D ) may be utilized to determine luminance levels. For example, the display system may be configured to take an average (including a weighted average) of the luminance levels determined using the cameras  510  and  112 . As noted above, the luminance level determined using the camera  510  may be extrapolated from the size of the pupil area of the user&#39;s eye, based on imaging the user&#39;s eye using that camera  510 . 
     It will be appreciated that rods and cones have different levels of visual acuity and different sensitivities to color and contrast. Consequently, because ambient luminance levels impact whether rods and/or cones are active, there are differences in visual acuity and sensitivities to color and contrast at different ambient luminance levels. Advantageously, the light-level differences in visual acuity and sensitivities to color and contrast may be applied to provide additional bases for reducing resolution, which may be utilized in conjunction with changes in resolution based on the fixation point as described above (e.g., regarding  FIGS.  12 A and  14   ), or may be utilized separately even without specifically making changes in resolution based on the fixation point. 
     With reference now to  FIG.  18   , a diagram is shown of an example of a process  1800  for adjusting virtual content based on the amount of light incident on an eye of a user. For convenience, the process may be described as being performed by a display system (e.g., the wearable display system  60  ( FIG.  9 D ), which may include processing hardware and software, and optionally may provide information to an outside system of one or more computers or other processing units, for instance to offload processing to the outside system, and receive information from the outside system). 
     At block  1810 , the display system determines the amount of light reaching the retina. Preferably, this determination is an estimate of the amount of light reaching the retina rather than a direct measurement of light that impinges on the retina. This estimate may be made as discussed herein using the methods disclosed for determining luminance levels. For example, luminance levels may be assumed to correspond to the amount of light reaching the retina. As result, determining the amount light reaching the retina may include determining a size of the user&#39;s pupil and/or determining ambient luminance levels using a sensor configured to detect light, such as an outwardly-facing camera on a display device. 
     At block  1820 , the display system adjusts the resolution of virtual content to be presented to the user based on the amount of light found to be reaching the retina at block  1810 . In some embodiments, adjusting the resolution of the virtual content comprises adjusting one or more of the spatial resolution, color depth, and light intensity resolution of the virtual content. It will be appreciated that the human visual system has the greatest acuity and sensitivity to spatial resolution, color, and light intensity under photopic illumination levels. The ability to perceive differences in spatial resolution, color, and light intensity decrease under mesotopic illumination levels, and further decrease under scotopic illumination levels. 
     Consequently, in some embodiments, if the amount of light present is found to correspond to the levels for photopic vision, then virtual objects may be rendered at full or high spatial resolution (compared to spatial resolution which would be utilized for mesotopic or scotopic vision). If the amount of light present is found to correspond to mesotopic levels, then virtual objects may be rendered at may reduce spatial resolution compared to the spatial resolution utilized for virtual objects under photopic illumination levels. If the amount of light is found to correspond to scotopic levels, then the virtual objects may be rendered at a spatial resolution that is lower than that used under mesotopic or photopic illumination levels. Spatial resolution may be adjusted as described herein, e.g., by reducing the number of polygons, etc. 
     Color depth or bit depth may similarly be adjusted depending on illumination levels, with the highest color depth used under photopic illumination levels, an intermediate color depth used under mesotopic illumination levels, and the lowest color depth used under scotopic illumination levels. It will be appreciated that color depth may be adjusted by changing the number of bits used for each color component of a pixel, with fewer bits equating to lower color depth. 
     Likewise, without being limited by theory, gradations in light intensity are believed to become larger as illumination levels progress from photopic to mesotopic to scotopic illumination levels. Stated another way, the human visual system is believed to be able to discern fewer differences in light intensity as the ambient illumination level decreases. In some embodiments, the display system may be configured to display fewer gradations in light intensity as illumination levels progress from photopic to mesotopic to scotopic illumination levels. As a result, the largest number of gradations in light intensity levels are presented under photopic illumination levels, fewer gradations are presented under mesotopic illumination levels, and yet fewer gradations are presented under scotopic illumination levels. 
     In addition, in some embodiments, the display system may be able to provide a larger number of gradations in light intensity than the user is able to perceive. An example of this illustrated in  FIGS.  22   a - 22   c   , discussed further below. For example, the display system may be able to display  256  different levels of intensity for a given image pixel, but the user may only be able to perceive a lower number of levels, e.g., 64 levels. In this instance, multiple possible light intensity levels are subsumed within a single one of the perceptible light intensity levels. For example, the display system may be able to display four different light intensity levels, but the user may perceive all four as being similar. In such circumstances, where multiple possible light intensities are perceived by the user as being the same, the display system may be configured to select the lowest intensity value, out of these values that are perceived to be similar, for display. As a result, the display system may be able to utilize lower intensities, thereby reducing the amount of power used to illuminate a display to achieve the desired light intensities. This may have particular advantages in display systems in which individual pixels of a spatial light modulator are themselves light emitters, such as organic and inorganic LEDs. In some embodiments, the number of gradations decrease with decreases in ambient illumination levels, and the display system is configured to group a larger number of possible light intensity levels together, to display the lowest light intensity of the group. 
     It will be appreciated that, for virtual content that is to be displayed, one, two, or all three of spatial resolution, color depth, and light intensity resolution may be changed based on the light conditions to which a user is subjected (the amount of light reaching the user&#39;s retina). These adjustments to spatial resolution, color depth, and/or light intensity resolution based on light conditions may be made to virtual content overall, without making adjustments to resolution based on distance from the fixation point of the user&#39;s eyes, as disclosed herein. In some other embodiments, the adjustments to spatial resolution, color depth, and/or light intensity resolution based on light conditions may be made in conjunction with adjustments to resolution based on distance from the fixation point (see, e.g.,  FIGS.  12 A and  14   ). In some embodiments, if resolution decreases with distance from the fixation point, the profile of the decrease on a given plane (on the x and y-axes) preferably matches the profile of changes in cone density across corresponding portions of the retina. 
     In some embodiments, as noted herein, adjustments to spatial resolution, color depth, and/or light intensity resolution are preferably tied to the mode of vision (photopic, mesotopic, or scotopic vision) active at a given time. These adjustments may dynamically change if the mode of vision changes. For example, when the user progresses from photopic vision to scotopic vision, resolution may decrease as discussed herein. Conversely, when the user progresses from scotopic vision to mesotopic vision, the resolution of virtual content may increase. It will be appreciated that tying resolution adjustments to a particular mode of vision does not require a specific determination that the user is in that particular mode; rather, the display system may be configured to simply associate particular ranges of ambient illumination levels or pupil size with particular resolutions, whether spatial resolution, color depth, or light intensity resolution. In addition, while the resolution adjustments are preferably tied to three levels of light conditions (corresponding to three modes of vision) as discussed herein, in some embodiments, the resolution adjustments may be tied to two levels of light conditions, or more than three levels of light conditions. 
     It will also be appreciated that the resolution adjustment may occur in real time (e.g., as ambient light conditions change), or may be delayed for a set duration to allow the human visual system to adapt to existing light conditions before the resolution adjustment to virtual content is made. Without being limited by theory, it is believed that the human visual system requires a period of time to adapt to different illumination levels, with that period of time increasing as illumination levels decrease. Consequently, in some embodiments, adjustments in resolution due to changing illumination levels are not made until the user has been exposed (e.g., substantially continuously exposed) to a particular illumination level for a set amount of time. For example, the set amount time may be 5 minutes, 10 minutes, 15 minutes, or 20 minutes. 
     With continued reference to  FIG.  18   , at block  1830 , virtual content is presented to the user. The presentation of this virtual content may be conducted as discussed herein, e.g., as in block  1208  of  FIG.  12 A  or block  1412  of  FIG.  14   . 
     With reference now to  FIG.  19   , an example is graphically illustrated of a change in resolution detectable by the eye of a user as the amount of light incident on the eye changes. This figure illustrates an example of the sensitivity of the human visual system to spatial resolution under different vision modes. Scotopic vision occurs in the low-light region  1910 , mesotopic vision occurs in the medium-light region  1920 , and photopic vision occurs in the bright light region  1930 . As shown, sensitivity to spatial resolution decreases substantially as ambient illumination levels decrease. In some embodiments, the adjustments to spatial resolution discussed above regarding  FIG.  18    correspond to the contours of the illustrated curve. For example, for a given light level in the photopic or scotopic vision mode, the virtual content is rendered with sufficient spatial resolution to meet or exceed the resolution values shown on the y-axis. 
     With reference now to  FIG.  20   , it will be appreciated that different photoreceptors may be used to perceive light of different wavelengths or colors.  FIG.  20    graphically illustrates an example of differences in sensitivity of the eye to light of different colors at different levels of illumination. The differences in time duration on the x-axis are reflective of the amount of time typically needed for the human visual system to adapt to a particular ambient illumination level, such that a particular mode of vision is activated. Notably, at ambient illumination levels corresponding to scotopic vision and a portion of mesotopic vision, photoreceptors for red light may no longer be active, while photoreceptors for blue light are active under the lowest light conditions. It will be appreciated that red, green, and blue light correspond to the colors most typically used as component colors in a display system to form full color images (e.g., as discussed herein regarding  FIG.  8 - 9 B ). In some embodiments, the display system may be configured to vary the rendering of images of different colors depending upon the ambient illumination levels. 
     With reference now to  FIG.  21   , a diagram is shown of an example of a process  2100  for adjusting virtual content formed using multiple component color images, where the resolution adjustment is made based on the color of the component color image. At block  2110 , the display system provides virtual content to be presented using multiple component images. These may be different images of different component colors to be directed to different waveguides, as discussed regarding  FIG.  8 - 9 B . Consequently, in some embodiments, each of the streams of images of different component colors may be separately rendered. Providing virtual content to be presented using multiple component images may include utilizing a display system that outputs image streams of different component colors to form a full color image. 
     At block  2120 , the display system may adjust resolutions of component color images based on their color. For example, the display system may select color images of one of these component colors for resolution adjustment. For example, the selection may be made based on a determination of illumination levels, as discussed above regarding block  1810  of  FIG.  18   . As shown in  FIG.  19   , some component colors may not be perceived by a user at some illumination levels. The display system may have stored within it information regarding illumination levels and component colors that are not visible at those levels. If there is a match between the illumination level and the component color not visible at those levels, then images of that component color may be selected for adjustment. In some environments, one adjustment may be to simply not render or display that component color image if the ambient illumination levels are such that the user is not expected to perceive that color. For example, under scotopic illumination levels, the display system may be configured to not render or display images of the component color red. 
     With continued reference to  FIG.  21   , at block  2130 , virtual content is presented to the user. The presentation of the virtual content may be conducted as discussed herein, e.g., as in block  1208  of  FIG.  12 A  or block  1412  of  FIG.  14   . 
     With reference now to  FIGS.  22 A- 22 C , as discussed above and without being limited by theory, the ability of the human visual system to perceive gradations in light intensity is believed to change with ambient illumination levels.  FIGS.  22 A- 22 C  show examples of changing contrast sensitivity as the amount of light incident on the eye of the user decreases. For example,  FIG.  22 A  may be understood to show the contrast sensitivity under photopic light conditions,  FIG.  22 B  may be understood to show the contrast sensitivity under mesotopic light conditions, and  FIG.  22 C  may be understood to show the contrast sensitivity under scotopic light conditions.  FIG.  22 A  shows a progression  2100  of gradations  2110   1  to  2110   i , proceeding from high light intensity at the top to low with light intensity at the bottom. Similarly,  FIG.  22 B  shows a progression  2102  of gradations  2110   1  to  2110   i , proceeding from high light intensity to low with light intensity. Likewise,  FIG.  22 C  shows a progression  2104  of gradations  2110   1  to  2110   i , proceeding from high light intensity to low light intensity. The boxes  2120 ,  2130 ,  2140 , indicate the groups of intensity gradations which are perceived by the user is being the same. The sizes of these groups are expected to increase with decreasing ambient illumination levels, as illustrated. Consequently, as discussed above regarding  FIG.  18   , in some embodiments, the display system may be configured to use the lowest intensity value within each group (e.g., within each of the boxes  2120 ,  2130 ,  2140 ). 
     With reference now to  FIG.  23   , an example of a representation of the optic nerve and peripheral blind spots of the eyes of a user is illustrated. In some embodiments, in addition to or as an alternative to any of the resolution adjustments disclosed herein, the display system may be configured to refrain from rendering content in various locations where content is not expected to be perceptible by the user.  FIG.  23    illustrates left and right eyes  210   L  and  210   R , respectively. Each eye has a respective optical axis  1003 A and  1003 B and optical nerve  2300   L  and  2300   R . There is a blind spot of the point where each of the optical nerves  2300   L  and  2300   R  contact their respective eyes  210   L , and  210   R . These blind spots prevent the viewer from seeing content in the direction of the rays  2302   L  and  2302   R . In addition, at the periphery of each eye there exists a region in which content cannot be seen by the opposite eye. For example, content in the left peripheral region P L  may be seen by the left eye  210   L , but is not seen by the right eye  210   R . On the other hand, content in the right peripheral region P R  may be seen by the right eye  210   R , but is not seen by the left eye  210   L . Consequently, in some embodiments, the display system may be configured to omit rendering content that would be mapped to the blind spots of each eye  210   L  and  210   R , e.g., content falling on the rays  2302   L  and  2302   R . In addition or alternatively, in some embodiments, the display system may be configured to omit rendering content to the left eye  210   L  if that content falls within the right peripheral region P L ; and/or the display system may be configured to omit rendering content to the right eye  210   R  if that content falls within the left peripheral region P L . It will be appreciated that the locations of the blind spots and/or the peripheral regions may be preset, e.g., based on averages for a population of users and/or may be tailored and calibrated for a particular user by test using content displayed at various locations and inputs from the user indicating whether or not a virtual object is visible. 
     III. Multiple Image Streams for Providing Content Having Different Resolutions 
     In some embodiments, a foveated image having high and low spatial resolution regions may be formed by spatially overlapping two or more image streams, each having a different resolution (e.g., a different perceived pixel density). For example, one of the image streams, e.g., the low resolution image stream, may form images having a large field of view and another of the image streams, e.g., the high-resolution image stream, may form images having a narrow field of view. The narrow field of view image and the high field of view image may contain similar content, although at different resolutions or pixel densities as seen by the user. These images may be overlaid one another (e.g., occupy the same location in space simultaneously or in close temporal proximity, such that the viewer perceives the images are being present simultaneously). Thus, the viewer may receive an aggregate image having high-resolution in a confined part of their field of view and low resolution over a larger portion of their field of view. Preferably, as discussed herein, the high-resolution portion maps to the foveal vision region of the user&#39;s eyes while the low resolution portion maps to the peripheral vision region of the user&#39;s eyes. As such, the differences in resolution between the high-resolution portion and the low resolution portion of the image is preferably not readily perceptible to the user. 
     In some environments, the display system for displaying the high and low resolution images utilizes the same spatial light modulator to form both images. Thus, the spatial light modulator has a fixed size and density of pixels. In display systems with a fixed size and density of pixels, an increase in angular field of view (FOV) comes at the cost of spatial or angular resolution, e.g., as governed by the Lagrange invariant. For example, if an SLM having a fixed number of pixels is used to form both the high and low resolution images, then spreading those pixels across the entire field of view would provide an image with a lower apparent resolution than confining those pixels to a small portion of the total field of view; the pixel density of the high-resolution images is higher than the pixel density of the low-resolution images. Consequently, there is generally an inverse relationship between FOV and angular resolution. Because FOV and angular resolution affect image visibility and quality, this tradeoff places constraints on user experience and the ultimate achievable FOV and angular resolution in AR or VR systems. As will be apparent from the discussion herein, in some embodiments, the term “resolution” may be used to refer to “angular resolution.” 
     Head-mounted display devices or wearable display devices can be configured to provide an immersive user experience by projecting virtual content directly into the eyes of a user. Although it can be beneficial to provide wide FOV images at a uniformly high resolution across the FOV, the physiological limitations of the human visual system can prevent a user from appreciating or even noticing high resolution imagery positioned in the peripheral regions of the user&#39;s field of view. This inability to perceive high resolution imagery within the peripheral regions is caused by characteristics of the retina of a human eye, which contains two types of photoreceptors, namely rod cells and cone cells. The cones are more responsible for acute (detailed) vision. The rods and cones are distributed differently in the human eye. The highest concentration of cone cells is found within the fovea (i.e., the center of the retina), while the highest concentration of rod cells is found in the region immediately surrounding the fovea (i.e., the periphery of the retina). Because of this non-uniform distributions of the rod cells and cone cells, the fovea is responsible for sharp central vision (also called foveal vision). Visual acuity decreases as distance from the fovea increases. 
     For AR or VR applications, a headset is generally worn by one user at a time. The headset can be configured to take advantage of the user&#39;s inability to perceive all the details of a wide field of view stream of images at once by limiting the display of high-resolution content to regions within the wide field of view currently being focused on by the user. In this way, the headset can provide the user with the appearance of a high-resolution wide FOV stream of images without the need for the processing power that would otherwise be required to generate high-resolution content across the entire field of view. The stream of images presented to the user can take many forms and will be generally referred to as an image stream. For example, the image stream can show a static image by continuously displaying the same image to the user or can show motion by displaying a stream of different images. In some embodiments, the headset can be configured to display more than one image stream at the same time; the different image streams can have different angular resolutions and can extend across different regions of the user&#39;s FOV. It should be noted that an image stream associated with an AR system might not display content entirely across a particular region to which it is assigned since AR systems are designed to mix virtual content with real-world content. 
     According to some embodiments, a first image stream and a second image stream can be presented to a user simultaneously, or in rapid succession such that the two image streams appear to be displayed simultaneously. The first image stream can have a wide FOV and low resolution that can encompass the user&#39;s vision to evoke an immersion experience to the user. A portion of the first image stream corresponding to an instantaneous portion of the FOV covered by the second image stream may be turned off in some embodiments. The second image stream can have a narrow FOV and a high resolution that can be dynamically displayed within the boundaries of the first image stream according to the user&#39;s current fixation point as determined in real-time using eye-gaze tracking techniques. In other words, the second image stream can be shifted around as the user&#39;s eye gaze changes, such that the second image stream persistently covers the user&#39;s foveal vision. In some embodiments, the first image stream is presented to the user at a fixed position, as the second image stream is shifted around relative to the first image stream. In some other embodiments, both the first image stream and the second image stream are shifted according to the user&#39;s current fixation point. 
     The content of the second image stream can include a subset of the content of the first image stream with a higher resolution than the first image stream, and can be overlaid on and properly aligned with respect to the first image stream. Because the higher resolution second image stream overlays the portion of the first image stream within the user&#39;s foveal vision, the modulation transfer function (MTF) in the area that includes the higher resolution image stream is increased. In some embodiments, the subset of the content of the first image stream overlaid by the second image stream can be turned off or be presented with a lower intensity. In this way, the user can perceive the combination of the first image stream and the second image stream as having both a wide FOV and high resolution. Such a display system can afford several advantages. For example, the display system can provide a superior user experience while having a relatively small form factor and saving computing resources and computing power. 
     In certain embodiments light intensity in border regions of the first image stream and the second image streams are tapered down to values below the intended image brightness and the border regions of the first image stream and second image stream are overlapped. In the overlapping area, the sum of light intensities attributed to the two image streams may be relatively constant and equal to the intended image brightness. Traversing the overlapping region from the first image stream side the second image stream side, the MTF changes from a first value equal or closer to the MTF of the first image stream to a second value equal or closer to the MTF of the second image stream. In this manner it is possible to avoid creating a sharp boundary between the regions served by the two image streams which might in certain circumstances be perceptible to the user. 
     According to some embodiments, a first light beam associated with the first image stream and a second light beam associated with the second image stream can be multiplexed into a composite light beam using certain multiplexing methods. For example, time-division multiplexing, polarization-division multiplexing, wavelength-division multiplexing, and the like, can be used according to various embodiments. The composite light beam can be directed to one or more optical elements that serve to de-multiplex the composite light beam into two separate optical paths. For example, a beam splitter such as a polarization beam splitter (PBS) or a dichroic beam splitter, or optical switching elements can be used to separate the composite light beam depending on the method of multiplexing used. Once separated, the first light beam associated with the first image stream and the second light beam associated with the second image stream can be routed through their respective optical paths and ultimately provided as output to the user. 
     According to some embodiments, the first light beam associated with the first image stream can be angularly magnified by optical elements in a first optical path so that the first image stream can be presented with a wider FOV and lower angular resolution (as governed by the Lagrange invariant); whereas the second light beam associated with the second image stream is not angularly magnified, demagnified, or magnified by an amount less than the amount of magnification applied to the first light beam associated with the first image stream. In this way, the second image stream can be presented with a narrower FOV and higher angular resolution (as governed by the Lagrange invariant) than the first image stream. 
       FIG.  24    shows a visual field diagram depicting the outer perimeter of an exemplary monocular field of view  3002  for a human eye in two-dimensional angular space. As shown in  FIG.  24   , temporal-nasal and inferior-superior axes of the visual field diagram serve to define the two-dimensional angular space within which the outer perimeter of the monocular field of view  3002  is mapped. In this way, the visual field diagram of  FIG.  24    may be seen as being equivalent or similar to a “Goldmann” visual field map or plot for a human eye. As indicated by the depicted arrangement of the temporal-nasal and inferior-superior axes, the visual field diagram shown in  FIG.  24    represents a visual field diagram for the left eye of a human. While field of view can vary slightly from person to person, the depicted field of view is close to what many humans are capable of viewing with their left eye. It follows that a visual field diagram depicting the outer perimeter of an exemplary monocular field of view of the right eye might resemble something of a version of the visual field diagram of  FIG.  24    in which the temporal-nasal axis and the outer perimeter of the monocular field of view  3002  have been mirrored about the inferior-superior axis. 
     The visual field diagram of  FIG.  24    further depicts the outer perimeter of an exemplary field of regard  3004  for the human eye, which represents a portion of the monocular field of view  30022  in angular space within which the person can fixate. In addition, the visual field diagram of  FIG.  24    also depicts the outer perimeter of an exemplary foveal field  3006  for the human eye, which represents a portion of the monocular field of view  3002  in angular space in direct view of the fovea of the human eye at a given point in time. As depicted, a person&#39;s foveal field  3006  can move anywhere within field of regard  3004 . Portions of the monocular field of view  3002  outside of foveal field  3006  in angular space can be referred herein as the peripheral region of the person&#39;s field of view. Because of the ability of human eyes to distinguish a high level of detail outside of the foveal field  3006  is quite limited, displaying reduced resolution imagery outside of the foveal field  3006  is unlikely to be noticed and can allow for substantial savings on power expenditure for processing components responsible for generating content for the display. 
       FIG.  25 A  shows an exemplary wearable display device  4050  configured to provide virtual content to a user according to some embodiments. Wearable display device  4050  includes main displays  4052  supported by frame  4054 . Frame  4054  can be attached to the head of a user using an attachment member taking the form of temple arms  4006 . 
     Referring now to  FIG.  25 B , an exemplary embodiment of an AR system configured to provide virtual content to a user will now be described. In some embodiments, the AR system of  FIG.  25 B  may represent a system to which the wearable display device  4050  of  FIG.  25 A  belongs. The AR system of  FIG.  25 B  uses stacked light-guiding optical element assemblies  4000  and generally includes an image generating processor  4010 , a light source  4020 , a controller  4030 , a spatial light modulator (“SLM”)  4040 , an injection optical system  4060 , and at least one set of stacked eyepiece layers or light guiding optical elements (“LOEs”; e.g., a planar waveguide)  4000  that functions as a multiple plane focus system. The system may also include an eye-tracking subsystem  4070 . It should be appreciated that other embodiments may have multiple sets of stacked LOEs  4000 , but the following disclosure will focus on the exemplary embodiment of  FIG.  25 B . 
     The image generating processor  4010  is configured to generate virtual content to be displayed to the user. The image generating processor may convert an image or video associated with the virtual content to a format that can be projected to the user in 3-D. For example, in generating 3-D content, the virtual content may need to be formatted such that portions of a particular image are displayed at a particular depth plane while others are displayed at other depth planes. In one embodiment, all of the image may be generated at a particular depth plane. In another embodiment, the image generating processor may be programmed to provide slightly different images to the right and left eyes  210  such that when viewed together, the virtual content appears coherent and comfortable to the user&#39;s eyes. 
     The image generating processor  4010  may further include a memory  4012 , a GPU  4014 , a CPU  4016 , and other circuitry for image generation and processing. The image generating processor  4010  may be programmed with the desired virtual content to be presented to the user of the AR system of  FIG.  25 B . It should be appreciated that in some embodiments, the image generating processor  4010  may be housed in the wearable AR system. In other embodiments, the image generating processor  4010  and other circuitry may be housed in a belt pack that is coupled to the wearable optics. The image generating processor  4010  is operatively coupled to the light source  4020  which projects the light associated with the desired virtual content and one or more spatial light modulators (described below). 
     The light source  4020  is compact and has high resolution. The light source  4020  includes a plurality of spatially separated sub-light sources  4022  that are operatively coupled to a controller  4030  (described below). For instance, the light source  4020  may include color specific LEDs and lasers disposed in various geometric configurations. Alternatively, the light source  4020  may include LEDs or lasers of like color, each one linked to a specific region of the field of view of the display. In another embodiment, the light source  4020  may comprise a broad-area emitter such as an incandescent or fluorescent lamp with a mask overlay for segmentation of emission areas and positions. Although the sub-light sources  4022  are directly connected to the AR system of  FIG.  2 B  in  FIG.  2 B , the sub-light sources  222  may be connected to system via optical fibers (not shown), as long as the distal ends of the optical fibers (away from the sub-light sources  4022 ) are spatially separated from each other. The system may also include condenser (not shown) configured to collimate the light from the light source  4020 . 
     The SLM  4040  may be reflective (e.g., a DLP DMD, a MEMS mirror system, an LCOS, or an FLCOS), transmissive (e.g., an LCD) or emissive (e.g. an FSD or an OLED) in various exemplary embodiments. The type of spatial light modulator (e.g., speed, size, etc.) can be selected to improve the creation of the 3-D perception. While DLP DMDs operating at higher refresh rates may be easily incorporated into stationary AR systems, wearable AR systems typically use DLPs of smaller size and power. The power of the DLP changes how 3-D depth planes/focal planes are created. The image generating processor  4010  is operatively coupled to the SLM  4040 , which encodes the light from the light source  4020  with the desired virtual content. Light from the light source  4020  may be encoded with the image information when it reflects off of, emits from, or passes through the SLM  4040 . 
     Referring back to  FIG.  25 B , the AR system also includes an injection optical system  4060  configured to direct the light from the light source  4020  (i.e., the plurality of spatially separated sub-light sources  4022 ) and the SLM  4040  to the LOE assembly  4000 . The injection optical system  4060  may include one or more lenses that are configured to direct the light into the LOE assembly  4000 . The injection optical system  4060  is configured to form spatially separated and distinct pupils (at respective focal points of the beams exiting from the injection optical system  4060 ) adjacent the LOEs  4000  corresponding to spatially separated and distinct beams from the sub-light sources  4022  of the light source  4020 . The injection optical system  4060  is configured such that the pupils are spatially displaced from each other. In some embodiments, the injection optical system  4060  is configured to spatially displace the beams in the X and Y directions only. In such embodiments, the pupils are formed in one X, Y plane. In other embodiments, the injection optical system  4060  is configured to spatially displace the beams in the X, Y and Z directions. 
     Spatial separation of light beams forms distinct beams and pupils, which allows placement of in-coupling gratings in distinct beam paths, so that each in-coupling grating is mostly addressed (e.g., intersected or impinged) by only one distinct beam (or group of beams). This, in turn, facilitates entry of the spatially separated light beams into respective LOEs  4000  of the LOE assembly  4000 , while minimizing entry of other light beams from other sub-light sources  4022  of the plurality (i.e., cross-talk). A light beam from a particular sub-light source  4022  enters a respective LOE  4000  through an in-coupling grating (not shown in  FIG.  25 B , see  FIGS.  24 - 26   ) thereon. The in-coupling gratings of respective LOEs  4000  are configured to interact with the spatially separated light beams from the plurality of sub-light sources  4022  such that each spatially separated light beam only intersects with the in-coupling grating of one LOE  4000 . Therefore, each spatially separated light beam mainly enters one LOE  4000 . Accordingly, image data encoded on light beams from each of the sub-light sources  4022  by the SLM  4040  can be effectively propagated along a single LOE  4000  for delivery to an eye  210  of a user. 
     Each LOE  4000  is then configured to project an image or sub-image that appears to originate from a desired depth plane or FOV angular position onto a users retina. The respective pluralities of LOEs  4000  and sub-light sources  4022  can therefore selectively project images (synchronously encoded by the SLM  4040  under the control of controller  4030 ) that appear to originate from various depth planes or positions in space. By sequentially projecting images using each of the respective pluralities of LOEs  4000  and sub-light sources  4022  at a sufficiently high frame rate (e.g., 360 Hz for six depth planes at an effective full-volume frame rate of 60 Hz), the system of  FIG.  25 B  can generate a 3-D image of virtual objects at various depth planes that appear to exist simultaneously in the 3-D image. 
     The controller  4030  is in communication with and operatively coupled to the image generating processor  4010 , the light source  4020  (sub-light sources  4022 ) and the SLM  4040  to coordinate the synchronous display of images by instructing the SLM  4040  to encode the light beams from the sub-light sources  4022  with appropriate image information from the image generating processor  4010 . 
     The AR system also includes an optional eye-tracking subsystem  4070  that is configured to track the users eyes  4002  and determine the users focus. In one embodiment, only a subset of sub-light sources  4022  may be activated, based on input from the eye-tracking subsystem, to illuminate a subset of LOEs  4000 , as will be discussed below. Based on input from the eye-tracking subsystem  4070 , one or more sub-light sources  4022  corresponding to a particular LOE  4000  may be activated such that the image is generated at a desired depth plane that coincides with the users focus/accommodation. For example, if the users eyes  210  are parallel to each other, the AR system of  FIG.  25 B  may activate the sub-light sources  4022  corresponding to the LOE  4000  that is configured to deliver collimated light to the users eyes, such that the image appears to originate from optical infinity. In another example, if the eye-tracking sub-system  4070  determines that the users focus is at 1 meter away, the sub-light sources  4022  corresponding to the LOE  4000  that is configured to focus approximately within that range may be activated instead. It should be appreciated that, in this particular embodiment, only one group of sub-light sources  4022  is activated at any given time, while the other sub-light sources  4020  are deactivated to conserve power. 
       FIG.  25 C  illustrates schematically the light paths in an exemplary viewing optics assembly (VOA) that may be used to present a digital or virtual image to a viewer, according to some embodiments. In some embodiments, the VOA could be incorporated in a system similar to wearable display device  4050  as depicted in  FIG.  25 A . The VOA includes a projector  4001  and an eyepiece  200  that may be worn around a viewer&#39;s eye. The eyepiece  4000  may, for example, may correspond to LOEs  4000  as described above with reference to  FIG.  25 B . In some embodiments, the projector  4001  may include a group of red LEDs, a group of green LEDs, and a group of blue LEDs. For example, the projector  201  may include two red LEDs, two green LEDs, and two blue LEDs according to an embodiment. In some examples, the projector  4001  and components thereof as depicted in  FIG.  25 C  (e.g., LED light source, reflective collimator, LCoS SLM, and projector relay) may represent or provide the functionality of one or more of light source  4020 , sub-light sources  4022 , SLM  4040 , and injection optical system  4060 , as described above with reference to  FIG.  25 B . The eyepiece  4000  may include one or more eyepiece layers, each of which may represent one of LOEs  4000  as described above with reference to  FIG.  25 B . Each eyepiece layer of the eyepiece  4000  may be configured to project an image or sub-image that appears to originate from a respective desired depth plane or FOV angular position onto the retina of a viewer&#39;s eye. 
     In one embodiment, the eyepiece  4000  includes three eyepiece layers, one eyepiece layer for each of the three primary colors, red, green, and blue. For example, in this embodiment, each eyepiece layer of the eyepiece  4000  may be configured to deliver collimated light to the eye that appears to originate from the optical infinity depth plane (0 diopters). In another embodiment, the eyepiece  4000  may include six eyepiece layers, i.e., one set of eyepiece layers for each of the three primary colors configured for forming a virtual image at one depth plane, and another set of eyepiece layers for each of the three primary colors configured for forming a virtual image at another depth plane. For example, in this embodiment, each eyepiece layer in one set of eyepiece layers of the eyepiece  4000  may be configured to deliver collimated light to the eye that appears to originate from the optical infinity depth plane (0 diopters), while each eyepiece layer in another set of eyepiece layers of the eyepiece  4000  may be configured to deliver collimated light to the eye that appears to originate from a distance of 2 meters (0.5 diopter). In other embodiments, the eyepiece  4000  may include three or more eyepiece layers for each of the three primary colors for three or more different depth planes. For instance, in such embodiments, yet another set of eyepiece layers may each be configured to deliver collimated light that appears to originate from a distance of 1 meter (1 diopter). 
     Each eyepiece layer comprises a planar waveguide and may include an incoupling grating  4007 , an orthogonal pupil expander (OPE) region  4008 , and an exit pupil expander (EPE) region  4009 . More details about incoupling grating, orthogonal pupil expansion, and exit pupil expansion are described in U.S. patent application Ser. No. 14/555,585 and U.S. patent application Ser. No. 14/726,424, the contents of which are hereby expressly and fully incorporated by reference in their entirety, as though set forth in full. Still referring to  FIG.  25 C , the projector  4001  projects image light onto the incoupling grating  4007  in an eyepiece layer  4000 . The incoupling grating  4007  couples the image light from the projector  4001  into the waveguide propagating in a direction toward the OPE region  4008 . The waveguide propagates the image light in the horizontal direction by total internal reflection (TIR). The OPE region  4008  of the eyepiece layer  4000  also includes a diffractive element that couples and redirects a portion of the image light propagating in the waveguide toward the EPE region  4009 . More specifically, collimated light propagates horizontally (i.e., relative to view of  FIG.  25 C ) along the waveguide by TIR, and in doing so repeatedly intersects with the diffractive element of the OPE region  4008 . In some examples, the diffractive element of the OPE region  4008  has a relatively low diffraction efficiency. This causes a fraction (e.g., 10%) of the light to be diffracted vertically downward toward the EPE region  4009  at each point of intersection with the diffractive element of the OPE region  4008 , and a fraction of the light to continue on its original trajectory horizontally along the waveguide via TIR. In this way, at each point of intersection with the diffractive element of the OPE region  4008 , additional light is diffracted downward toward the EPE region  4009 . By dividing the incoming light into multiple outcoupled sets, the exit pupil of the light is expanded horizontally by the diffractive element of the OPE region  4008 . The expanded light coupled out of the OPE region  4008  enters the EPE region  4009 . 
     The EPE region  4009  of the eyepiece layer  4000  also includes a diffractive element that couples and redirects a portion of the image light propagating in the waveguide toward a viewer&#39;s eye  210 . Light entering the EPE region  4009  propagates vertically (i.e., relative to view of  FIG.  25 C ) along the waveguide by TIR. At each point of intersection between the propagating light and the diffractive element of the EPE region  4009 , a fraction of the light is diffracted toward the adjacent face of the waveguide allowing the light to escape the TIR, emerge from the face of the waveguide, and propagate toward the viewer&#39;s eye  210 . In this fashion, an image projected by projector  4001  may be viewed by the viewer&#39;s eye  210 . In some embodiments, the diffractive element of the EPE region  4009  may be designed or configured to have a phase profile that is a summation of a linear diffraction grating and a radially symmetric diffractive lens. The radially symmetric lens aspect of the diffractive element of the EPE region  4009  additionally imparts a focus level to the diffracted light, both shaping the light wavefront (e.g., imparting a curvature) of the individual beam as well as steering the beam at an angle that matches the designed focus level. Each beam of light outcoupled by the diffractive element of the EPE region  4009  may extend geometrically to a respective focus point positioned in front of the viewer, and may be imparted with a convex wavefront profile with a center of radius at the respective focus point to produce an image or virtual object at a given focal plane. 
     Descriptions of such a viewing optics assembly and other similar set-ups are further provided in U.S. patent application Ser. No. 14/331,218. U.S. patent application Ser. No. 15/146,296, and U.S. patent application Ser. No. 14/555,585, all of which are incorporated by reference herein in their entireties. It follows that, in some embodiments, the exemplary VOA may include and/or take on the form of one or more components described in any of the patent applications mentioned above with reference to  FIG.  25 C  and incorporated herein by reference. 
     IV. High Field of View and High Resolution Foveated Display Using Multiple Optical Paths 
       FIGS.  26 A- 26 D  illustrate exemplary render perspectives to be used and light fields to be produced in an AR system for each of two exemplary eye orientations. In  FIG.  26 A , a viewer&#39;s eye  210  is oriented in a first manner with respect to an eyepiece  5000 . In some embodiments, the eyepiece  5000  may be similar to the stack of LOEs or eyepiece  4000  as described above with reference to  FIGS.  25 B and  25 C . More specifically, in this example, the viewer&#39;s eye  210  is oriented such that the viewer may be able to see the eyepiece  5000  in a relatively straightforward direction. The AR system to which the eyepiece  5000  belongs, which in some examples may be similar to the AR system as described above with reference to  FIG.  25 B , may perform one or more operations to present virtual content on one or more depth planes positioned within the viewer&#39;s FOV at one or more distances in front of the viewer&#39;s eye  210 . 
     The AR system may determine a perspective within render space from which the viewer is to view 3-D virtual contents of the render space, such as virtual objects, based on the position and orientation of the viewer&#39;s head. As described in further detail below with reference to  FIG.  29 A , in some embodiments, such an AR system may include one or more sensors and leverage data from these one or more sensors to determine the position and/or orientation of the viewer&#39;s head. The AR system may include such one or more sensors in addition to one or more eye-tracking components, such as one or more components of the eye-tracking sub-system  4070  described above with reference to  FIG.  25 B . With such data, the AR system may effectively map the position and orientation of the viewer&#39;s head within the real world to a particular location and a particular angular position within a 3D virtual environment, create a virtual camera that is positioned at the particular location within the 3D virtual environment and oriented at the particular angular position within the 3D virtual environment relative to at the particular location within the 3D virtual environment, and render virtual content for the viewer as it would be captured by the virtual camera. Further details discussing real world to virtual world mapping processes are provided in U.S. patent application Ser. No. 15/296,869, entitled “SELECTING VIRTUAL OBJECTS IN A THREE-DIMENSIONAL SPACE,” which is expressly incorporated herein by reference in its entirety for all purposes. 
     In some examples, the AR system may create or dynamically reposition and/or reorient one such head-tracked virtual camera for the viewer&#39;s left eye or eye socket, and another such head-tracked virtual camera for the viewer&#39;s right eye or eye socket, as the viewer&#39;s eyes and or eye sockets are physically separated from one another and thus consistently positioned at different locations. It follows that virtual content rendered from the perspective of a head-tracked virtual camera associated with the viewers left eye or eye socket may be presented to the viewer through an eyepiece on the left side of a wearable display device, such as that described above with reference to  FIGS.  25 A- 25 C , and that virtual content rendered from the perspective of a head-tracked virtual camera associated with the viewer&#39;s right eye or eye socket may be presented to the viewer through an eyepiece on the right side of the wearable display device. Although a head-tracked virtual camera may be created and/or dynamically repositioned for each eye or eye socket based on information regarding the current position and orientation of the viewer&#39;s head, the position and orientation of such a head-tracked virtual camera may neither depend upon the position nor the orientation of each eye of the viewer relative to the respective eye socket of the viewer or the viewer&#39;s head. Further details discussing the creation, adjustment, and use of virtual cameras in rendering processes are provided in U.S. patent application Ser. No. 15/274,823, entitled “METHODS AND SYSTEMS FOR DETECTING AND COMBINING STRUCTURAL FEATURES IN 3D RECONSTRUCTION,” which is expressly incorporated herein by reference in its entirety for all purposes. 
     The AR system of  FIG.  26 A  may create or dynamically reposition and/or reorient such a head-tracked virtual camera, render virtual content from the perspective of the head-tracked virtual camera (perspective  5010 ), and project light representing renderings of the virtual content through the eyepiece  5000  and onto the retina of the viewer&#39;s eye  210 . As shown in  FIG.  26 A , the head-tracked render perspective  5010  may provide an FOV spanning a region of ±θ 310  angular units diagonally, horizontally, and/or vertically. As described in further detail below, in some embodiments, the head-tracked render perspective  5010  may provide a relatively wide FOV. In such embodiments, the AR system may also create or dynamically reposition and/or reorient another virtual camera for each eye or eye socket different from and in addition to a head-tracked virtual camera. In the example of  FIG.  26 A , the AR system may render and present virtual content from the perspective of the head-tracked virtual camera  5010  along with virtual content from the perspective of another virtual camera in render space. 
     For instance, in such embodiments, the AR system of  FIG.  26 A  may create or dynamically reposition and/or reorient such a fovea-tracked virtual camera based on the current gaze of the viewer&#39;s eye  210 . As described in further detail below with reference to  FIG.  29 A , in some examples, such an AR system may include one or more eye-tracking components, such as one or more components of the eye-tracking sub-system  4070  described above with reference to  FIG.  25 B , to determine the viewer&#39;s current gaze, the current position and/or orientation of the viewer&#39;s eye  210  relative to the viewer&#39;s head, and the like. With such data, the AR system of  FIG.  26 A  may create or dynamically reposition and/or reorient such a fovea-tracked virtual camera, render virtual content from the perspective of the fovea-tracked virtual camera (perspective  5020 A), and project light representing virtual content as rendered from perspective  5020 A through the eyepiece  5000  and onto the fovea of the viewer&#39;s eye  210 . 
     As shown in  FIG.  26 A , the fovea-tracked render perspective  5020 A may provide for an FOV that is narrower than that of the head-tracked render perspective  5010 . In this way, the FOV of the fovea-tracked render perspective  5020 A can be seen as occupying a conical subspace of the FOV of the head-tracked render perspective  5010 . That is, the FOV of the fovea-tracked render perspective  5020 A may be a subfield of the FOV of the head-tracked render perspective  5010 . For instance, as shown in  FIG.  26 A , the fovea-tracked render perspective  320 A may provide an FOV spanning a region of ±θ 320A  angular units diagonally, horizontally, and/or vertically, such that the relationship between the FOV of the head-tracked render perspective  5010  and the fovea-tracked render perspective  5020 A is given by −θ 310 ≤−θ 320A ≤θ 320A ≤θ 310 . In some examples, the FOV of the head-tracked render perspective  5010  may be at least as wide as the viewer&#39;s field of regard, which in this example would be the total conical space within which the viewer&#39;s eye  210  can fixate when the viewer&#39;s head is held in a given position and orientation. As such, in these examples, the head-tracked virtual camera and the fovea-tracked virtual camera may be positioned at substantially the same location within render space or may be positioned at locations within render space that are a fixed distance away from one another, such that both virtual cameras may be linearly and/or angularly translated in unison within render space when the position and/or orientation of the viewer&#39;s head changes. For example, the head-tracked virtual camera may be positioned at a location in render space that corresponds to the center-of-rotation of the viewer&#39;s eye  210 , while the fovea-tracked virtual camera may be positioned at a location in render space that corresponds to a region of the viewer&#39;s eye  210  between the center-of-rotation and cornea. Indeed, the Euclidean distance between the two virtual cameras may remain substantially constant when translated in render space in much the same way that the Euclidean distance between two specific regions of the viewer&#39;s eye  210  or another rigid body may remain substantially constant at all times. 
     Although the spatial relationship between each virtual camera in such a pair of virtual cameras may remain substantially fixed within render space throughout use of the AR system in these examples, the orientation of the fovea-tracked virtual camera may, however, vary relative to the head-tracked virtual camera when the viewer rotates their eye  210 . In this way, the conical subspace of the FOV of the head-tracked virtual camera that is occupied by the FOV of the fovea-tracked virtual camera may dynamically change as the viewer rotates their eye  210 . 
     Furthermore, virtual objects and other content that fall within the fovea-tracked render perspective  5020 A may be rendered and presented by the AR system in relatively high resolution. More specifically, the resolution at which virtual content within the FOV of the fovea-tracked virtual camera is rendered and presented may be higher than the resolution at which virtual content within the FOV of the head-tracked virtual camera is rendered and presented. In this way, the highest-resolution subfield of a given light field that is outcoupled by the eyepiece  5000  and projected onto the retina of the viewer&#39;s eye  210  may be that which reaches the fovea of the viewer&#39;s eye  210 . 
       FIG.  3 B  illustrates an exemplary light field  5030 A that is outcoupled by the eyepiece  5000  and projected onto the retina of the viewer&#39;s eye  210  while the viewer&#39;s eye  210  is oriented in the first manner as depicted in  FIG.  26 A  and described above with reference thereto. The light field  5030 A may include various angular light components representative of virtual content as would be captured in render space by the abovementioned pair of virtual cameras. As described in further detail below with reference to  FIG.  26 A  and onward, light representative of virtual content as would be captured in render space by the head-tracked virtual camera and light representative of virtual content as would be captured in render space by the fovea-tracked virtual camera may be multiplexed by the AR system according to any of a variety of different multiplexing schemes. Employment of such multiplexing schemes may, at least in some instances, allow for the AR system to operate with greater efficiency and/or occupy less physical space. 
     Still referring to  FIG.  26 B , angular light components of the light field  5030 A that are representative of virtual content as would be captured in render space by the head-tracked virtual camera (e.g., virtual objects and other content that fall within the head-tracked render perspective  5010 ) may include those which are to be projected onto the retina of the viewer&#39;s eye  210  at angles ranging from −θ 310  to +θ 310  angular units relative to the viewer&#39;s eye  210 . Similarly, angular light components of the light field  5030 A that are representative of virtual content as would be captured in render space by the fovea-tracked virtual camera (e.g., virtual objects and other content that fall within the fovea-tracked render perspective  5020 A) may include those which are to be projected onto the retina of the viewer&#39;s eye  210  at angles ranging from −θ 320A  to +θ 320A  angular units relative to the viewer&#39;s eye  210 . The intervals between −θ 320A  and +θ 320A  angular units at which such angular light components associated with the fovea-tracked render perspective  5020 A occur within the light field  5030 A may be higher in regularity than the intervals between −θ 310  and +θ 310  angular units at which angular light components associated with the head-tracked render perspective  5010  occur within the light field  5030 A. In this way, the resolution at which virtual content associated with the fovea-tracked render perspective  5020 A may be rendered and presented to the viewer may be higher than the resolution at which virtual content associated with the head-tracked render perspective  5010  may be rendered and presented to the viewer. 
     In some embodiments, angular light components associated with the head-tracked render perspective  5010  that occur within the light field  5030 A may further include those which are to be projected onto the retina of the viewer&#39;s eye  210  at angles ranging from −θ 320A  to +θ 320A  angular units relative to the viewer&#39;s eye  210 . In such embodiments, the intervals between −θ 320A  and +θ 320A  angular units at which such angular light components associated with the head-tracked render perspective  5010  occur within the light field  5030 A may be lower in regularity than the intervals between −θ 320A  and +θ 320A  angular units at which angular light components associated with the fovea-tracked render perspective  5020 A occur within the light field  5030 A. In other embodiments, angular light components associated with the head-tracked render perspective  5010  that occur within the light field  5030 A may exclude those which are to be projected onto the retina of the viewer&#39;s eye  210  at angles ranging from −θ 320A  to +θ 320A  angular units relative to the viewer&#39;s eye  210 . As such, in these other embodiments, angular light components associated with the head-tracked render perspective  5010  that occur within the light field  5030 A may be those which are to be projected onto the retina of the viewer&#39;s eye  210  at angles between −θ 310  and −θ 320A  angular units or angles between θ 320A  and θ 310 . 
     In  FIG.  26 C , the viewer&#39;s eye  210  is oriented in a second manner with respect to the eyepiece  5000  different from the first manner in which the viewer&#39;s eye  210  is oriented with respect to the eyepiece  5000  in  FIGS.  26 A- 26 B . For purposes of example, the position and orientation of the viewer&#39;s head in  FIGS.  26 C- 26 D  may be treated as being the same as the position and orientation of the viewer&#39;s head as described above with reference to  FIGS.  26 A- 26 B . As such,  FIGS.  26 A- 26 B  and  FIGS.  26 C- 26 D  may represent the abovementioned viewer and AR system in first and second time-sequential stages, respectively. More specifically, in this example, the viewer&#39;s eye  210  has rotated off-center from the relatively straightforward orientation as depicted in  FIGS.  26 A- 26 B . 
     In transitioning from the first stage to the second stage, the AR system of  FIG.  26 C  may, for instance, function to maintain the head-tracked virtual camera at the same position and orientation as described above with reference to  FIGS.  26 A- 26 B , as the viewer&#39;s head pose (e.g., position and orientation) has not changed. As such, in the second stage depicted in  FIGS.  26 C- 26 D , the AR system may render virtual content from the perspective of the head-tracked virtual camera (i.e., head-tracked render perspective  5010 ) and project light representing renderings of the virtual content through the eyepiece  5000  and onto the retina of the viewer&#39;s eye  210 . While the head-tracked render perspective  5010  may remain static or relatively static throughout the first and second time-sequential stages of  FIGS.  26 A- 26 D , in transitioning from the first stage to the second stage, the AR system may function to adjust the orientation of a fovea-tracked virtual camera in render space based on the change in gaze of the viewer&#39;s eye  210  from the first stage to the second stage. That is, the AR system may replace or reorient the fovea-tracked virtual camera as employed in the first stage to provide the fovea-tracked render perspective  5020 A, such that the fovea-tracked virtual camera as employed in the second stage provides a fovea-tracked render perspective  5020 C different from the fovea-tracked render perspective  5020 A. It follows that, in the second stage, the AR system may also render virtual content from the perspective of the fovea-tracked virtual camera perspective  5020 C and project light representing renderings of the virtual content through the eyepiece  5000  and onto the fovea of the viewer&#39;s eye  201 . 
     In the example of  FIGS.  26 C- 26 D , the fovea-tracked render perspective  5020 C may occupy a different conical subspace of the head-tracked render perspective  5010  than that of the fovea-tracked render perspective  5020 A. For instance, as shown in  FIG.  26 C , the fovea-tracked render perspective  5020 C may provide an FOV displaced θ 320C  angular units from the FOV of the fovea-tracked render perspective  5020 A and spanning a region of ±θ 320A  angular units diagonally, horizontally, and/or vertically. That is, the fovea-tracked render perspective  5020 C may provide an FOV spanning a region of θ 320C ±θ 320A  angular units diagonally, horizontally, and/or vertically. 
       FIG.  26 D  illustrates an exemplary light field  5030 C that is outcoupled by the eyepiece  5000  and projected onto the retina of the viewer&#39;s eye  201  while the viewer&#39;s eye  201  is oriented in the second manner as depicted in  FIG.  26 C  and described above with reference thereto. The light field  5030 C may include various angular light components representative of virtual content as would be captured in render space from the head-tracked render perspective  5010  and the fovea-tracked render perspective  5020 C. Angular light components of the light field  5030 C that are representative of virtual content as would be captured in render space from the head-tracked render perspective  5010  may include those which are to be projected onto the retina of the viewer&#39;s eye  210  at angles ranging from −θ 310  to +θ 310  angular units relative to the viewer&#39;s eye  210 . However, in a departure from the first stage as described above with reference to  FIGS.  26 A- 26 B , the angular light components of light field  5030 C that are representative of virtual content as would be captured in render space by the fovea-tracked virtual camera (e.g., virtual objects and other content that fall within the fovea-tracked render perspective  5020 C) may include those which are to be projected onto the retina of the viewer&#39;s eye  210  at angles ranging from θ 320C −θ 320A  angular units to θ 320C +θ 320A  angular units relative to the viewer&#39;s eye  210 . 
     The intervals between θ 320C −θ 320A  angular units and θ 320C +θ 320A  angular units at which such angular light components associated with the fovea-tracked render perspective  320 C occur within the light field  5030 C may be higher than the intervals between −θ 310  and +θ 310  angular units at which angular light components associated with the head-tracked render perspective  5010  occur within the light field  5030 C. In this way, the resolution at which virtual content associated with the fovea-tracked render perspective  5020 C may be rendered and presented to the viewer may be higher than the resolution at which virtual content associated with the head-tracked render perspective  5010  may be rendered and presented to the viewer, which notably includes virtual content represented by angular light components that are to be projected onto the retina of the viewer&#39;s eye  210  at angles ranging from −θ 320A  to +θ 320A  angular units relative to the viewer&#39;s eye  210 . 
     In some embodiments, angular light components associated with the head-tracked render perspective  5010  that occur within the light field  5030 C may further include those which are to be projected onto the retina of the viewer&#39;s eye  210  at angles ranging from θ 320C −θ 320A  angular units and θ 320C +θ 320A  angular units relative to the viewer&#39;s eye  210 . In such embodiments, the intervals between −θ 320C −θ 320A  angular units and θ 320C +θ 320A  angular units at which such angular light components associated with the head-tracked render perspective  310  occur within the light field  5030 C may be lower in regularity than the intervals between θ 320C −θ 320A  angular units and θ 320C +θ 320A  angular units angular units at which angular light components associated with the fovea-tracked render perspective  5020 C occur within the light field  5030 C. In other embodiments, angular light components associated with the head-tracked render perspective  5010  that occur within the light field  5030 C may exclude those which are to be projected onto the retina of the viewer&#39;s eye  210  at angles ranging from θ 320C −θ 320A  angular units and θ 320C +θ 320A  angular units relative to the viewer&#39;s eye  210 . As such, in these other embodiments, angular light components associated with the head-tracked render perspective  5010  that occur within the light field  5030 C may be those which are to be projected onto the retina of the viewer&#39;s eye  210  at angles between −θ 310  and θ 320C −θ 320A  angular units and angular units or angles between θ 320C +θ 320A  angular and θ 310  angular units. 
       FIGS.  26 E- 26 F  illustrate schematically an exemplary configuration of images that can be presented to a user according to some embodiments. It should be noted that the grid squares in  FIGS.  26 E- 26 F  represent schematically image points that, much like fields  3002 ,  3004  and  3006  as described above with reference to  FIG.  24   , are defined in two-dimensional angular space. A low-resolution first image stream  5010 E having a wide FOV can be displayed at a static location. A low-resolution first image stream  5010 E having a wide FOV can represent one or more images of virtual content as would be captured by a first virtual camera having a static position and orientation in render space. For instance, the low-resolution first image stream  5010 E can represent one or more images of virtual content as would be captured by a head-tracked virtual camera such as the head-tracked virtual camera described above with reference to  FIGS.  26 A- 26 D . The first image stream  5010 E can encompass the user&#39;s vision to evoke an immersion experience to the user. 
     A high-resolution second image stream  5020 E having a relatively narrow FOV can be displayed within the boundaries of the first image stream  5010 E. In some examples, the second image stream  5020 E can represent one or more images of virtual content as would be captured by a second, different virtual camera having an orientation in render space that can be dynamically adjusted in real-time based on data obtained using eye-gaze tracking techniques to angular positions coinciding with the user&#39;s current fixation point. In these examples, the high-resolution second image stream  5020 E can represent one or more images of virtual content as would be captured by a fovea-tracked virtual camera such as the fovea-tracked virtual camera described above with reference to  FIGS.  26 A- 26 D . In other words, the perspective in render space from which one or more images of virtual content represented by the second image stream  5020 E is captured can be reoriented as the user&#39;s eye gaze changes, such that the perspective associated with the second image stream  5020 E is persistently aligned with the user&#39;s foveal vision. 
     For example, the second image stream  5020 E can encompass virtual content located within a first region of render space when the user&#39;s eye gaze is fixed at the first position as illustrated in  FIG.  26 E . As the user&#39;s eye gaze moves to a second position different from the first position, the perspective associated with the second image stream  5020 E can be adjusted such that the second image stream  5020 E can encompass virtual content located within a second region of render space, as illustrated in  FIG.  26 F . In some embodiments, the first image stream  5010 E has a wide FOV, but a low angular resolution as indicated by the coarse grid. The second image stream  5020 E has a narrow FOV, but a high angular resolution as indicated by the fine grid. 
       FIG.  26 G  illustrates schematically an exemplary configuration of images that can be presented to a user according to some other embodiments. Like  FIGS.  26 E- 26 F , the grid squares in  FIG.  26 G  represent schematically image points that are defined in two-dimensional angular space. Similar to the configuration illustrated in  FIGS.  26 E- 26 F , a low resolution first image stream  5010 G having a wide FOV encompasses virtual content as viewed from a head-tracked render perspective, while a high resolution second image stream  5020 G having a narrow FOV encompasses virtual content as viewed from a fovea-tracked render perspective that may be dynamically reoriented so as to coincide with the user&#39;s current fixation point. Here, the outer perimeter of the FOV associated with the first image stream  5010 G can form a rectangular boundary with rounded corners, and the outer perimeter of the FOV associated with the second image stream  5020 G can form a circular boundary. 
       FIG.  26 H  illustrates schematically an exemplary configuration of images that can be presented to a user according to yet some other embodiments. Like  FIGS.  26 E- 26 G , the grid squares in  FIG.  26 H  represent schematically image points that are defined in two-dimensional angular space. Here, both the outer perimeter of the FOV associated with the first image stream  5010 H and the outer perimeter of the FOV associated with the second image stream  5020 H can form circular boundaries. In some other embodiments, either the outer perimeter of the FOV associated with the first image stream  5010 H and the outer perimeter of the FOV associated with the second image stream  5020 H, or both, can form an elliptical boundary or other shapes. In some embodiments, an image source of the AR system of  FIG.  26 H  may include a scanning fiber that can be scanned in a predetermined pattern to provide light beams for the first image stream  5010 H and the second image stream  5020 H with desired boundary shapes. 
       FIG.  27    illustrates a field of view  3002  and a field of regard  3004  as shown in  FIG.  24   , overlaid upon one of the displays  4052  in the wearable display device  4050  as shown in  FIG.  25 A . According to some embodiments, the wide FOV and low resolution first image stream  5010 E illustrated in  FIGS.  26 E- 26 F  can be displayed across the entire area of the display  4052  (the relatively low resolution of the first image stream  5010 E is illustrated with a coarse grid), while the narrow FOV and high resolution second image stream  5020 E can be displayed at the user&#39;s current foveated region  3006  (the relatively high resolution of the second image stream  5020 E is illustrated with a fine grid). While in  FIG.  27    the first image stream  5010 E and the second image stream  5020 E are illustrated as displayed in the “plane” of the displays  4052 , in a see-through augmented reality (AR) display system the first image stream  5010 E and the second image stream  5020 E can also be presented to the user as light fields within certain angular fields of view. Such an AR display system can produce display planes that appear to be “floating” at some distance (e.g., 2 meters) in front of the user. The display plane can appear to be much larger than the glasses. This floating distanced display is used for overlaying information on the real world. 
       FIGS.  28 A- 28 B  illustrate some of the principles described in  FIGS.  26 A- 26 D  using exemplary virtual content that can be presented to a user according to some embodiments. As such,  FIGS.  28 A- 28 B  may represent a viewer and an AR system in first and second time-sequential stages, respectively. Furthermore, some or all of the components shown in  FIGS.  28 A- 28 B  may be the same as or at least similar to components as described above with reference to  FIGS.  26 A- 26 D . 
     The AR system of  FIGS.  28 A- 28 B  may create or dynamically reposition and/or reorient a head-tracked virtual camera similar to the head-tracked virtual camera described above with reference to  FIGS.  26 A- 26 D , render virtual content from the perspective of the head-tracked virtual camera, and project light representing renderings of the virtual content through the eyepiece  6000  and onto the retina of the viewer&#39;s eye  210 . The AR system of  FIGS.  28 A- 28 B  may also create or dynamically reposition and/or reorient a fovea-tracked virtual camera similar to the fovea-tracked virtual camera described above with reference to  FIGS.  26 A- 26 D , render virtual content from the perspective of the fovea-tracked virtual camera, and project light representing renderings of the virtual content through the eyepiece  400  and onto the fovea of the viewer&#39;s eye  210 . As shown in  FIGS.  28 A- 28 B , such virtual content may include 3-D virtual objects  6011 ,  6012 , and  6013 . In some examples, the AR system of  FIGS.  28 A- 28 B  may perform one or more of the operations described immediately above regarding the head-tracked render perspective and one or more of the operations described immediately above regarding the fovea-tracked render perspective simultaneously. In other examples, the AR system of  FIGS.  28 A- 28 B  may perform such operations in rapid succession. 
     In this example, the FOV of the head-tracked render perspective employed by the AR system in  FIGS.  28 A- 28 B  may be diagonally, horizontally, and/or vertically wide enough in angular space to encompass each of virtual objects  6011 ,  6012 , and  6013 . For purposes of example, the position and orientation of the viewer&#39;s head may be treated as being static throughout the first and second stages as depicted in  FIGS.  28 A and  28 B , respectively, such that the position and orientation of the head-tracked render perspective remain the same throughout the two stages. In order for the FOV of the head-tracked render perspective employed by the AR system to be large enough to encompass virtual objects  6011 - 6013 , it must at least span a region of α+ζ angular units diagonally, horizontally, and/or vertically. More specifically, in the example of  FIGS.  28 A- 28 B , it can be seen that virtual objects  6011 ,  6012 , and  6013  may span regions of α−β, γ+δ, and ζ−ε angular units, respectively. 
     In  FIG.  28 A , a viewer&#39;s eye  210  is oriented in a first manner with respect to an eyepiece  6000 , such that the viewer may be able to see the eyepiece  6000  in a relatively straightforward direction. The orientation of the viewer&#39;s eye  210  in  FIG.  28 A  may, for instance, be the same as or similar to the orientation of the viewer&#39;s eye  210  as described above with reference to  FIGS.  26 A- 26 B , and may be determined by the AR system using one or more of the sensing components and/or techniques described herein. As such, in the stage depicted in  FIG.  28 A , the AR system may employ head-tracked and fovea-tracked render perspectives at relative positions and orientations similar to those of the head-tracked and fovea-tracked render perspectives  5010  and  5020 A, respectively. In the particular example of  FIG.  28 A , the FOV of the fovea-tracked render perspective employed by the AR system may, for instance, encompass virtual object  6012 , but may not encompass either of virtual objects  6011  and  6013 . It follows that, in  FIG.  28 A , the AR system may render virtual object  6012  as it would be captured from the perspective of the fovea-tracked virtual camera in high definition, and may render virtual objects  6011  and  6013  as they would be captured from the perspective of the head-tracked virtual camera in lower definition. In addition, the AR system may project light representing such renderings of virtual objects  6011 ,  6012 , and  6013  through the eyepiece  6000  and onto the retina of the viewer&#39;s eye  210 . In some embodiments, the AR system may also render virtual object  6012  as it would be captured from the perspective of the head-tracked virtual camera in lower definition. 
       FIG.  28 A  also illustrates an exemplary light field  6030 A that is outcoupled by the eyepiece  6000  and projected onto the retina of the viewer&#39;s eye  210 . The light field  6030 A may include various angular light components representative of one or more of the abovementioned renderings of virtual objects  6011 ,  6012 , and  6013 . For example, angular light components of the light field  6030 A that are representative of the virtual object  6011  as it would be captured from the perspective of the head-tracked virtual camera may include those which are to be projected onto the retina of the viewer&#39;s eye  210  at angles ranging from −α to −β, angular units relative to the viewer&#39;s eye  210 , and angular light components of the light field  6030 A that are representative of the virtual object  6013  as it would be captured from the perspective of the head-tracked virtual camera may include those which are to be projected onto the retina of the viewer&#39;s eye  210  at angles ranging from ε to ζ angular units relative to the viewer&#39;s eye  210 . Similarly, angular light components of the light field  6030 A that are representative of the virtual object  6012  as it would be captured from the perspective of the fovea-tracked virtual camera may include those which are to be projected onto the fovea of the viewer&#39;s eye  210  at angles ranging from −γ to δ angular units relative to the viewer&#39;s eye  210 . As such, components of the light field  6030 A that are representative of virtual object  6012  (i.e., components to be projected at angles ranging from −γ to δ angular units relative to the viewer&#39;s eye  210 ) may be more densely distributed in angular space than components of the light field  6030 A that are representative of virtual object  6011  or  6013  (i.e., components to be projected at angles ranging from −α to −β or ε to ζ angular units relative to the viewer&#39;s eye  210 ). In this way, the resolution at which the virtual object  6012  may be rendered and presented to the viewer may be higher than the resolution at which virtual object  6011  or  6013  may be rendered and presented to the viewer. 
     In  FIG.  28 B , the viewer&#39;s eye  210  is oriented in a second manner with respect to the eyepiece  6000  different from the first manner in which the viewer&#39;s eye  210  is oriented with respect to the eyepiece  6000  in  FIG.  28 A . The orientation of the viewer&#39;s eye  210  in  FIG.  28 B  may, for instance, be the same as or similar to the orientation of the viewer&#39;s eye  210  as described above with reference to  FIGS.  26 C- 26 D , and may be determined by the AR system using one or more of the sensing components and/or techniques described herein. As such, in the stage depicted in  FIG.  28 B , the AR system may employ head-tracked and fovea-tracked render perspectives at relative positions and orientations similar to those of the head-tracked and fovea-tracked render perspectives  5010  and  5020 C, respectively. In the particular example of  FIG.  28 B , the FOV of the fovea-tracked render perspective employed by the AR system may, for instance, encompass virtual object  6013 , but may not encompass either of virtual objects  6011  and  6012 . It follows that, in  FIG.  28 B , the AR system may render virtual object  6013  as it would be captured from the perspective of the fovea-tracked virtual camera in high definition, and may render virtual objects  6011  and  6012  as they would be captured from the perspective of the head-tracked virtual camera in lower definition. In addition, the AR system may project light representing such renderings of virtual objects  6011 ,  6012 , and  6013  through the eyepiece  6000  and onto the retina of the viewer&#39;s eye  210 . In some embodiments, the AR system may also render virtual object  6013  as it would be captured from the perspective of the head-tracked virtual camera in lower definition. 
       FIG.  28 B  also illustrates an exemplary light field  6030 B that is outcoupled by the eyepiece  6000  and projected onto the retina of the viewer&#39;s eye  210 . The light field  6030 B may include various angular light components representative of one or more of the abovementioned renderings of virtual objects  6011 ,  6012 , and  6013 . For example, angular light components of the light field  6030 B that are representative of the virtual object  6011  as it would be captured from the perspective of the head-tracked virtual camera may include those which are to be projected onto the retina of the viewer&#39;s eye  210  at angles ranging from −α to −β angular units relative to the viewer&#39;s eye  210 , and angular light components of the light field  6030 B that are representative of the virtual object  6012  as it would be captured from the perspective of the head-tracked virtual camera may include those which are to be projected onto the retina of the viewer&#39;s eye  210  at angles ranging from −γ to δ angular units relative to the viewer&#39;s eye  210 . Similarly, angular light components of the light field  6030 B that are representative of the virtual object  6013  as it would be captured from the perspective of the fovea-tracked virtual camera may include those which are to be projected onto the fovea of the viewer&#39;s eye  210  at angles ranging from ε to ζ angular units relative to the viewer&#39;s eye  210 . As such, components of the light field  6030 B that are representative of virtual object  6013  (i.e., components to be projected at angles ranging from ε to ζ angular units relative to the viewer&#39;s eye  210 ) may be more densely distributed in angular space than components of the light field  6030 A that are representative of virtual object  6011  or  6012  (i.e., components to be projected at angles ranging from −α to −β or −γ to δ angular units relative to the viewer&#39;s eye  210 ). In this way, the resolution at which the virtual object  6013  may be rendered and presented to the viewer may be higher than the resolution at which virtual object  6011  or  6012  may be rendered and presented to the viewer. Indeed, from the stage of  FIG.  28 A  to the stage of  FIG.  28 B , the AR system described herein with reference thereto has effectively reoriented the perspective from which virtual content may be viewed in high resolution in accordance with the change in gaze of the viewer&#39;s eye  402  between stages. 
       FIGS.  28 C- 28 F  illustrate some of the principles described in  FIGS.  3 E- 3 F  using some exemplary images that can be presented to a user according to some embodiments. In some examples, the one or more of the images and/or image streams depicted in  FIGS.  28 C- 28 F  may represent two-dimensional images or portions thereof that are to be displayed at a particular depth plane, such as one or more of the depth planes described above with reference to  FIG.  25 B . That is, such images and/or image streams may represent 3-D virtual content having been projected onto at least one two-dimensional surface at a fixed distance away from the user. In such examples, it is to be understood that such images and/or image streams may be presented to the user as one or more light fields with certain angular fields of view similar to those described above with reference to  FIGS.  26 A- 26 D and  28 A- 28 B . 
     As depicted, a first image stream  6010  includes a tree. During a first period of time represented by  FIG.  28 C , eye-tracking sensors can determine a user&#39;s eye gaze (i.e., the foveal vision) is focused within a first region  6010 - 1  of the tree that includes the trunk of the tree. In response to determining the user&#39;s eye gaze is focused within the first region  6010 - 1 , a second image stream  6020  that includes high-resolution imagery associated with the first region  6010 - 1  of the first image stream  6010  can be positioned within the first region  410 - 1  concurrent with the display of the first image stream  6010 . The first image stream  410  can have a lower resolution than the second image stream  6020 , as illustrated in  FIG.  28 C . 
     During a second period of time represented by  FIG.  28 D , eye-tracking sensors can determine the user&#39;s eye gaze has moved to a second region  6010 - 2  of the tree that includes a branch of the tree as illustrated in  FIG.  28 D . Accordingly, the second image stream  420  can be shifted to the second region  6010 - 2  and have its content changed to correspond to the content within second region  6010 - 2  of the first image stream  6010 . Because the higher resolution second image stream  6020  overlays the portion of the first image stream  6010  within the user&#39;s foveal vision, the lower resolution of the portion of the first image stream  6010  surrounding the second image stream  6020  may not be perceived or noticed by the user. In this way, the user may perceive the combination of the first image stream  6010  and the second image stream  6020  as having both a wide FOV and high resolution. Such a display system can afford several advantages. For example, the display system can provide a superior user experience while maintaining a relatively small form factor and keeping computation resource requirement relatively low. The small form factor and low computation resource requirement can be due to the device only having to generate high-resolution imagery in a limited region of the display. 
     The second image stream  6020  can be overlaid on the first image stream  6010  simultaneously, or in rapid succession. As discussed above, in some embodiments, the subset of the content of the first image stream  6010  overlaid by the second image stream  6020  can be turned off or be presented with a lower intensity for more uniform brightness and for better resolution perception. It should also be noted that in some embodiments the second image stream associated with the second image stream  6020  can differ from the first image stream associated with the first image stream  6010  in other ways. For example, a color resolution of the second image stream could be higher than the color resolution of the first image stream. A refresh rate of the second image stream could also be higher than the refresh rate of the first image stream. 
       FIG.  28 E  illustrates an exemplary high-FOV low-resolution image frame (i.e., the first image stream), and  FIG.  28 F  illustrates an exemplary low-FOV high-resolution image frame (i.e., the second image stream), according to some embodiments. As illustrated in  FIG.  28 E , the region  6030  of the high-FOV low-resolution image frame, which would be overlaid by the low-FOV high-resolution image frame, can be devoid of virtual content. By omitting the portion of the high-FOV image that corresponds to region  6030 , any image blurring or smearing resulting from slight differences in the two images can be avoided. The content of the low-FOV high-resolution image frame (e.g., as illustrated in  FIG.  28 F ) can include a high resolution version of the content corresponding to region  6030 . 
       FIG.  29 A  shows a simplified block diagram of a display system  7000 A according to some embodiments. The display system  7000 A can include one or more sensors  7002  for detecting the position and movement of the head of a user, as well as the eye position and inter-ocular distance of the user. Such sensors may include image capture devices (such as cameras), microphones, inertial measurement units, accelerometers, compasses, GPS units, radio devices, gyroscopes, and the like. In an augmented reality system, the one or more sensors  7002  can be mounted on a head-worn frame. 
     For example, in some implementations, the one or more sensors  7002  of the display system  7000 A may be part of a head worn transducer system and include one or more inertial transducers to capture inertial measures indicative of movement of the head of the user. As such, in these implementations the one or more sensors  7002  may be used to sense, measure, or collect information about the head movements of the user. For instance, such may be used to detect measurement movements, speeds, acceleration, and/or positions of the head of the user. 
     In some embodiments, the one or more sensors  7002  can include one or more forward facing cameras, which may be used to capture information about the environment in which the user is located. The forward facing cameras may be used to capture information indicative of distance and orientation of the user with respect to that environment and specific objects in that environment. When head worn, the forward facing cameras is particularly suited to capture information indicative of distance and orientation of the head of the user with respect to the environment in which the user is located and specific objects in that environment. The forward facing cameras can be employed to detect head movement, speed, and acceleration of head movements. The forward facing cameras can also be employed to detect or infer a center of attention of the user, for example, based at least in part on an orientation of the head of the user. Orientation may be detected in any direction (e.g., up and down, left and right with respect to the reference frame of the user). 
     The one or more sensors  7002  can also include a pair of rearward facing cameras to track movement, blinking, and depth of focus of the eyes of the user. Such eye-tracking information can, for example, be discerned by projecting light at the users eyes, and detecting the return or reflection of at least some of that projected light. Further details discussing eye-tracking devices are provided in U.S. Provisional Patent Application No. 61/801,219, entitled “DISPLAY SYSTEM AND METHOD,” U.S. Provisional Patent Application No. 62/005,834, entitled “METHODS AND SYSTEM FOR CREATING FOCAL PLANES IN VIRTUAL AND AUGMENTED REALITY,” U.S. Provisional Patent Application No. 61/776,771, entitled “SYSTEM AND METHOD FOR AUGMENTED AND VIRTUAL REALITY,” and U.S. Provisional Patent Application No. 62/420,292, entitled “METHOD AND SYSTEM FOR EYE TRACKING USING SPECKLE PATTERNS,” which are expressly incorporated herein by reference. 
     The display system  7000 A can further include a user orientation determination module  7004  communicatively coupled to the one or more sensors  7002 . The user orientation determination module  7004  receives data from the one or more sensors  7002  and uses such data to determine the user&#39;s head pose, cornea positions, inter-pupillary distance, and the like. The user orientation determination module  7004  detects the instantaneous position of the head of the user and may predict the position of the head of the user based on position data received from the one or more sensors  7002 . The user orientation determination module  7004  also tracks the eyes of the user based on the tracking data received from the one or more sensors  7002 . 
     The display system  7000 A may further include a control subsystem that may take any of a large variety of forms. The control subsystem includes a number of controllers, for instance one or more microcontrollers, microprocessors or central processing units (CPUs), digital signal processors, graphics processing units (GPUs), other integrated circuit controllers, such as application specific integrated circuits (ASICs), programmable gate arrays (PGAs), for instance field PGAs (FPGAs), and/or programmable logic controllers (PLUs). 
     In the example depicted in  FIG.  29 A , the display system  7000 A includes a central processing unit (CPU)  7010 , a graphics processing unit (GPU)  7020 , and frame buffers  7042  and  7044 . Briefly, and as described in further detail below, the CPU  7010  controls overall operation, while the GPU  7020  renders frames (i.e., translating a three-dimensional scene into a two-dimensional image) from three-dimensional data stored in database  7030  and stores these frames in the frame buffers  7042  and  7044 . While not illustrated, one or more additional integrated circuits may control the reading into and/or reading out of frames from the frame buffers  7042  and  7044  and operation of one or more other components of the display system  7000 A, such as components of the image multiplexing subsystem  7060 , foveal-tracking beam-steering components  7080 , and the like. Reading into and/or out of the frame buffers  542  and  544  may employ dynamic addressing, for instance, where frames are over-rendered. The display system  7000 A further comprises a read only memory (ROM) and a random access memory (RAM). The display system  7000 A further comprises a three-dimensional data base  7030  from which the GPU  7020  can access three-dimensional data of one or more scenes for rendering frames. 
     The CPU  7010  can include a high-FOV low-resolution render perspective determination module  7012  and a low-FOV high-resolution render perspective determination module  7014 . In some embodiments, the user orientation determination module  7004  can be part of the CPU  7010 . 
     The high-FOV low-resolution render perspective determination module  7012  can include logic for mapping the data output by the user orientation determination module to the location in 3D space and the angle from which high-FOV low-resolution images are to be perceived. That is, the CPU  7010  determines the perspective of a virtual camera fixed with respect to the user&#39;s head at any given time based on the data received from the user orientation determination module  7004 . Within the context of the examples described above with reference to  FIGS.  26 A- 26 D and  28 A- 28 B , the high-FOV low-resolution render perspective determination module  7012  may serve to monitor head position and orientation, as indicated by the user orientation determination module  7004 , and control the position and orientation of at least the head-tracked virtual camera within render space accordingly. 
     The low-FOV high-resolution render perspective determination module  7014  can include logic for mapping the data output by the user orientation determination module (e.g., data indicating the user&#39;s gaze and foveal positioning) to the location in 3D space and the angle from which low-FOV high-resolution images are to be perceived. That is, the CPU  7010  determines the perspective of a virtual camera fixed with respect to the user&#39;s fovea at any given time based on the data received from the user orientation determination module  7004 . Within the context of the examples described above with reference to  FIGS.  26 A- 26 D and  28 A- 28 B , the low-FOV high-resolution render perspective determination module  7014  may serve to monitor eye gaze, as indicated by the user orientation determination module  7004 , and control the position and orientation of at least the fovea-tracked virtual camera within render space accordingly. 
     The display system  7000 A can further include a graphics processing unit (GPU)  7020  and a database  7030 . The database  7030  can store 3D virtual content. The GPU  7020  can access the 3D virtual content stored in the database  7030  for rendering frames. The GPU  7020  can render frames of virtual content in low FOV and high resolution from the perspective of the virtual camera fixed with respect to the user&#39;s fovea (e.g., fovea-tracked render perspective), as determined and provided as output by the CPU  7010 . The GPU  7020  can also render frames of virtual content in high FOV and low resolution from the perspective of the virtual camera fixed with respect to the user&#39;s head (e.g., head-tracked/non-foveated perspective), as determined and provided as output by the CPU  7010 . Further details discussing the creation, adjustment, and use of virtual cameras in rendering processes are provided in U.S. patent application Ser. No. 15/274,823, entitled “METHODS AND SYSTEMS FOR DETECTING AND COMBINING STRUCTURAL FEATURES IN 3D RECONSTRUCTION,” which is expressly incorporated herein by reference in its entirety for all purposes. 
     The high-FOV low-resolution rendered frames of virtual content can be stored in a high-FOV low-resolution rendered frame buffer  7042 . Similarly, the low-FOV high-resolution rendered frames of virtual content can be stored in a low-FOV high-resolution rendered frame buffer  7044 . In some embodiments, the high-FOV low-resolution rendered frame buffer  7042  and the low-FOV high-resolution rendered frame buffer  7044  can be part of the GPU  7020 . 
     The display system  7000 A can further include an image multiplexing subsystem  7060  and an image multiplexing subsystem controller  7050  communicatively coupled to the image multiplexing subsystem  7060 . The image multiplexing subsystem  7060  can include an image source  7062  and multiplexing components  7064  for multiplexing high-FOV low-resolution image frames and low-FOV high-resolution image frames, substantially as described in further detail below with reference to  FIGS.  30 A- 30 B . The image source  7062  can include, for example, a light source in combination with fiber scanning components, liquid crystal on silicon (LCoS), MEMs scanning mirror, and the like. The multiplexing components  7064  can include optical elements, such as polarization rotators, switchable optics, liquid crystal arrays, varifocal lenses, and the like. The multiplexing components  7064  can be internal or external to the image source  7062 . 
     The image multiplexing subsystem controller  7050  is communicatively coupled to the image multiplexing subsystem  7060 , the high-FOV low-resolution rendered frame buffer  7042 , and the low-FOV high-resolution rendered frame buffer  7044 . The control circuitry can send control signals to the image source  562 , so that appropriate image content is presented from each render perspective, as discussed above. The image multiplexing subsystem controller  7050  can also control the multiplexing components  7064  in conjunction with the image source  7062  in a manner so as to yield a multiplexed image stream. 
     The display system  7000 A can further include foveal-tracking beam-steering components  7080  and a foveal-tracking controller  7070  communicatively and/or operatively coupled to foveal-tracking beam-steering components  7080 . The foveal-tracking controller  7070  can receive output data from the CPU  7010  regarding the position of the user&#39;s fovea (e.g., as determined by the low-FOV high-resolution render perspective determination module  7014  and/or the user orientation determination module  7004 ), and use such data to control the position of the foveal-tracking beam-steering components  7080 . The foveal-tracking beam-steering components  7080  can serve to dynamically steer or otherwise direct low-FOV high-resolution portions of the multiplexed image stream (produced by the image source  7062  and the multiplexing components  7064 ) toward the user&#39;s fovea. Such low-FOV high-resolution portions of the image stream may, for instance, represent virtual content as would be captured from the perspective of a fovea-tracked virtual camera. 
     The display system  7000 A can also include a storage medium for storing computer-readable instructions, databases, and other information usable by the CPU  7010 , GPU  7020 , and/or one or more other modules or controllers of the display system  7000 A. The display system  7000 A can further include input-output (I/O) interfaces, such as buttons, that a user may use for interaction with the display system. The display system  7000 A can also include a wireless antenna for wireless communication with another part of the display system  7000 A, or with the Internet. 
       FIG.  29 B  illustrates schematically a cross-sectional view of an AR system  7000 B according to some embodiments. The AR system  7000 B can incorporate at least some of the components of the display system  7000 A as described above with reference to  FIG.  29 A , and can be fitted into one of the displays  4052  in the wearable display device  4050  as shown in  FIG.  25 A  according to some embodiments. For instance, the AR system  7000 B can include an image multiplexing subsystem  560 , which can include an image source  7062  and one or more multiplexing components. In addition, the AR system  7000 B can also include foveal-tracking beam-steering components  7080 , which in this example may an electromechanical optical device, such as a MEMs scanning mirror. Much like the display system  7000 A, the image multiplexing subsystem  7060  may be communicatively and/or operatively coupled to an image multiplexing subsystem controller, and the foveal-tracking beam-steering components  7080  may be communicatively and/or operatively coupled to a foveal-tracking controller. The AR system  7000 B can further include one or more incoupling gratings (ICGs)  7007 , and one or more eyepieces  7008 . Each incoupling grating  7007  can be configured to couple the first light beam and the second light beam into a respective eyepiece  7008 . Each eyepiece  7008  can include outcoupling gratings for outcoupling the first light beam and the second light beam into a user&#39;s eye. The incoupling gratings  7007  and the eyepieces  7008  may be referred herein as a “viewing assembly.” It will be appreciated that the various incoupling gratings (ICG&#39;s) disclosed herein may correspond to the in-coupling optical elements  700 ,  710 ,  720  of  FIGS.  9 A- 9 C . 
       FIGS.  30 A- 30 B  illustrate schematically a display system  8000  for projecting images to an eye of a user according to some embodiments. The display system  8000  includes an image source  8010 . The image source  8010  can be configured to project a first light beam  8052  associated with a first image stream, as shown in  FIG.  30 A , and project a second light beam  8054  associated with a second image stream, as shown in  FIG.  30 B . It should be noted that, the first light beam  8052  and the second light beam  8054  are depicted in  FIGS.  30 A- 30 B  as schematic light rays, which are not intended to represent accurate ray-traced rays. The first light beam  8052  can be angularly magnified to cover a wider FOV, resulting in a lower angular resolution image stream. The second light beam  8054  can have a narrower FOV with a higher angular resolution, as discussed above with reference to  FIGS.  26 A- 26 F and  28 A- 28 D . 
     The image source  8010  may include a liquid crystal on silicon (LCoS or LCOS) display (can also be referred to as a spatial light modulator), a scanning fiber, or a scanning mirror according to various embodiments. For example, the image source  8010  may include a scanning device that scans an optical fiber in a predetermined pattern in response to control signals. The predetermined pattern can correspond to certain desired image shape, such as rectangle or circular shapes. 
     According to some embodiments, the first light beam  8052  associated with the first image stream and the second light beam  8054  associated with the second image stream can be multiplexed and output by the image source  8010  as composite light beams. For example, polarization-division multiplexing, time-division multiplexing, wavelength-division multiplexing, and the like, can be used for multiplexing the light beams associated with the first image stream and the light beams associated with the second image stream. 
     In embodiments where polarization-division multiplexing is used, the first light beam  8052  can be in a first polarization state, and the second light beam  8054  can be in a second polarization state different from the first polarization state. For example, the first polarization state can be a linear polarization oriented in a first direction, and the second polarization state can be a linear polarization oriented in a second direction orthogonal to the first direction. In some other embodiments, the first polarization state can be a left-handed circular polarization, and the second polarization state can be a right-handed circular polarization, or vice versa. The first light beam  8052  and the second light beam  8054  can be projected by the image source  8010  simultaneously or sequentially. 
     The display system  8000  can further include a polarization beam splitter (PBS)  8030  configured to de-multiplex the first light beam  8052  from the second light beam  8054  according to some embodiments. The polarization beam splitter  8030  can be configured to reflect the first light beam  8052  along a first optical path toward a viewing assembly as illustrated in  FIG.  30 A , and to transmit the second light beam  8054  along a second optical path as illustrated in  FIG.  30 B . 
     Alternatives to polarization beam splitter  8030  may also be used for de-multiplexing light beams. As an example, the beam splitters described herein, including but not limited to polarization beam splitter  8030  of  FIGS.  30 A and  30 B , may be replaced or implemented with a switchable reflector, such as a liquid crystal switchable reflector. In embodiments with such a switchable reflector, all other aspects disclosed herein apply and may be similar, except that the polarization beam splitter is replaced by the switchable reflector. As an example, a switchable reflector, such as switchable reflector  50042  of  FIG.  53 A , may switch between a reflective state and a transparent state in response to control signals. By coordinating the switching of the switchable reflector, the switchable reflector may operate to de-multiplex light beams. As an example, the switchable reflector may be made reflective at times when a first light beam is incident on the switchable reflector and may be made transparent at times when a second light beam is incident on the switchable reflector, thus permitting de-multiplexing of the first and second light beams. In some embodiments, the switchable reflector may be positioned at an angle (e.g., a 45° angle) relative to the light beams  8052 ,  8054 . As a result, in a transmissive state, one of the light beams  8052 ,  8054  is transmitted through the switchable reflector; and in a reflective state, the other one of the light beams  8054 ,  8052  is reflected such that it travels in a different direction away from the switchable reflector than the light beam that was transmitted through the reflector. 
     Referring to  FIG.  30 B , the display system  8000  can further include a scanning mirror  8060  positioned downstream from the polarization beam splitter  8030  along the second optical path. The scanning mirror  8060  is configured to reflect the second light beam  8054  toward the viewing assembly to be projected to the user&#39;s eye. According to some embodiments, the scanning mirror  8060  can be controlled based on the fixation position of the user&#39;s eye for dynamically projecting the second image stream. For example, the scanning mirror  8060  can be in electrical communication via control circuitry with an eye-gaze tracker that tracks the user&#39;s eye movement. The control circuitry can send a control signal to tilt and/or translate the scanning mirror  8060  based on the user&#39;s current fixation point, such that the second light beam  8054  project the second image stream to a region determined to cover the user&#39;s foveal vision. In some embodiments, the scanning mirror  8060  can be a microelectromechanical systems (MEMS) scanner with two degrees of freedom (i.e., capable of being scanned in two independent angles). 
     In some other embodiments, instead of using a scanning mirror  8060 , the display system  8000  can use a fixed mirror. Controlling the position of the second image stream can be achieved by transversely displacing a third optical lens  8046  (see the description of the third optical lens  8046  below). For example, the third optical lens  8046  can be displaced up and down as indicated by the arrow, as well as in and out of the page, to shift the position of the second image stream in two dimensions. 
     In some embodiments, the display system  8000  can further include a polarization rotator  8022  positioned between the polarization beam splitter  8030  and the scanning mirror  8060 . The polarization rotator  8022  can be configured to rotate the polarization of the second light beam  8054 , so that the second light beam can have approximately the same polarization as that of the first light beam  8052  as they enter the viewing assembly. The polarization rotator  8022  can include, for example, a half-wave plate. 
     In some embodiments, the display system  8000  can further include a first relay lens assembly for the first optical path, and a second relay lens assembly for the second optical path. The first relay lens assembly can include a first optical lens  8042  disposed between the image source  8010  and the polarization beam splitter  8030 , and a second optical lens  8044  disposed downstream from the polarization beam splitter  8030  along the first optical path. The second relay lens assembly can include the first optical lens  8042 , and a third optical lens  8046  disposed downstream from the polarization beam splitter  8030  along the second optical path. 
       FIG.  30 C  illustrates schematically a cross-sectional view of an augmented reality (AR) system according to some embodiments. The AR system can be fitted into one of the displays  4052  in the wearable display device  4050  as shown in  FIG.  25 A  according to some embodiments. The AR system can include a light projector  8000  for projecting a first light beam associated with a first image stream and a second light beam associated with a second image stream. The light projector  8000  can be similar to the display system illustrated in  FIGS.  30 A- 30 B . The AR system can further include one or more incoupling gratings (ICGs)  8070 , and one or more eyepieces  8080 . Each incoupling grating  8070  can be configured to couple the first light beam and the second light beam into a respective eyepiece  8080 . Each eyepiece  8080  can include outcoupling gratings for outcoupling the first light beam and the second light beam into a user&#39;s eye. The incoupling gratings  8070  and the eyepieces  8080  may be referred herein as a “viewing assembly.” 
       FIG.  30 D  shows a simplified block diagram of a display system according to some embodiments. The display system can include an image source  8010 , and a scanning mirror  8060 , substantially as described above with reference to  FIGS.  30 A- 30 C . The display system can also include an eye-gaze tracker  8071  and control circuitry  8081 . The control circuitry  8081  can be communicatively coupled to the image source  8010 , the scanning mirror  8060 , and the eye-gaze tracker  8071 . The control circuitry  8081  can send control signals to tilt and/or translate the scanning mirror  8060  based on the user&#39;s current fixation point as determined by the eye-gaze tracker  8071 , so that the second light beam  8054  project the second image stream to a region determined to cover the user&#39;s foveal vision. The control circuitry  8081  can also send control signals to the image source  8010 , so that appropriate image content is presented in the first image stream and the second image stream, as discussed above. The display system can also include a central processing unit (CPU)  8096 , a graphics processing unit (GPU)  8098 , a storage medium  8090  for storing computer-readable instructions, databases, and other information usable by the control circuitry  8081 , the CPU  8096 , and the GPU  8098 . The display system can further include input-output (I/O) interfaces  8092 , such as buttons, that a user may use for interaction with the display system. The display system can also include a wireless antenna  8094  for wireless communication with another part of the display system, or with the Internet. The display system can also include other sensors, such as cameras. 
       FIG.  31 A  illustrates schematically the operating principles of the first relay lens assembly according to some embodiments. The first relay lens assembly can operate in a manner similar to a telescope. A collimated first light beam  8052  associated with the first image stream is incident on the first optical lens  8042  at an angle of incidence θ A , and is focused by the first optical lens  8042  to a real image point P 0  located approximately at a focal plane of the first optical lens  8042 . The real image point P 0  is also located approximately at a focal plane of the second optical lens  8044 . Thus, the first light beam  8052  emitted from the real image point P 0  is collimated by the second optical lens  80044  and exits from the second optical lens  8044  at an angle of transmittance θ B . 
     The ratio of θ B  and θ A  can give rise to a first angular magnification M 1 , where 
               M   1     =         θ   B       θ   A       .           
The magnitude of first angular magnification M 1  can be approximately equal to the ratio of the focal length of the first optical lens  8042  f A  and the focal length of the second optical lens  8044  f B . Thus
 
               M   1     ≈         f   A       f   B       .           
In some embodiments, the first relay lens assembly is configured such that the magnitude of the first angular magnification M 1  is greater than one, e.g., by having f A &gt;f B . Therefore, referring again to  FIG.  30 A , the collimated first light beam  8052  associated with the first image stream can be angularly magnified by the first relay lens assembly as it exits the second optical lens  8044 , which is then projected to a viewing assembly for presenting the first image stream with a first field of view FOV 1  that is relatively wide.
 
       FIG.  31 B  illustrates schematically the operating principles of the second relay lens assembly according to some embodiments. The second relay lens assembly can also operate in a similar manner as a telescope. A collimated second light beam  8054  associated with the second image stream is incident on the first optical lens  8042  at an angle of incidence θ A , and is focused by the first optical lens  8042  to a real image point P 0  located approximately at a focal plane of the first optical lens  8042 . The real image point P 0  is also located approximately at a focal plane of the third optical lens  8046 . Thus, the second light beam  8054  emitted from the real image point P 0  is collimated by the third optical lens  8046  and exits from the third optical lens  8046  at an angle of transmittance θ C . 
     The ratio of θ C  and θ A  can give rise to a second angular magnification M 2 , where 
               M   2     =         θ   C       θ   A       .           
The magnitude of second angular magnification M 2  can be approximately equal to the ratio of the focal length of the first optical lens  8042  f A  and the focal length of the third optical lens  644  f C . Thus
 
               M   2     ≈         f   A       f     C   ⁢                 .           
The second lens assembly can be configured such that the magnitude of the second angular magnification M 2  is less than the first angular magnification M 1 . In some embodiments, the second angular magnification M 2  can have a value of unity (i.e., no magnification) or less than one (i.e., demagnification), e.g., by having f A ≤f C . Therefore, referring again to  FIG.  30 B , the collimated second light beam  8054  associated with the second image stream can have a second field of view FOV 2  as it exits the third optical lens  8046 , the second field of view FOV 2  being less than the first field of view FOV 1  of the first light beam  8052  associated with the first image stream.
 
     Note in  FIG.  31 A  that the collimated first light beam  8052  has an initial beam width w A  as it is incident on the first optical lens  8042 , and a final beam width w B  as it exits the second optical lens  8044 , where the final beam width w B  is narrower than the initial beam width w A . Note also in  FIG.  31 B  that the collimated second light beam  8054  has an initial beam width w A  as it is incident on the first optical lens  8042 , and a final beam width w C  as it exits the third optical lens  8046 , where the final beam width w C  is about the same as the initial beam width w A . In other words, the final beam width w C  of the second light beam  8054  is wider than the final beam width w B  of the first light beam  8052 . A wider beam width would result in a sharper angular resolution perceived by the eye. This can be explained by Gaussian beam physics, where a collimated beam with a wider beam waist has lower angular divergence over propagation to infinity. Therefore, increasing the FOV can reduce the beam width, and hence can reduce the angular resolution, which is consistent with the Lagrange invariant. 
     In some embodiments, the first angular magnification M 1  can have a magnitude of about 3, and the second angular magnification M 2  can have a magnitude of about unity. Referring to  FIGS.  30 A- 30 B , assume that the collimated first light beam  8052  associated with the first image stream and the collimated second light beam  8054  associated with the second image stream have the same initial FOV of about 20 degrees as projected by the image source  8010 . The collimated first light beam  8052  exiting the second optical lens  644  can have a first field of view FOV 1  of about 60 degrees, whereas the collimated second light beam  654  exiting the third optical lens  8046  can have a second field of view FOV 2  of about 20 degrees. In some embodiments, the first FOV can range from about 30 degrees to about 90 degrees; and the second FOV can range from about 10 degrees to about 30 degrees. 
     As illustrated in  FIGS.  28 C- 28 D , the second image stream  6020  can be a high resolution version of a portion of the first image stream  6010  and is overlaid on and properly aligned with respect to the wide FOV and low resolution first image stream  6010 . The content of the second image stream  6020  changes as the second image stream shifts relative to the first image stream  6010 , so that the content of the second image stream  6020  corresponds to the portion of the first image stream  6010  overlaid by the second image stream  6020 . Because the second image stream  6020  persistently covers the user&#39;s foveal vision, the user can perceive the combination of the first image stream  6010  and the second image stream  6020  as a composite image stream that has both a wide FOV and a high resolution. 
       FIGS.  31 C- 31 D  illustrate schematically a display system  10000  according to some other embodiments. The display system  10000  includes an image source  9010  and a beam splitter  9030 . The image source  9010  can provide a first light beam  8052  associated with a first image stream and a second light beam  8054  associated with a second image stream. The first light beam  8052  and the second light beam  8054  can be time-division multiplexed, polarization-division multiplexed, wavelength-division multiplexed, or the like. The beam splitter  9030  can serve as a de-multiplexer to separate the first light beam  8052  and the second light beam  8054  toward a first optical path and a second optical path, as depicted in  FIGS.  31 C and  31 D , respectively. 
     The display system  10000  can also include a first optical lens  9042  and a second optical lens  9044  disposed downstream from the beam splitter  9030  along the first optical path. The combination of the first optical lens  9042  and the second optical lens  9044  can serve as a first relay lens assembly for the first light beam  8052 . In some embodiments, the first relay lens assembly can provide an angular magnification for the first light beam  8052  that is greater than one, as described above in relation to  FIG.  31 A . 
     The display system  10000  can also include a third optical lens  9045  and a fourth optical lens  9046  disposed downstream from the beam splitter  9030  along the second optical path. The combination of the third optical lens  9045  and the fourth optical lens  9046  can serve as a second relay lens assembly for the second light beam  8054 . In some embodiments, the second relay lens assembly can provide an angular magnification for the second light beam  8054  that is substantially unity or less than one, as described above in relation to  FIG.  31 B . 
     The display system  10000  can also include a scanning mirror  9060  positioned downstream from the second relay lens assembly along the second optical path. The scanning mirror  9060  is configured to reflect the second light beam  8054  toward a viewing assembly to be projected to the user&#39;s eye. According to some embodiments, the scanning mirror  9060  can be controlled based on the fixation position of the user&#39;s eye for dynamically projecting the second image stream. 
     The display system  10000  can also include a fifth optical lens  9047  and a sixth optical lens  9048  disposed downstream from scanning mirror  9060  along the second optical path. The combination of the fifth optical lens  9047  and the sixth optical lens  9048  can serve as a third relay lens assembly for the second light beam  8054 . In some embodiments, the third relay lens assembly can provide an angular magnification for the second light beam  8054  that is substantially unity or less than one, as described above in relation to  FIG.  31 B . 
     In some embodiments, the display system  10000  can also include a polarizer  9080  and a switching polarization rotator  9090 . The image source  9010  can provide an unpolarized first light beam  8052  and an unpolarized second light beam  8054 , which are time-division multiplexed. The first light beam  652  and the second light beam  654  may become polarized after passing through the polarizer  9080 . The switching polarization rotator  9090  can be operated in synchronization with the time-division multiplexing of the first light beam  8052  and the second light beam  8054 . For example, the switching polarization rotator  9090  can be operated such that the polarization of the first light beam  8052  is unchanged after passing through the switching rotator  9090 , whereas the polarization of the second light beam  8054  is rotated by 90 degrees after passing through the switching polarization rotator  9090 , or vice versa. Therefore, the first light beam  8052  can be reflected by the polarization beam splitter  9030  along the first optical path as illustrated in  FIG.  31 C , and the second light beam  8054  can be transmitted by the polarization beam splitter  9030  along the second optical path, as illustrated in  FIG.  31 D . 
       FIGS.  32 A- 32 C  illustrate schematically a display system  10000  according to some other embodiments. In some examples, one or more components of display system  10000  may be the same as or similar to one or more components of the display system as described above with reference to  FIGS.  31 C- 31 D . The display system  10000  includes an image source  10010 , a beam splitter  10030 , a first optical lens  10042 , a second optical lens  10044 , a third optical lens  10045 , a fourth optical lens  10046 , a fifth optical lens  10047 , a sixth optical lens  10048 , a scanning mirror  10060 , a polarizer  10080 , a switching polarization rotator  10090  that, in some examples, may be the same as or similar to elements  9010 ,  9030 ,  9042 ,  9044 ,  9045 ,  9046 ,  9047 ,  9048 ,  9060 ,  9080 , and  9090 , respectively, of the display system as described above with reference to  FIGS.  31 C- 31 D . 
     More specifically,  FIGS.  32 A- 32 C  illustrate a display system  10000  in each of three different stages. In each of the three stages, the image source  10010  can output a range of angular light field components representative of virtual content as would be captured from the perspective of a head-tracked virtual camera and a range of angular light field components representative of virtual content as would be captured from the perspective of a fovea-tracked virtual camera. The two sets of angular light field components may, for instance, be time-division multiplexed, polarization-division multiplexed, wavelength-division multiplexed, or the like. As such, the angular light field components associated with the head-tracked virtual camera can be diverted upward by the polarization beam splitter  10030  along a first optical path through the first and second optical lenses  10042  and  10044 , and the angular light field components associated with the fovea-tracked virtual camera can pass through the polarization beam splitter  10030  along a second optical path through third and fourth optical lenses  10045  and  10046  toward the scanning mirror  10060  and reflected upward through fifth and sixth optical lenses  10047  and  10048 . 
     The virtual content represented by the angular light field components associated with the head-tracked virtual camera may be rendered upstream from the image source  10010  at a relatively low resolution, while the virtual content represented by the angular light field components associated with the fovea-tracked virtual camera may be rendered upstream from the image source  10010  at a relatively high resolution. And, as shown in  FIGS.  32 A- 32 C , the display system  10000  may be configured to output the angular light field components associated with the head-tracked render perspective and the angular light field components associated with the fovea-tracked render perspective as high FOV and low FOV light fields, respectively. In each of  FIGS.  32 A- 32 C , the light field components that propagate along the first optical path are output by the display system  10000  as a relatively wide cone of light  10052 . 
     In the stage depicted in  FIG.  32 A , the scanning mirror  10060  is in a first position. As such, it can be seen that the light field components that pass through the polarization beam splitter  10030  and propagate along the second optical path are output by the display system  10000  as a relatively narrow cone of light  10054 A spanning a substantially central region of angular space. Within the context of the examples described above with reference to  FIGS.  28 A- 28 B , the display system  10000  could, for instance, place the scanning mirror  10060  in the first position shown in  FIG.  32 A  when the user&#39;s eye is oriented in a manner similar to that of the viewer&#39;s eye  210  in  FIG.  28 A . In this way, the light components  10054 A may represent virtual content in a relatively centralized region of render space, such as virtual object  6012 . Further to the examples of  FIGS.  28 A- 28 B , the relatively wide cone of light  10052  may, for instance, include virtual content in off-centered regions of render space, such as virtual objects  6011  and  6013 . In some examples, the relatively wide cone of light  10052  may further include light components that represent the same virtual content as is represented by the light components  10054 A, but in lower resolution. 
     In the stage depicted in  FIG.  32 B , the scanning mirror  10060  is in a second position different from the first position. As such, it can be seen that the light field components that pass through the polarization beam splitter  10030  and propagate along the second optical path are output by the display system  10000  as a relatively narrow cone of light  10054 B spanning one substantially off-centered region of angular space. Within the context of the examples described above with reference to  FIGS.  28 A- 28 B , the display system  10000  could, for instance, place the scanning mirror  10060  in the second position shown in  FIG.  32 B  when the user&#39;s eye is oriented in a manner similar to that of the viewer&#39;s eye  210  while the viewer is looking at virtual object  6011 . In this way, the light components  10054 B may represent virtual content in one relatively off-centered region of render space, such as virtual object  6011 . Further to the examples of  FIGS.  28 A- 28 B , the relatively wide cone of light  10052  may, for instance, include virtual content in the other off-centered region of render space, such as virtual object  6013 , as well as virtual content in the centralized region of render space, such as virtual object  6012 . In some examples, the relatively wide cone of light  10052  may further include light components that represent the same virtual content as is represented by the light components  10054 B, but in lower resolution. 
     In the stage depicted in  FIG.  32 C , the scanning mirror  10060  is in a third position different from the first and second positions. As such, it can be seen that the light field components that pass through the polarization beam splitter  10030  and propagate along the second optical path are output by the display system  10000  as a relatively narrow cone of light  10054 C spanning another, different substantially off-centered region of angular space. Within the context of the examples described above with reference to  FIGS.  28 A- 28 B , the display system  10000  could, for instance, place the scanning mirror  10060  in the second position shown in  FIG.  32 C  when the user&#39;s eye is oriented in a manner similar to that of the viewer&#39;s eye  210  in  FIG.  28 B . In this way, the light components  10054 C may represent virtual content in the other relatively off-centered region of render space, such as virtual object  6013 . Further to the examples of  FIGS.  28 A- 28 B , the relatively wide cone of light  10052  may, for instance, include virtual content in the off-centered region of render space described above with reference to  FIG.  32 B , such as virtual object  6011 , as well as virtual content in the centralized region of render space, such as virtual object  6012 . In some examples, the relatively wide cone of light  10052  may further include light components that represent the same virtual content as is represented by the light components  10054 C, but in lower resolution. 
       FIGS.  33 A- 33 B  illustrate schematically a display system  11000  for presenting a first image stream and second image stream, where time-division multiplexing is used for multiplexing the first light beam  8052  associated with the first image stream and the second light beam  8054  associated with the second image stream, according to some embodiments. The display system  11000  is similar to the display system  8000 . The image source  11010  can be configured to provide time-division multiplexed first light beam  8052  and second light beam  8054 . The first light beam  8052  and the second light beam  8054  can be in the same polarization state as output from the image source  8010 . It should be noted that the first light beam  8052  and the second light beam  8054  are depicted in  FIGS.  33 A- 33 B  as schematic light rays, which are not intended to represent accurate ray-traced rays. 
     The display system  11000  can further include a switching polarization rotator  11020 , whose operation can be synchronized with the time-division multiplexing of the first light beam  8052  and the second light beam  8054 . For example, the switching polarization rotator  11020  can be operated such that the polarization of the first light beam  8052  is unchanged after passing through the switching rotator  11020 , whereas the polarization of the second light beam  8054  is rotated by 90 degrees after passing through the switching polarization rotator  11020 , or vice versa. Therefore, the first light beam  8052  can be reflected by the polarization beam splitter  8030  along the first optical path as illustrated in  FIG.  33 A , and the second light beam  8054  can be transmitted by the polarization beam splitter  8030  along the second optical path, as illustrated in  FIG.  33 B . 
     In some other embodiments, the switching polarization rotator  11020  can be part of the image source  11010 . In such cases, the first light beam  8052  and second light beam  8054  would be emitted sequentially and the first light beam  8052  projected from the image source  8010  would be polarized in a first direction, and the second light beam  8054  projected from the image source  8010  would be polarized in a second direction. 
     According to some embodiments, in cases where the first light beam  8052  associated with the first image stream and the second light beam  8054  associated with the second image stream are time-division multiplexed, a switchable mirror can be used in place of the polarization beam splitter  8030  shown in  FIGS.  30 A- 30 B,  31 C- 31 D, and  33 A- 33 B . The switching of the switchable mirror can be synchronized with the time-division multiplexing of the first light beam  8052  and the second light beam  8054 . For example, the switchable mirror can be switched to a first state for the first light beam  8052  so that it operates as a mirror reflecting the first light beam  8052  along the first optical path as illustrated in  FIGS.  30 A,  31 C, and  33 A , and be switched to a second state for the second light beam  8054  so that it operates as a transparent optical element transmitting the second light beam  8054  along the second optical path as illustrated in  FIGS.  30 B,  31 D, and  33 B . 
     According to some embodiments, wavelength-division multiplexing can be used for multiplexing the first light beam associated with the first image stream and the second light beam associated with the second image stream. For example, the first light beam can be composed of light in a first set of wavelength ranges in red, green, and blue, and the second light beam can be composed of light in a second set of wavelength ranges in red, green, and blue light. The two sets of wavelength ranges can be shifted with respect to each other, but the composite of the second set of wavelength ranges produces a white light that is substantially the same as the white light produced by the composite of the first set of wavelength ranges. 
     In cases where wavelength-division multiplexing is used, a display system can include a dichroic beam splitter that takes the place of the polarization beam splitter to separate the first light beam associated with the first image stream and the second light beam associated with the second image stream. For example, the dichroic beam splitter can be configured to have a high reflectance value and a low transmittance value for the first set of wavelength ranges, and a low reflectance value and a high transmittance value for the second set of wavelength ranges. In some embodiments, the first light beam and the second light beam can be projected concurrently without the need for a switchable polarization rotator. 
       FIGS.  34 A- 34 B  illustrate schematically a display system  12000  according to some other embodiments. The display system  12000  includes an image source  12010 . The image source  12010  can be configured to project first light beam  12052  associated with a first image stream as illustrated in  FIG.  34 A , and second light beam  12054  associated with a second image stream as illustrated in  FIG.  34 B . The first image stream can be a wide FOV and low resolution image stream, and the second image stream can be a narrow FOV and high resolution image stream, as discussed above with reference to  FIGS.  26 E- 26 F . The first light beam  12052  and the second light beam  12054  can be multiplexed using, for example, polarization-division multiplexing, time-division multiplexing, wavelength-division multiplexing, and the like. In  FIGS.  34 A- 34 B , the first light beam  12052  and the second light beam  12054  are depicted as schematic light rays, which are not intended to represent accurate ray-traced rays. 
     The display system  12000  can further include a beam splitter  12030  configured to de-multiplex the first light beam  12052  and the second light beam  12054  according to some embodiments. For example, the beam splitter  12030  can be a polarization beam splitter (PBS) or a dichroic beam splitter. The beam splitter  12030  can be configured to reflect the first light beam  12052  along a first optical path as illustrated in  FIG.  34 A , and to transmit the second light beam  12054  along a second optical path as illustrated in  FIG.  34 B . 
     The display system  12000  can further include a switchable optical element  12040 . Although the switchable optical element  12040  is illustrated as a single element, it can include a pair of sub switchable optical elements that functions as a switchable relay lens assembly. Each sub switchable optical element can be switched to a first state such that it operates as an optical lens with a first optical power, or be switched to a second state such that it operates as an optical lens with a second optical power different than the first optical power. As such, the switchable optical element  12040  can provide a first angular magnification when the sub switchable optical elements are switched to the first state, as illustrated in  FIG.  34 A , and a second angular magnification different from the first angular magnification when the sub switchable optical elements are switched to the first state, as illustrated in  FIG.  34 B . 
     Each sub switchable optical element can take many forms, including e.g., liquid crystal varifocal lenses, tunable diffractive lenses, or deformable lenses. In general, any lens that could be configured to change shape or configuration to adjust its optical power could be applied. In some embodiments, each sub switchable optical element can be a multifocal birefringent lens that has a first optical power for a light with a first polarization and a second optical power substantially different from the first optical power for light with a second polarization. For example, a multifocal birefringent lenses can comprise a polymer that has been made birefringent by an orienting process by stretching the polymer under defined conditions, such that the polymer exhibits an ordinary refractive index n o  and an extraordinary refractive index n e . 
     In cases where the first light beam  12052  and the second light beam  12054  are time-division multiplexed, the switching of the switchable optical element  12040  can be synchronized with the time-division multiplexing of the first light beam  12052  and the second light beam  12054 , so that each sub switchable optical element operates as an optical lens with the first optical power for the first light beam  12052  as illustrated in  FIG.  34 A , and operates as an optical lens with the second optical power for the second light beam  12054  as illustrated in  FIG.  34 B . Therefore, the first light beam  12052  associated with the first image stream can be angularly magnified by the switchable optical element  12040  as they exit the switchable optical element  12040 , and can be subsequently projected to a viewing assembly for presenting the first image stream with a first field of view FOV 1  that is relatively wide. 
     The display system  12000  can further include a first mirror  12060  positioned downstream from the beam splitter  12030  along the second optical path as illustrated in  FIG.  34 B . The first mirror  12060  can reflect the second light beam  12054  back toward the beam splitter  12030 , which can be subsequently reflect by the beam splitter  12030  towards a second mirror  12070 . 
     The second mirror  12070  is positioned below the beam splitter  12030  as illustrated in  FIG.  34 B . The second mirror  12070  can reflect the second light beam  12054  back toward the beam splitter  12030 , which can be subsequently transmitted by the beam splitter  12030  toward the switchable optical element  12040 . As described above, each sub switchable optical element can be switched to the second state so that it can operate as an optical lens with the second optical power for the second light beam  12054 . The second optical power can be less than the first optical power associated with the first state, or be substantially zero or negative. Therefore, the second light beam  12054  can be angularly magnified by an amount less than the first light beam  12052 , or be not magnified or be demagnified as they exit the switchable optical element  12040 . Thus, the second light beam  12054  can be subsequently projected to the viewing assembly for presenting the second image stream with a second field of view FOV 2  that is relatively narrow. 
     In some embodiments, the second mirror  12070  can be configured as a two-dimensional (2D) scanning mirror (i.e., a scanning mirror with two degrees of rotational freedom), such as a 2D MEMS scanner, that can be tilted in two directions as illustrated in  FIG.  34 B . The tilting of the second mirror  12070  can be controlled based on the fixation position of the user&#39;s eye, such that the second light beam  12054  can project the second image stream at the user&#39;s foveal vision. In some other embodiments, the second mirror  12070  can be a fixed mirror, and the first mirror  12060  can be a 2D scanning mirror. In some further embodiments, the first mirror can be a one-dimensional (1D) scanning mirror (i.e., a scanning mirror with one degree of rotational freedom) that can be tilted in a first direction, and the second mirror can be a 1D scanning mirror that can be tilted in a second direction. 
       FIG.  35    illustrates schematically a display system  13000  according to some other embodiments. The display system  13000  includes an image source  13010 . The image source  13010  can be configured to provide a first light beam associated with a first image stream in right-handed circular polarization (RHCP) and a second light beam associated with a second image stream in left-handed circular polarization (LHCP) (or vice versa). 
     The display system  13000  can further include a beam splitter  13030  configured to de-multiplex the first light beam and the second light beam. For example, the beam splitter  13030  can comprise a liquid crystal material that reflects the right-handed circularly polarized first light beam and transmits the left-handed circularly polarized second light beam. 
     The display system  13000  can further include a first switchable optical element  13042  and a second switchable optical element  13044 , the combination of which can serve as a relay lens assembly. Each of the first switchable optical element  13042  and the second switchable optical element  13044  can comprise a liquid crystal material such that it has a first focal length f RHCP  for right-handed circular polarized light and a second focal length f LHCP  for left-handed circularly polarized light. Therefore, the combination of the first switchable optical element  13042  and the second switchable optical element  13044  can provide a first angular magnification to the first light beam, and a second angular magnification to the second light beam that is different from the first angular magnification. For example, the first angular magnification can be greater than one, and the second angular magnification can equal to unity or less than one. 
       FIG.  36 A  illustrates schematically an augmented reality near-eye display system  14000  according to some embodiments.  FIG.  36 A  shows a portion of the display systems  14000  for one eye  210 . In practice a second such system would be provided for a user&#39;s other eye. Two such systems are incorporated in augmented reality glasses according to embodiments. Referring to  FIG.  36 A , a red laser diode  14002  is optically coupled through a red laser collimating lens  14004  into a red light input face  14006  of a Red-Green-Blue (RGB) dichroic combiner cube  14008 . A green laser diode  14010  is optically coupled through a green laser collimating lens  14012  into a green light input face  14014  of the RGB dichroic combiner cube  14008 . Similarly, a blue laser diode  14016  is optically coupled through a blue laser collimating lens  14018  into a blue light input face  14020  of the RGB dichroic combiner cube  14008 . The RGB dichroic combiner cube  14008  has an output face  14022 . The RGB dichroic combiner cube  14008  includes a red reflecting dichroic mirror (short wavelength pass mirror)  14024  set at 45 degrees so as to reflect light from the red laser diode  14002  through the output face  14022 . The RGB dichroic combiner cube  14008  also includes blue reflecting dichroic mirror (long wavelength pass)  14026  set at 135 degrees (perpendicular to red reflecting dichroic mirror  14024 ) so as to reflect light from the blue laser diode  14016  to the output face  14022 . Light from the green laser diode  14010  passes through (is transmitted by) the red reflecting dichroic mirror  14024  and the blue reflecting dichroic mirror  14026  to the output face  14022 . The red reflecting dichroic mirror  14024  and the blue reflecting dichroic mirror  14026  can be implemented as thin film optical interference films. 
     The red, green, and blue laser diodes  14002 ,  14010 ,  14016  are separately modulated with red, blue and green color channel image information. A cycle including a first period in which image information to be directed to the fovea of a user&#39;s retina is output and a subsequent period in which image information to be directed to a larger portion of the user&#39;s retina is repeated sequentially. There can be some angular overlap between image information directed to user&#39;s retina in the first period and the image information directed to the user&#39;s retina during the subsequent period of the cycle. In other words, certain portions of the user&#39;s eye may receive light during both periods. Rather than trying to achieve a sharp boundary, overlapping boundaries characterized by a tapering intensity may be used. The optical arrangement to achieve the aforementioned functionality will be described below. 
     The dichroic combiner cube  14008  outputs a collimated beam  14028  that includes red, blue and green components. The collimated beam  14028  is incident on a first two degree of freedom image scanning mirror  14030 . The image scanning mirror  14030  has two degrees of freedom of rotation and can be oriented to angles within a predetermined angular range. Each orientation of the image scanning mirror  14030  effectively corresponds to angular coordinates in an image space. The orientation of the image scanning mirror  14030  is scanned in coordination with modulation of the red, green and blue laser diodes  14002 ,  14010 ,  14016  based on image information so as to present an image, ultimately, to a user&#39;s eye. 
     Light deflected by the image scanning mirror  14030  is coupled through a first relay lens element  14032  to a polarization rotation switch  14034 . Alternatively, the polarization rotation switch could be located closer to the laser diodes  14002 ,  14010 ,  14016 . The polarization rotation switch  14034  is electrically controlled by electronics (not shown in  FIG.  36 A ). The polarization rotation switch  14034  can be implemented as a liquid crystal polarization rotation switch. The polarization rotation switch  14034  receives light of a specific linear polarization that is output by the laser diodes  14002 ,  14010 ,  14016  and transferred through the collimating lenses  14004 ,  14012 ,  14018  and the RGB dichroic combiner cube  14008  without altering the polarization. The polarization rotation switch  14034  under the control of external electrical signals either passes the incoming light without altering its polarization or rotates the polarization of the light by 90 degrees. 
     Light exiting the polarization rotation switch  14034  is coupled to a polarization beam splitter (PBS)  14036 . The PBS  14036  has embedded therein a polarization selective reflector  14038  arranged diagonally across the PBS  14036 . The polarization selective reflector  14038  can be of the type including an array of parallel metal conductive lines (not visible in  FIG.  36 A ). Light polarized (i.e., have an electric field direction) parallel to the metal conductive lines is reflected and light polarized perpendicular to the conductive metal lines is transmitted. In the case of the embodiment shown in  FIG.  36 A  it is assumed that the conductive metal lines are oriented perpendicular to the plane of the drawing sheet. With such an orientation the polarization selective reflector  14038  will reflect S-polarized light and transmit P-polarized light. 
     Considering first the case in which the polarization rotation switch  14034  is in a state that outputs P-polarized light, such P-polarized light will pass through the polarization selective reflector  14038  and through the PBS  14036  entirely reaching a first quarter wave plate (QWP)  14040 . The first QWP  14040  is oriented so as to convert P-polarized light to right hand circularly polarized (RHCP) light. (Alternatively the first QWP could have been oriented so as to convert P-polarized light to LHCP, in which changes to other components described below will also be made as will be apparent after considering the remaining description of  FIG.  36 A .) After passing through the first QWP  14040  light will reach a second relay lens element  14042 . The first relay lens element  14032  and the second relay lens element  14042  for a unity magnification afocal compound lens. Note that the image scanning mirror  14030  is spaced from the first relay lens element  14032  by a distance equal to the focal length of the first relay lens element  14032 . The second relay lens element  14032  will recollimate the light (the light having been initially collimated by collimating lenses  14004 ,  14012 ,  14018 ). Note also that light propagating from the second relay lens element  14042  will cross an optical axis OA near a point P 1  that is spaced from the second relay lens element  14042  by the focal length of the second relay lens element  14042 . In the embodiment shown in  FIG.  36 A  the first relay lens element  14032  and the second relay lens element  14042  have the same focal length. 
     After exiting the second relay lens element  14042  the light will be incident on a first group positive refractive lens  14044  of a first group  14046  of a dual magnification afocal magnifier  14048 . In addition to the first group positive refractive lens  14044 , the first group  14046  also includes a first group geometric phase lens  14050 . After passing through the first group geometric phase lens  14050 , the light passes through a second group  14052  that includes a second group positive refractive lens  14054  and a second group geometric phase lens  14056 . The geometric phase lenses  14050 ,  14056  include patternwise aligned liquid crystal material. Geometric phase lenses (also known as “polarization directed flat lenses”) are available from Edmund Optics of Barrington, N.J. The geometric phase lenses  14050 ,  14056  have the property that they are positive lenses for circularly polarized light that has a handedness (RH or LH) that matches their handedness and are negative lenses for circularly polarized light of opposite handedness. Geometric phase lenses also have the property that in transmitting light they reverse the handedness of circularly polarized light. In the embodiment shown in  FIG.  36 A , the geometric phase lenses  14050 ,  14056  are right handed. It should be noted that this system could be modified to accommodate use with left handed geometric phase lenses. 
     In operation when RHCP light is passed through the first group  14046 , the first group geometric phase lens  14050  will act as a negative lens, so that the positive optical power of the first group  14046  will be less than the positive optical power of the first group refractive lens  14044  alone and the first group  14046  will have focal length about equal to a distance to point F RHCP  indicated in  FIG.  36 A  from a principle plane of the first group  14046 . Propagating through the first group geometric phase lens  14050  will convert the light to the left handed circularly polarized (LHCP) state. For light of the LHCP state the second group geometric phase lens  14056  will have positive refractive power, and therefore the positive refractive power of the second group  14052  will be greater than the positive refractive power of the second group positive refractive lens  14054  alone. In this case a focal length of the second group  14052  will also equal a distance from the principle plane of the second group  14052  to the point F RHCP , with the subscript “RHCP” referring to the polarization state of the light entering the magnifier  14048 . Because the point F RHCP  is closer to the second group  14052  than the first group  14046 , the dual magnification afocal magnifier  14048  will be a magnifier (have a magnification greater than 1) for RHCP light received from the second relay lens element  14042 . 
     Now considering a second case in which the polarization rotation switch  14034  is in a state that outputs S-polarized light, such S-polarized light is reflected by the polarization selective reflector  14038  nominally 90 degrees and then passes through a second QWP  14058  and thereafter passes through a third relay lens element  14060  which deflects the light toward a fixed mirror  14062 . Note that for S-polarized light the first relay lens element  14032  in combination with the third relay lens element  14060  form a unity magnification afocal relay. The fixed mirror  14062  reflects the light back through third relay lens element  14060  and second QWP  14058  changing the sign but not the absolute value of the angle of the light beam with respect to the optical axis OA. After the first pass through the second QWP  14058  the S-Polarized light is converted to circularly polarized light of a particular handedness (which can be chosen to be either RHCP or LHCP by choosing the orientation of the fast and slow axes of the second QWP  14058 ). Upon reflection by the fixed mirror  14062  the handedness of the circularly polarized light is reversed. Upon the second pass through the second QWP the circularly polarized light which was S-polarized is converted (temporarily) to P-polarized light which then passes through the polarization selective reflector  14038 . 
     After passing through the polarization selective reflector  14038 , the light passes through a third QWP  14064  and a fourth relay lens element  14066  and is directed to a fovea tracking mirror  14068 . In the system  14000 , because the image scanning mirror  14030 , the fixed mirror  14060  and the fovea tracking mirror  14068  are spaced from respectively from the relay lens elements  14032 ,  14066 ,  14060  by the focal length of the relay lens element  14032 ,  14066 ,  14060  and the QWPs  14040 ,  14058 ,  14064  are positioned after the relay lens elements  14032 ,  14042 ,  14060 ,  14066  the angle of light incidence on the QWPs  14040 ,  14058 ,  14064  is relatively low which leads to improved performance of the QWPs  14040 ,  14058 ,  14064 . According to an alternative embodiment, rather than having a single fovea tracking mirror  1268  that tracks two angular degrees of freedom of eye movement (e.g., azimuth and elevation), the fixed mirror  14062  can be replaced with a second fovea tracking mirror (not shown) and one of the two fovea tracking mirrors can be used to track one degree of freedom of eye movement and the second fovea tracking mirror can be used to track a second degree of freedom of eye movement. In such an alternative, single degree of freedom fovea tracking mirrors may be used. Referring again to  FIG.  36 A , the third relay lens element  14060  in combination with the forth relay lens element  14066  forms a unity magnification afocal relay. The fovea tracking mirror  14068  can add to the deflection of the light beam  14028  produced by the image scanning mirror  14030  and thereby deflect the mean angle of the entire solid angle range of beam angles produced by the image scanning mirror  14030  off axis in order to track the fovea (not shown) of a user&#39;s eye  210 . An eye-tracking camera  14098  tracks the eye gaze of a user&#39;s eye  210 . The eye-tracking camera  14098  is coupled to a fovea tracking control system  14097 . The eye-tracking camera  14098  outputs information indicative of the eye gaze which is input to the fovea tracking control system  14097 . The fovea tracking control system  14097  is drivingly coupled to the fovea tracking mirror  14068 . Based on the eye gaze information received from the eye-tracking camera  14098 , the fovea tracking control system  14097  outputs a signal to the fovea tracking mirror  14068  in order to orient the fovea tracking mirror  14068  to track the fovea of the user&#39;s eye  14099 . The fovea tracking control system  14097  can use image processing to determine the user&#39;s eye gaze and generate the signal to control the fovea tracking mirror based on the eye gaze. 
     After being reflected by the fovea tracking mirror  14068  the light passes back through the fourth relay lens element  14066  and the third QWP  14064 . The first pass of light through the third QWP  14064  converts the light to circularly polarized light, the reflection by the fovea tracking mirror  14068  reverses the handeness of the circularly polarized light and the second pass through the third QWP  14064  converts the light back to the S-polarized state. Because the light is now S-polarized it is reflected by the polarization selective reflector  14038  and deflected nominally 90 degrees toward the first QWP  14040 . The first QWP  14040  converts the S-Polarized light to left hand circularly polarized (LHCP) light. The light then passes through second relay lens element  14042 . The fourth relay lens element  14066  in combination with the second relay lens element  14042  forms a unity magnification afocal compound lens. The relay lens elements  14032 ,  14042 ,  14060 ,  14066  are symmetrically placed at 90 degree intervals about the center of the polarization selective mirror  14038 . Generally successive (in the order of light propagation) relay lens elements  14032 ,  14042 ,  14060 ,  14066  form unity magnification afocal relays. Successive relay lens elements positioned so as to be confocal, sharing a common focal point halfway across the PBS  14036 . The relay lens elements  14032 ,  14042 ,  14060 ,  14066  can include, by way of non-limiting examples, aspheric lenses, aplanatic lenses, hybrid refractive and diffractive lenses and achromatic lenses, compound lenses including for example refractive lenses along with diffractive lenses. As used in the present description “relay lens element” includes a single lens or compound lens. 
     For LHCP light the first group geometric phase lens  14050  has a positive refractive power which increases the refractive power of the first group  14046 . For LHCP the focal length of the first group  14044  is equal to a distance from the principal plane of the first group  14044  to a point F LHCP . Upon passing through the first group geometric phase lens  14050  the LHCP light is converted to RHCP light. Subsequently the light passes through the second group  14052 . For RHCP light the second group geometric phase lens  14056  has a negative refractive power so that the positive refractive power of the second group  14052  will be lower than the refractive power of the second group positive refractive lens  14054  alone. For RHCP light the second group  14052  has a focal length equal to a distance from a principal plane of the second group  14052  to the point F LHCP . Accordingly for LHCP light entering the dual magnification afocal magnifier  14048 , the dual magnification afocal magnifier  14048  serves as a demagnifier with a magnification less than one. Thus a solid angle range of light beam directions produced by the image scanning mirror  14030  which is deflected by the fovea tracking mirror  14068  is demagnified to cover a reduced angular range which tracks a user&#39;s fovea as the user&#39;s gaze is shifted. Recall that for incoming RHCP the dual magnification afocal magnifier  14048  has a magnification greater than one. The magnification greater than one is used to provide a wider field of view corresponding to a portion of the user&#39;s retina outside the fovea. 
     In certain embodiments the second group  14052  is a mirror image of the first group  14046 , in which case the first group geometric phase lens  14050  and the second group geometric phase lens  14056  are identical, and the first group positive refractive lens  14044  and the second group positive refractive lens  14054  are identical. If the refractive lenses  14044 ,  14054  have surfaces of different refractive power, they can be positioned so that surfaces of the same refractive power face each other in order to maintain the mirror image symmetry of the dual magnification afocal magnifier  14048 . In this case although each group  14046 ,  14052  can have two different principal planes depending on whether the geometric phase lenses  14050 ,  14056  are acting as positive or negative lenses, nonetheless two groups  14046 ,  14052  can be spaced from each other at a fixed distance that maintains the confocal relation of the two groups  14046 ,  14052  in order to maintain the afocal magnification of the magnifier  14048  regardless of whether LHCP or RHCP light entering the magnifier  14048 . 
     A set of three augmented reality glasses eyepiece waveguides including a first eyepiece waveguide  14070 , a second eyepiece waveguide  14072  and a third eyepiece waveguide  14074  are positioned beyond and optically coupled (through free space, as shown) to the second group  14052  of the dual magnification afocal magnifier  14048 . Although three eyepiece waveguides  14070 ,  14072 ,  14074  disposed in overlying relation are shown, alternatively a different number of eyepiece waveguides are provided. For example multiple sets of three eyepiece waveguides, with each set configured to impart a different wavefront curvature (corresponding to a different virtual image distance) to exiting light may be provided. The three eyepiece waveguides  14070 ,  14072 ,  14074  are respectively provided with three light incoupling elements  14076 ,  14078 ,  14080  including a first light incoupling element  14076 , a second light incoupling element  14078  and a third light incoupling element  14080 . Each of the three eyepiece waveguides  14070 ,  14072 ,  14074  can be configured to transfer light in a particular color channel, e.g., red, green or blue light. Additionally each of the incoupling elements  14076 ,  14078 ,  14080  can be wavelength selective so as to only couple light in one color channel into its associated eyepiece waveguide  14070 ,  14072 ,  14074 . The incoupling elements  14076 ,  14078 ,  14080  can for example comprise spectrally selective reflective diffraction gratings, such as for example diffraction gratings made of cholesteric liquid crystal material. Such cholesteric liquid crystal material has a helical pitch which determines a spectral reflectivity band. Each of the incoupling elements can for example include two superposed layers of cholesteric liquid crystal material with one being reflective of LHCP light and the other being reflective of RHCP light. Diffraction gratings generally have a profile pitch which determines light deflection angles. In the case that the incoupling elements  14076 ,  14078 ,  14080  are implemented as diffraction gratings the grating profile pitch of each grating is suitably selected in view of an associated the wavelength of light to be incoupled such that light is diffracted to angles above the critical angle for total internal reflection for the associated eyepiece waveguide  14070 ,  14072 ,  14074 . The first, second and third eyepiece waveguides  14070 ,  14072 ,  14074  respectively include a first exit pupil expander (EPE)  14082 , a second EPE  14084  and a third EPE  14086 . The EPEs  14082 ,  14084 ,  14086  may be implemented as transmissive and/or reflective diffraction gratings. The EPEs  14082 ,  14084 ,  14086  incrementally couple light that is propagating within the waveguides  14070 ,  14072 ,  14074  out of the waveguides  14070 ,  14072 ,  14074  such that light exits the waveguides  14070 ,  14072 ,  14074  over a relatively wide area compared to the transverse extent of the incoupling elements  14076 ,  14078 ,  14080 . Orthogonal pupil expanders (OPEs) not visible in  FIG.  36 A  can also be provided on the eyepiece waveguides  14070 ,  14072 ,  14074  and located behind the EPEs  14082 ,  14084 ,  14086 . The OPEs serve to deflect light from the incoupling elements  14076 ,  14078 ,  14080  that is propagating within the eyepiece waveguides  14070 ,  14072 ,  14074  toward the EPEs  14082 ,  14084 ,  14086 . The OPEs may be located in the path of light emanating from the incoupling elements  14076 ,  14078 ,  14080  and the EPEs  14082 ,  14084 ,  14086  may be outside the path of light emanating from the incoupling elements  14076 ,  14078 ,  14080 , but the OPEs may deflect light from the incoupling elements  14076 ,  14078 ,  14080  toward the EPEs  14082 ,  14084 . 
     According to an alternative embodiment the first relay lens element  14032  has a longer focal length than the second  14042 , third  14060  and fourth  14066  relay lens elements, and is spaced from the center of the PBS  14036  (taking into account the index of refraction of the PBS  14036 ) by a distance equal to the longer focal length. In this case the longer focal length first relay lens element  14032  in combination with the second relay lens  14042  imparts an angular magnification greater than 1:1 to the non-fovea tracked light; and the longer focal length first relay lens element  14032  in combination with the third relay lens element  14060  imparts an angular magnification greater than 1:1 to fovea tracked light. Recall that the dual magnification afocal magnifier  14048  will demagnify the fovea tracked light and the magnify the non-fovea tracked light. Thus changing the focal length of the first relay lens element  14032  provides another degree of design freedom that can be used to set the magnifications achieved in the system  14000  without disturbing the symmetry of the design of the dual magnification afocal magnifier  14048 . Introducing asymmetry into the design of the dual magnification afocal magnifier  14048  is another possible alternative. 
     According to an alternative embodiment in lieu of the geometric phase lenses  14050 ,  14056  other types of dual state lenses are used. According to one alternative actively driven electrowetting liquid lenses may be used. According to another alternative lenses that include a liquid crystal with its ordinary axis aligned in a specific direction overlying a diffractive optic made of a material that matches the ordinary axis and exhibits a lens power for light polarized parallel to the extraordinary axis may be used. In the latter case the first QWP  14040  may be eliminated as the anisotropic performance of the lenses will be dependent on the linear polarization differences between the fovea tracked and non-fovea tracked light. 
     Each orientation of the image scanning mirror  14030  corresponds to certain angular coordinates in the image space when the polarization rotation switch  14034  is configured to transmit non-fovea-tracked P-polarized light. When the polarization rotation switch  14034  is configured to output S-polarized light that is fovea-tracked, the orientation of the image scanning mirror  14030  in combination with the orientation of the fovea tracking mirror  14068  determine angular coordinates in the image space. The angles of light beam propagation determined by the orientation of the image scanning mirror and the fovea tracking mirror  14068  are multiplied by the magnifications of the dual magnification afocal magnifier  14048  and optionally by magnification determined by the relative focal lengths of the relay lenses  14032 ,  14042 ,  14060 ,  14066 . The effective size of pixel defined in angular image space is related to the inverse of the modulation rates of the laser diodes  14002 ,  14010 ,  14016  and the angular rate of motion of the image scanning mirror  14030 . To the extent that the motion of the image scanning mirror  14030  may be sinusoidal, the modulation rate of the laser diodes  14002 ,  14010 ,  14016  may be made inversely related to the angular rate of the image scanning mirror  14030  in order to reduce or eliminate pixel size variation. When both fovea tracked and non-fovea tracked are being generated the laser diodes  14002 ,  14010 ,  14016  the full potential modulation rate of laser diodes  14002 ,  14010 ,  14010  (limited by characteristics of available lasers) can be used (at least for certain points in the field of view), and the full angular range of the image scanning mirror can be used such that resolution imagery of imagery produced for the fovea tracked region which subtends a relatively small solid angle range can be higher (smaller pixel size) than the resolution of imagery produced for the wider field of view. 
     According to certain embodiments in an augmented reality system in which the system  14000  is used virtual content is superimposed on the real world which is visible to the user through the eyepiece waveguides  14070 ,  14072 ,  14074 . The virtual content is defined as 3D models (e.g., of inanimate objects, people, animals, robots, etc.). The 3D models are positioned and oriented in a 3D coordinate system. In an augmented reality system, through the provision of, for example, an inertial measurement unit (IMU) and/or visual odometry the aforementioned 3D coordinate system is maintained registered to a real world environment (inertial reference frame) of the user of the augmented reality system. A game engine processes the 3D models taking into account their position and orientation in order to render a left eye image and a right eye image of the 3D models, for output to the user via the system  14000  (and a like systems for the user&#39;s other eye). To the extent that the 3D models are defined in a coordinate system that is fixed to user&#39;s environment and to the extent that the user may move and turn his or her head (which carriers the augmented reality glasses) within the environment, the rendering of the left eye image and the right eye image is updated to take into account the user&#39;s head movement and turning. So for example if a virtual book is displayed resting on a real table and the user&#39;s rotates his or her head by 10 degrees to the left in response to information of the rotation from the IMU or a visual odometry subsystem (not shown), the game engine will update the left and right images to shift the image of the virtual book being output by the system  14000  10 degrees to the right so that the book appears to maintain its position notwithstanding the user&#39;s head rotation. In the present case imagery for a wider portion of the retina extending beyond the fovea and imagery for more limited portion of retina including the fovea are time multiplexed through the system  14000  using the polarization rotation switch  14034 . Imagery is generated and output by the game engine in synchronism with the operation of the polarization rotation switch  14034 . As mentioned above the game engine generates left eye imagery and right eye imagery. The game engine also generates narrower FOV left fovea and right fovea imagery which are output when the polarization rotation switch  14034  is configured to output S-polarization light that is fovea tracked using the fovea tracking mirror  14068 . As discussed above such fovea tracked imagery is converted to LHCP light and is demagnified by the dual magnification afocal magnifier  14048 . Such demagnification limits the angular extent to a narrow range including the fovea (or at least a portion thereof). The demagnification reduces pixel size thereby increasing angular resolution for the fovea tracked imagery. 
       FIG.  37 A  is a schematic illustration of a dual magnification afocal magnifier  14048  used in augmented reality near eye display system shown in  FIG.  36 A  according to one embodiment. 
       FIG.  37 B  is a schematic illustration of a dual focal magnification afocal magnifier  15000  that may be used in the augmented reality near eye display system  14000  shown in  FIG.  36 A  in lieu of the afocal magnifier  14048  according to other embodiments. The afocal magnifier  15000  includes a lens group  15002  that includes a positive refractive lens  15004  and a first geometric phase lens  15006 . The afocal magnifier  15000  further includes a second geometric phase lens  15008  spaced at a distance from the first lens group  15002 . The first geometric phase lens  15006  and the second geometric phase lens  15008  have opposite handedness. For light having a handedness matching the handedness of a geometric phase lens the geometric phase lens acts as a positive lens and for light having a handedness opposite to the handedness of the geometric phase lens the geometric phase lens acts as a negative lens. Additionally upon propagating through a geometric phase lens the handedness of the light is reversed. Accordingly when the first geometric phase lens  15006  is acting as a positive lens the second geometric phase lens  15008  will also be acting as a positive lens and when the first geometric phase lens  15006  is acting as a negative lens the second geometric phase lens  15008  will also be acting as a negative lens. When the first geometric phase lens  15006  is acting as a negative lens the lens group  15002  will have a longer focal length than the focal length of the positive refractive lens  15004  alone. When the first geometric phase lens  15006  is acting as a positive lens the lens group  15002  will have a shorter focal length than the focal length of the positive refractive lens  15004  alone. 
     Recall that in the augmented reality near eye display system  14000  shown in  FIG.  36 A , the P-polarized light output by the polarization switch  14034  passes directly through the PBS  14036 , is not foveal tracked and is converted to RHCP light by the first QWP  14040 ; whereas S-polarized light output from the polarization rotation switch  14034  is routed so as to be reflected by the foveal tracking mirror  14068  and is eventually converted to LHCP light. 
     The embodiment shown in  FIG.  37 B  will be further described with the assumption that the first geometric phase lens  15006  is left handed and the second geometric phase lens  15008  is right handed. It is further assumed, that as in the case of the system  14000  shown in  FIG.  36 A , LHCP light is foveal tracked and RHCP is not foveal tracked light and carries imagewise modulated light for a wider FOV (a wider portion of the retina). For LHCP light the first geometric phase lens  15006  acts as a positive lens and the lens group  15002  has a relatively short focal length corresponding to a distance from the lens group  15002  to a focal point F LHCP . In transmitting light the first geometric phase lens  15006  converts the LHCP light to RHCP light for which the second geometric phase lens  15008  has a positive refractive power and a focal length equal to a distance from the second geometric phase lens  15008  to the point F LHCP . In this case the afocal magnifier  15000  forms a Keplerian afocal magnifier. By proper selection (as will be described further below) of the focal lengths of the positive refractive lens  15004 , the first geometric phase lens  15006  and the second geometric phase lens  15008 , the magnification of the afocal magnifier  15000  in the Keplerian configuration can be chosen to be about 1:1 or another desired value. Assuming for example that image scanning mirror  14030  has an optical angular scan range of +/−10 degrees, such an angular range can substantially cover the fovea region of the retina. 
     For RHCP light entering the afocal magnifier  15000  the first geometric phase lens  15006  has a negative optical power and the lens group  15002  has a relatively longer focal length corresponding to a distance from the lens group  15002  to a point F RHCP . The first geometric phase lens  15006  converts the RHCP light to LHCP light for which the second geometric phase lens  15008  has a negative focal length corresponding to a distance from the second geometric phase lens  15008  to the point F RHCP . In this case, the afocal magnifier  15000  is configured as a Galilean afocal magnifier and can have a magnification substantially greater than 1:1 for example 3:1. Thus the RHCP light entering the afocal magnifier (which is not fovea tracked) can provide imagewise modulated light to a larger portion of the retina beyond the fovea (compared to the portion illuminated by the LHCP light. It should be noted that the systems  14000 ,  15000  can be reconfigured to reverse the roles the RHCP and LHCP light. 
     For a given focal length of the positive refractive lens  15004  and given magnitude of focal length of the first geometric phase lens  15004 , the lens group  15002  will have one of two focal lengths equal to distances from the lens group  15002  to the points F LHCP  and F RHCP , depending on the handedness of incoming light (as described above). The second geometric phase lens  15008  should be positioned about half way between the points F LHCP  and F RHCP  and the focal length of the second geometric phase lens  15008  should be set to about one-half of the distance between F LHCP  and F RHCP . The magnification of the Keplerian configuration is equal to about minus the ratio of the distance from the lens group  15002  to point F LHCP  divided by the distance from the point F LHCP  to the second geometric phase lens  15008 . The magnification of the Galilean configuration is about equal to the ratio of the distance from the lens group  15002  to the point F RHCP  divided by the distance from the second geometric phase lens  15008  to the point F RHCP . 
     The dual magnification afocal magnifiers  14048 ,  15000  can be used in other types of optical devices, including, by way of non-limiting example, telescopes, binoculars, cameras and microscopes. In systems in which a real image is to be formed the afocal magnifiers  14048 ,  15000  can be used in combination with additional optical elements (e.g., lenses, convex mirrors). 
     Referring to  FIG.  36 A , according to an alternative embodiment, the fixed mirror  14062  is replaced with a second image scanning mirror, and a second subsystem (like what is shown in  FIG.  36 A ) including laser diodes, collimating lenses and RGB dichroic combining cube can be used to provide RGB image modulated light to the second scanning mirror. The second subsystem and second scanning mirror would be dedicated to providing fovea-tracked light. In this case the polarization rotation switch  14034  and the second QWP  14058  can be dispensed with and both fovea-tracked and non-fovea-tracked light can be simultaneously produced. In such an alternative all of the laser diodes would be oriented to inject P-polarized light into the PBS  14036 . 
       FIG.  36 B  illustrates schematically another augmented reality near-eye display system  14000 B. To the extent that the system  14000 B has certain aspects in common with the system  14000  shown in  FIG.  36 A , the following description of the embodiment shown in  FIG.  36 B  will focus on the differences. In the system  14000 B the dual magnification afocal magnifier  14048 , the second QWP  14058  and the polarization rotation switch  14034  are eliminated. A longer focal length first relay lens element  14032 B is used such that the combination of the first relay lens element  14032 B and the second relay lens element  14042  magnifies the angular field of view of light scanned by the scanning mirror  14030 . The scanning mirror  14030  is used to cover a full field of view of the system  14000 B minus a high resolution fovea tracked portion of the FOV. The second scanning mirror  14030  can be placed at a distance away from the first relay lens element  14032 B equal to the focal length of the first relay lens element  14032 B. The first RGB light engine  14095  is configured to output P-polarized light and in the absence of the polarization rotation switch  14034  light scanned by the scanning mirror  14030  will be coupled through the first relay lens element  14032 B and the second relay lens element  14042 . 
     The fixed mirror  14062  used in system  14000  ( FIG.  36 A ) is replaced with a second scanning mirror  14030 B. A second component color (e.g., Red-Blue-Green (RGB)) light engine  14096  complements the first component color (e.g., Red-Blue-Green (RGB))  14095 . The second RGB light engine  14095  includes second red, green and blue laser diodes  14002 B,  14010 B,  14016 B laser diodes coupled through collimating lenses  14004 B,  14012 B,  14018 B and a second RGB dichroic combiner cuber  14008 B to the second scanning mirror  14030 B. Additional elements of the second RGB light engine  14096  correspond to elements of the first RGB light engine  14095  described above and are labeled with reference numerals having a common numeric portion and an added suffix ‘B’. P-polarized light that is output by the second RGB light enting  14096  and angularly scanned by the second scanning mirror  14030  is optically coupled through the afocal relay formed by the third relay lens element  14060  and the fourth relay lens element  14066  to the fovea tracking mirror  14068  and in reaching the fovea tracking mirror passes through the third QWP  14064 . Upon being angularly shifted by the fovea tracking mirror  14068  the light is reflected back through the fourth relay lens element  14066  and third QWP  14068  and now having its polarization state changed to S-polarization is reflected by the polarization selective mirror towards first QWP  14040  and the second relay lens element  14042  and thereafter impinges the incoupling elements  14076 ,  14078 ,  14080 . It be appreciated that the first and second RGB light engines  14095 ,  14096  may utilize light of component colors other than, or in addition to, red, blue, and green. 
     The augmented reality near-eye display system  14000 B is able to simultaneously output fovea tracked high resolution imagery and nonfovea tracked wider field of view imagery. By avoiding the need to time multiplex higher resolution fovea tracked imager with wider field of view imagery (as in the case of the system shown in  FIG.  36 A ) the system  14000 B is more readily able to achieve a higher frame rate. 
     V. Tracking the Entire Field of View with Eye Gaze 
     According to some embodiments, instead of presenting the first image stream at a static position as illustrated in  FIGS.  26 E- 26 F , both the first image stream and the second image stream can be dynamically shifted around according to the user&#39;s current fixation point.  FIGS.  38 A- 38 B  illustrates schematically an exemplary configuration of images that can be presented to a user according to some embodiments.  FIG.  38 A  shows how the second image stream  16020  can be positioned substantially at the center of the first image stream  16010 . In some embodiments, it may be desirable to offset the second image stream  16020  from the center of the first image stream. For example, since a user&#39;s field of view extends farther in the temporal direction than the nasal direction it may be desirable to have the second image stream  16020  offset towards the nasal side of the first image stream. During operation, the first and second image stream can be persistently shifted in accordance with the user&#39;s current fixation point as determined in real-time using eye-gaze tracking techniques, as shown in  FIG.  38 B . That is, the first image stream  16010  and the second image stream  16020  can be shifted around in tandem such that the user is usually looking directly at the center of both image streams. It should be noted that the grid squares in  FIGS.  38 A- 38 B  represent schematically image points that, much like fields  3002 ,  3004  and  3006  as described above with reference to  FIG.  24   , are defined in two-dimensional angular space. 
     Similar to the embodiments depicted in  FIGS.  26 A- 26 B , the second image stream  16020  represents a high-resolution image stream having a relatively narrow FOV that can be displayed within the boundaries of the first image stream  16010 . In some embodiments, the second image stream  16020  can represent one or more images of virtual content as would be captured by a second, different virtual camera having an orientation in render space that can be dynamically adjusted in real-time based on data obtained using eye-gaze tracking techniques to angular positions coinciding with the user&#39;s current fixation point. In these examples, the high-resolution second image stream  16020  can represent one or more images of virtual content as would be captured by a fovea-tracked virtual camera such as the fovea-tracked virtual camera described above with reference to  FIGS.  26 A- 26 D . In other words, the perspective in render space from which one or more images of virtual content represented by the second image stream  16020  is captured can be reoriented as the user&#39;s eye gaze changes, such that the perspective associated with the second image stream  5020 E is persistently aligned with the user&#39;s foveal vision. 
     For example, the second image stream  16020  can encompass virtual content located within a first region of render space when the user&#39;s eye gaze is fixed at the first position as illustrated in  FIG.  38 A . As the user&#39;s eye gaze moves to a second position different from the first position, the perspective associated with the second image stream  16020  can be adjusted such that the second image stream  16020  can encompass virtual content located within a second region of render space, as illustrated in  FIG.  38 B . In some embodiments, the first image stream  16010  has a wide FOV, but a low angular resolution as indicated by the coarse grid. The second image stream  16020  has a narrow FOV, but a high angular resolution as indicated by the fine grid. 
       FIGS.  39 A- 39 B  illustrate some of the principles described in  FIGS.  38 A- 38 B  using some exemplary images that can be presented to a user according to some embodiments. In some examples, one or more of the images and/or image streams depicted in  FIGS.  39 A- 39 B  may represent two-dimensional images or portions thereof that are to be displayed at a particular depth plane, such as one or more of the depth planes described above with reference to  FIG.  25 B . That is, such images and/or image streams may represent 3-D virtual content having been projected onto at least one two-dimensional surface at a fixed distance away from the user. In such examples, it is to be understood that such images and/or image streams may be presented to the user as one or more light fields with certain angular fields of view similar to those described above with reference to  FIGS.  26 A- 26 D and  28 A- 28 B . 
     As depicted, the content of a first image stream  17010  includes a portion of a tree. During a first period of time represented by  FIG.  39 A , eye-tracking sensors can determine a user&#39;s eye gaze (i.e., the foveal vision) is focused at a first region  17010 - 1  within a viewable region  17000 . In this example, first region  17010 - 1  includes lower branches of the tree. A second image stream  17020  can be positioned within the first region  17010 - 1  and have a higher resolution than the first image stream. The first and second image streams can be displayed concurrently or in rapid succession in a position determined to correspond to the user&#39;s current eye gaze. 
     During a second period of time represented by  FIG.  39 B , the user&#39;s eye gaze can be detected shifting to a second region  17010 - 2  within the viewable region  1500  that corresponds to upper branches of the tree. As depicted, during the second period of time, the position and content of the first and second image streams changes to correspond to the second region  17010 - 2 . The content of both the first image stream  17010  and second image stream  17020  can include the second region  17010 - 2  of the tree. The first and second image streams can be displayed concurrently or in rapid succession. Further detected movements of the user&#39;s eye gaze can be accommodated in the same manner to keep both the first and second image streams aligned with the user&#39;s current eye gaze. 
     Similar to the embodiments illustrated in  FIGS.  28 C- 28 D , because the higher resolution second image stream  17020  overlays the portion of the first image stream  17010  within the user&#39;s foveal vision, the lower resolution of the first image stream  17010  may not be perceived or noticed by the user. Furthermore, because the first image stream  17010  having a wide field of view can encompass a substantial portion of the user&#39;s vision, the user may be prevented from fully perceiving the boundaries of the light field display. Therefore, this technique can provide an even more immersive experience to the user. 
       FIGS.  40 A- 40 D  illustrate schematically a display system  18000  for projecting images to an eye of a user according to some embodiments. The display system  18000  includes an image source  18010 . The image source  18010  can be configured to project first light beam  18052  associated with a first image stream and second light beam  18054  associated with a second image stream. The first image stream can be a wide FOV and low resolution image stream, and the second image stream can be a narrow FOV and high resolution image stream, as discussed above with reference to  FIGS.  38 A- 38 B . In some embodiments, the first light beam  18052  and the second light beam  18054  can be time-division multiplexed, polarization-division multiplexed, wavelength-division multiplexed, or the like. 
     The display system  18000  can further include a 2D scanning mirror  18020  configured to reflect the first light beam  18052  and the second light beam  18054 . In some embodiments, the 2D scanning mirror  18020  can be tilted in two directions based on the fixation position of the user&#39;s eye, such that both the first light beam  18052  and the second light beam  18054  can project the first image stream and the second image stream, respectively, at the user&#39;s foveal vision. 
     The display system  18000  can further include a switchable optical element  18040 . Although the switchable optical element  18040  is illustrated as a single element, it can include a pair of sub switchable optical elements that functions as a switchable relay lens assembly. Each sub switchable optical element can be switched to a first state such that it operates as an optical lens with a first optical power, as illustrated in  FIGS.  40 A and  40 C , or be switched to a second state such that it operates as an optical lens with a second optical power different from the first optical power, as illustrated in  FIGS.  40 B and  40 D . Each sub switchable optical element can be, for example, a liquid crystal varifocal lens, a tunable diffractive lens, a deformable lens, or a multifocal birefringent lens according to various embodiments. 
     In cases where the first light beam  18052  and the second light beam  18054  are time-division multiplexed, the switchable optical element  18040  and the scanning mirror  18020  can operate as follows. Assume that the user&#39;s eye gaze is fixed at a first position during a first time period. The scanning mirror  18020  can be in a first orientation during the first time period so that the first light beam  18052  and the second light beam  18054  are directed toward a first position, as illustrated in  FIGS.  40 A and  40 B . During a first time slot of the first time period (Stage A 1 ) when the image source  18010  outputs the first light beam  18052 , the switchable optical element  18040  can be switched to the first state where it operates as an optical lens with the first optical power as illustrated in  FIG.  40 A . During a second time slot of the first time period (Stage A 2 ) when the image source  18010  outputs the second light beam  18054 , the switchable optical element  18040  can be switched to the second state where it operates as an optical lens with the second optical power as illustrated in  FIG.  40 B . Thus, the first light beam  18052  are angularly magnified more than the second light beam  18054 , so that the first light beam  18052  can present the first image stream with a wider FOV than that of the second image stream presented by the second light beam  18054 . 
     Now assume that the user&#39;s eye gaze moves from the first position to a second position during a second time period. The scanning mirror  18020  can be in a second orientation during the second time period so that the first light beam  18052  and the second light beam  18054  are directed toward a second position, as illustrated in  FIGS.  40 C and  40 D . During a first time slot of the second time period (Stage B 1 ) when the image source  18010  outputs the first light beam  18052 , the switchable optical element  18040  can be switched to the first state where it operates as an optical lens with the first optical power as illustrated in  FIG.  40 C . During a second time slot of the second time period (Stage B 2 ) when the image source  18010  outputs the second light beam  18054 , the switchable optical element  18040  can be switched to a second state where it operates as an optical lens with the second optical power as illustrated in  FIG.  40 D . 
     In cases where the first light beam  18052  and the second light beam  18054  are polarization-division multiplexed, the switchable optical element  18040  can comprise a multifocal birefringent lens, so that it operates as an optical lens with the first optical power for the first light beam  18052  as illustrated in  FIGS.  40 A and  40 C , and operates as an optical lens with the second optical power for the second light beam  18054  as illustrated in  FIGS.  40 B and  40 D . 
     In cases where the first light beam  18052  and the second light beam  18054  are wavelength-division multiplexed, the switchable optical element  18040  can comprise a wavelength-dependent multifocal lens, so that it operates as an optical lens with the first optical power for the first light beam  18052  as illustrated in  FIGS.  40 A and  40 C , and operates as an optical lens with the second optical power for the second light beam  18054  as illustrated in  FIGS.  40 B and  40 D . 
       FIGS.  41 A- 41 D  illustrate schematically a display system  19000  for projecting images to an eye of a user according to some other embodiments. The display system  19000  can be similar to the display system  18000 , except that the switchable optical element  18040  can be disposed on the surface of the scanning mirror  18020 . For example, the switchable optical element  18040  can be one or more substrates layered on the surface of the scanning mirror  18020 . 
     In some further embodiments, the switchable optical element  18040  can be positioned elsewhere in the display system  19000 . For example, it can be positioned between the image source  18010  and the scanning mirror  18020 . 
     In some other embodiments, a polarization beam splitter or a dichroic beam splitter can be used to de-multiplex the first light beam  18052  and the second light beam  18054  into two separate optical paths, but both optical paths intersect the reflective surface of the scanning mirror  18020 . 
     In other embodiments, more than two image streams can be presented to the user so that the transition in resolution from the user&#39;s fixation point to the user&#39;s periphery vision is more gradual in appearance. For example, a third image stream having a medium FOV and medium resolution can be presented in addition to the first image stream and the second image stream. In such cases, additional relay lens assemblies and/or scanning mirrors can be utilized to provide additional optical paths for the additional image streams. 
     VI. Time Multiplexing Scheme 
     In some embodiments, the high-FOV low-resolution image stream (i.e., the first image stream) and the low-FOV high-resolution image stream (i.e., the second image stream) can be time-division multiplexed. 
       FIG.  42    shows a graph illustrating an exemplary time-division multiplexing pattern suitable for use with a high-FOV low-resolution image stream and a low-FOV high-resolution image stream. As illustrated, the high-FOV low-resolution image stream and the low-FOV high-resolution image stream are allocated at alternating time slots. For example, each time slot can be about one eighty-fifth of a second in duration. Thus, each of the high-FOV low-resolution image stream and the low-FOV high-resolution image stream may have a refresh rate of about 42.5 Hz. In some embodiments, an angular region corresponding to light fields of the low-FOV high-resolution image stream overlaps a portion of an angular region of the light fields corresponding to the high-FOV low-resolution image stream making the effective refresh rate in the overlapped angular region about 85 Hz (i.e., twice the refresh rate of each individual image stream). 
     In some other embodiments, the time slots for the high-FOV low-resolution image stream and the time slots for the low-FOV high-resolution image stream can have different durations. For example, each time slot for the high-FOV low-resolution image stream can have a duration longer than one eighty-fifth seconds, and each time slot for the low-FOV high-resolution image stream can have a duration shorter than one eighty-fifth seconds, or vice versa. 
       FIG.  43    illustrates schematically a display system  21000  for projecting image streams to an eye of a user according to some embodiments. The display system  21000  may share some elements in common with display system  8000  as illustrated in  FIGS.  30 A- 30 B ; for this reason, the description about those common elements in relation to  FIGS.  30 A- 30 B  are applicable here as well. An image source  21002  can be configured to provide a high-FOV low-resolution image stream in a first polarization state and a low-FOV high-resolution image stream in a second polarization state contemporaneously. For example, the first polarization state can be a linear polarization in a first direction, and the second polarization state can be a linear polarization in a second direction orthogonal to the first direction; or alternatively, the first polarization state can be a left-handed circular polarization and the second polarization state can be a right-handed circular polarization. Similar to the display system  8000  illustrated in  FIGS.  30 A- 30 B , the display system  21000  includes a polarization beam splitter  21004  for separating light beams projected by an image source (e.g., image source  21002 ) into a first light beam associated with the high-FOV low-resolution image stream propagating along a first optical path, and a second light beam associated with the low-FOV high-resolution image stream propagating along a second optical path. 
     Similar to the display system illustrated in  FIGS.  30 A- 30 B , the display system  21000  can include a first optical lens (lens A) positioned between the image source  21002  and the beam splitter  21004 , a second optical lens (lens B) positioned downstream from the beam splitter  21004  along the first optical path, and a third optical lens (lens C) positioned downstream from the beam splitter  21004  along the second optical path. In some embodiments, as described above in relation to  FIGS.  30 A- 30 B and  31 A- 31 B , the combination of the first optical lens (lens A) and the second optical lens (lens B) can provide an angular magnification for the first light beam that is greater than unity, and the combination of the first optical lens (lens A) and the third optical lens (lens C) can provide an angular magnification for the second light beam that is substantially equal to unity or less than unity. Thus, the first light beam can project an image stream that has a wider FOV than that projected by the second light beam. 
     Similar to the display system  8000  illustrated in  FIGS.  30 A- 30 B , the display system  21000  also includes a foveal tracker  21006  that can take the form of a scanning mirror (e.g., a MEMs mirror), which can be controlled based on the fixation position of the user&#39;s eye for dynamically projecting the second light beam associated with the low-FOV, high-resolution image stream. 
     The display system  21000  can also include a first in-coupling grating (ICG)  21010  and a second ICG  21020  coupled to an eyepiece  21008 . The eyepiece  21008  can be a waveguide plate configured to propagate light therein. Each of the first ICG  21010  and the second ICG  21020  can be a diffractive optical element (DOE) configured to diffract a portion of the light incident thereon into the eyepiece  21008 . The first ICG  21010  can be positioned along the first optical path for coupling a portion of the first light beam associated with the high-FOV low-resolution image stream into the eyepiece  21008 . The second ICG  21020  can be positioned along the second optical path for coupling a portion of the second light beam associated with the low-FOV high-resolution image stream into the eyepiece  21008 . 
     The display system  21000  can also include a first switchable shutter  21030 , and a second switchable shutter  21040 . The first switchable shutter  21030  is positioned along the first optical path between the second optical lens (lens B) and the first ICG  21010 . The second switchable shutter  21040  is positioned along the second optical path between the foveal tracker and the second ICG  21020 . The operation of the first switchable shutter  21030  and the second switchable shutter  21040  can be synchronized with each other such that the high-FOV low-resolution image stream and the low-FOV high-resolution image stream are time-division multiplexed according to a time-division multiplexing sequence (e.g. as illustrated in  FIG.  42   ). The first switchable shutter  21030  can be open for a time period corresponding to a first time slot associated with the high-FOV low-resolution image and closed during a second time slot associated with the low-FOV high-resolution image stream. Similarly, the second switchable shutter  21040  is open during the second time slot and is closed during the first time slot. 
     As such, the high-FOV low-resolution image stream is coupled into the eyepiece  21008  by way of the first ICG  21010  during the first time slot (e.g., when the first switchable shutter  21030  is open), and the low-FOV high-resolution image stream is coupled into the eyepiece  21008  by way of the second ICG  21020  during the second time slot (e.g., when the second switchable shutter  21040  is open). Once the high-FOV low-resolution image stream and the low-FOV high-resolution image stream are coupled into the eyepiece  21008 , they may be guided and out-coupled (e.g., by out-coupling gratings) into a user&#39;s eye. 
       FIG.  44    illustrates schematically a display system  22000  for projecting image streams to an eye of a user according to some embodiments. The display system  22000  may share some elements in common with the display system  8000  illustrated in  FIGS.  30 A- 30 B ; the description about those elements in relation to  FIGS.  30 A- 30 B  are applicable here as well. The high-FOV low-resolution image stream and the low-FOV high-resolution image stream provided by the image source  22002  can be time-division multiplexed and can be in a given polarized state. 
     The display system  22000  can include a switchable polarization rotator  22010  (e.g., ferroelectric liquid-crystal (FLC) cell with a retardation of half a wave). The operation of the switchable polarization rotator  22010  can be electronically programmed to be synchronized with the frame rates of the high-FOV low-resolution image stream and the low-FOV high-resolution image stream in the time-division multiplexing (e.g., as illustrated in  FIG.  42   ), so that the switchable polarization rotator  22010  does not rotate (or rotates by a very small amount) the polarization of the high-FOV low-resolution image stream, and rotates the polarization of the low-FOV high-resolution image stream by about 90 degrees (i.e., introducing a phase shift of π), or vice versa. Therefore, after passing through the switchable polarization rotator  22010 , the polarization of the high-FOV low-resolution image stream may be orthogonal to the polarization of the low-FOV high-resolution image stream. For example, the high-FOV low-resolution image stream can be s-polarized, and the low-FOV high-resolution image stream can be p-polarized, or vice versa. In other embodiments, the high-FOV low-resolution image stream can be left-handed circularly polarized, and the low-FOV high-resolution image stream can be right-handed circularly polarized, or vice versa. 
     The display system  22000  can include a polarization beam splitter  22004  for separating light beams into a first light beam associated with the high-FOV low-resolution image stream propagating along a first optical path toward the first ICG  21010 , and a second light beam associated with the low-FOV high-resolution image stream propagating along a second optical path toward the second ICG  21020 . 
     The display system  22000  can also include a static polarization rotator  22020  positioned along one of the two optical paths, for example along the second optical path as illustrated in  FIG.  44   . The static polarization rotator  22020  can be configured to rotate the polarization of one of the low-FOV high-resolution image stream and the high-FOV low-resolution image stream, so that the two image streams may have substantially the same polarization as they enter the first ICG  21010  and the second ICG  21020 , respectively. This may be advantageous in cases where the first ICG  21010  and the second ICG  21020  are designed to have a higher diffraction efficiency for a certain polarization. The static polarization rotator  22020  can be, for example, a half-wave plate. 
       FIG.  45    illustrates schematically a display system  23000  for projecting image streams to an eye of a user according to some embodiments. The display system  23000  may share some elements in common with the display system  8000  illustrated in  FIGS.  30 A- 30 B ; the description about those elements in relation to  FIGS.  30 A- 30 B  are applicable here as well. An image source  23002  can be configured to provide a high-FOV low-resolution image stream and a low-FOV and high-resolution image stream that are time-division multiplexed. 
     Here, instead of a beam splitter, the display system  23000  includes a switchable reflector  23004 . The switchable reflector  23004  can be switched to a reflective mode where an incident light beam is reflected, and to a transmission mode where an incident light beam is transmitted. The switchable reflector may include an electro-active reflector comprising liquid crystal embedded in a substrate host medium such as glass or plastic. Liquid crystal that changes refractive index as a function of an applied current may also be used. Alternatively, lithium niobate may be utilized as an electro-active reflective material in place of liquid crystal. The operation of the switchable reflector  23004  can be electronically programmed to be synchronized with the frame rates of the high-FOV low-resolution image stream and the low-FOV high-resolution image stream in the time-division multiplexing (for example as illustrated in  FIG.  42   ), so that the switchable reflector  23004  is in the reflective mode when the high-FOV low-resolution image stream arrives, and in the transmission mode when the low-FOV high-resolution image stream arrives. Thus, the high-FOV low-resolution image stream can be reflected by the switchable reflector  23004  along the first optical path toward the first ICG  21010 ; and the low-FOV high-resolution image stream can be transmitted by the switchable reflector  23004  along the second optical path toward the second ICG  21020 . 
     Alternatively, the switchable reflector  23004  can be replaced by a dichroic mirror configured to reflect light in a first set of wavelength ranges, and to transmit light in a second set of wavelength ranges. The image source  23002  can be configured to provide the high-FOV low-resolution image stream in the first set of wavelength ranges, and the low-FOV high-resolution image stream in the second set of wavelength ranges. For example, the first set of wavelength ranges can correspond to the red, green, and blue (RGB) colors, and the second set of wavelength ranges can correspond to the RGB colors in a different hue than that of the first set of wavelength ranges. In some embodiments, the high-FOV low-resolution image stream and the low-FOV high-resolution image stream are time-division multiplexed, for example as illustrated in  FIG.  42   . In some other embodiments, the high-FOV low-resolution image stream and the low-FOV high-resolution image stream are presented simultaneously. 
     VII. Polarization Multiplexing Scheme 
     In some embodiments, the high-FOV low-resolution image stream and the low-FOV high-resolution image stream can be polarization-division multiplexed. An image source can include a first set of RGB lasers for providing the high-FOV low-resolution image stream in a first polarization, and a second set of RGB lasers for providing the low-FOV high-resolution image stream in a second polarization different from the first polarization. For example, the high-FOV low-resolution image stream can be s-polarized, and the low-FOV high-resolution image stream can be p-polarized, or vice versa. Alternatively, the high-FOV low-resolution image stream can be left-handed circular polarized, and the low-FOV high-resolution image stream can be right-handed circular polarized, or vice versa. 
       FIG.  46    illustrates schematically a display system  25000  for projecting image streams to an eye of a user according to some embodiments. The display system  25000  may share some elements in common with the display system  8000  illustrated in  FIGS.  30 A- 30 B ; the description about those elements in relation to  FIGS.  30 A- 30 B  are applicable here as well. An image source  25002  can be configured to provide a high-FOV low-resolution image stream and a low-FOV and high-resolution image stream that are polarization-division multiplexed, as discussed above. 
     The display system  25000  can include a polarization beam splitter  25004  for separating light beams into a first light beam associated with the high-FOV low-resolution image stream propagating along a first optical path toward the first ICG  21010 , and a second light beam associated with the low-FOV high-resolution image stream propagating along a second optical path toward the second ICG  21020 . 
     The display system  25000  can also include a static polarization rotator  25020  positioned along one of the two optical paths, for example along the second optical path as illustrated in  FIG.  46   . The static polarization rotator  25020  can be configured to rotate the polarization of one of the low-FOV high-resolution image stream and the high-FOV low-resolution image stream, so that the two image streams may have substantially the same polarization as they enter the first ICG  21010  and the second ICG  21020 , respectively. This may be advantageous in cases where the first ICG  21010  and the second ICG  21020  are designed to have a higher diffraction efficiency for a certain polarization. The static polarization rotator  25020  can be, for example, a half-wave plate. 
     VIII. Optical Architectures for Incoupling Images Projected into Opposing Sides of the Eyepiece 
     In some embodiments, instead of having two ICGs laterally separated from each other (i.e., having separate pupils), a display system can be configured so that the high-FOV low-resolution image stream and the low-FOV high-resolution image stream are incident on opposing sides of the same ICG (i.e., having a single pupil). 
       FIG.  47    illustrates schematically a display system  26000  for projecting image streams to an eye of a user according to some embodiments. The display system  26000  can include a first image source  26002  configured to provide a high-FOV low-resolution image stream, and a second image source  26004  configured to provide a low-FOV high-resolution image stream. 
     The display system  26000  can also include a first optical lens (lens A) and a second optical lens (lens B) positioned along a first optical path of the high-FOV low-resolution image stream. In some embodiments, the combination of the first optical lens and the second optical lens can provide an angular magnification that is greater than unity for a first light beam associated with the high-FOV low-resolution image stream, thereby resulting in a wider FOV for the first light beam. 
     The display system  26000  also includes an eyepiece  26008  and an in-coupling grating (ICG)  26010  coupled to the eyepiece  26008 . The eyepiece  26008  can be a waveguide plate configured to propagate light therein. The ICG  26010  can be a diffractive optical element configured to diffract a portion of the light incident thereon into the eyepiece  26008 . As the first light beam associated with the high-FOV low-resolution image stream is incident on a first surface  26010 - 1  of the ICG  26010 , a portion of the first light beam is diffracted into the eyepiece  26008  in a reflection mode (e.g., a first order reflection), which may then be subsequently propagated through the eyepiece  26008  and be out-coupled toward an eye of a user. 
     The display system  26000  can also include a third optical lens (lens C) and a fourth optical lens (lens D) positioned along a second optical path of the low-FOV high-resolution image stream. In some embodiments, the combination of the third optical lens and the fourth optical lens can provide an angular magnification that is equal substantially to unity or less than unity for a second light beam associated with the low-FOV high-resolution image stream. Thus, the second light beam may have a narrower FOV than that of the first light beam. 
     The display system  26000  can further include a foveal tracker  26006 , such as a scanning mirror (e.g., a MEMs mirror), that can be controlled based on the fixation position of the user&#39;s eye for dynamically projecting the second light beam associated with the low-FOV and high-resolution image stream. 
     The second light beam associated with the low-FOV high-resolution image stream may be incident on the second surface  26010 - 1  of the ICG  26010  opposite the first surface  26010 - 2 . A portion of the second light beam can be diffracted into the eyepiece  2408  in a transmission mode (e.g., a first order transmission), which may then be subsequently propagated through the eyepiece  26008  and be out-coupled toward the eye of the user. 
     As described above, the display system  26000  uses a single ICG  26010 , instead of two separate ICGs as illustrated in  FIGS.  43 - 46   . This can simplify the design of the eyepiece. 
       FIG.  48    illustrates schematically a display system  27000  for projecting image streams to an eye of a user according to some embodiments. The display system  27000  may share some elements in common with the display system  8000  illustrated in  FIGS.  30 A- 30 B ; the description about those elements in relation to  FIGS.  30 A- 30 B  are applicable here as well. The display system  27000  can include an image source  27002  configured to provide a high-FOV low-resolution image stream and a low-FOV and high-resolution image stream that are time-division multiplexed. In some embodiments, the image source  27002  can take the form of a pico projector. 
     The display system  27000  can include a polarizer  27010  positioned downstream from the image source  27002  and configured to convert the high-FOV low-resolution image stream and the low-FOV and high-resolution image stream from an unpolarized state into a polarized state, such as S-polarized and P-polarized, or RHCP and LHCP polarized. 
     The display system  27000  can further include a switchable polarization rotator  27020  positioned downstream from the polarizer  27010 . The operation of the switchable polarization rotator  27020  can be electronically programmed to be synchronized with the frame rates of the high-FOV low-resolution image stream and the low-FOV high-resolution image stream in the time-division multiplexing, so that the switchable polarization rotator  27020  does not rotate (or rotates by a very small amount) the polarization of the high-FOV low-resolution image stream, and rotates the polarization of the low-FOV high-resolution image stream by about 90 degrees (i.e., introducing a phase shift of π), or vice versa. Therefore, after passing through the switchable polarization rotator  27020 , the polarization of the high-FOV low-resolution image stream may be orthogonal to the polarization of the low-FOV high-resolution image stream. For example, the high-FOV low-resolution image stream can be s-polarized, and the low-FOV high-resolution image stream can be p-polarized, or vice versa. In other embodiments, the high-FOV low-resolution image stream can be left-handed circular polarized, and the low-FOV high-resolution image stream can be a right-handed circular polarized, or vice versa. 
     The display system  27000  further includes a polarization beam splitter  27004  configured to reflect the high-FOV low-resolution image stream along a first optical path, and to transmit the low-FOV high-resolution image stream along a second optical path. 
     The display system  27000  can further include a first optical lens (lens A) positioned in in front of the polarization beam splitter  27004 , a second optical lens (lens B) positioned downstream from the polarization beam splitter  27004  along the first optical path, and a third optical lens (lens C) positioned downstream from the beam splitter  27004  along the second optical path. In some embodiments, as described above in relation to  FIGS.  30 A- 30 B and  31 A- 31 C , the combination of the first optical lens (lens A) and the second optical lens (lens B) can provide an angular magnification for the high-FOV low-resolution image stream that is greater than unity; and the combination of the first optical lens (lens A) and the third optical lens (lens C) can provide an angular magnification for the low-FOV high-resolution image stream that equals substantially to unity or less than unity. Thus, the high-FOV low-resolution image stream may be projected to an eye of a user with a wider FOV than that projected by the low-FOV high-resolution image stream. 
     The display system  27000  can further include a foveal tracker  27006 , such as a scanning mirror (e.g., a MEMs mirror), that can be controlled based on the fixation position of the user&#39;s eye for dynamically projecting the second light beam associated with the low-FOV and high-resolution image stream. 
     The display system  27000  can further include an eyepiece  27008  and an in-coupling grating (ICG)  27050  coupled to the eyepiece  27008 . The eyepiece  27008  can be a waveguide plate configured to propagate light therein. The ICG  27050  can be a diffractive optical element configured to diffract a portion of the light incident thereon into the eyepiece  27008 . 
     The display system  27000  can further include a first reflector  27030  positioned downstream from the second optical lens (lens B) along the first optical path. The first reflector  27030  can be configured to reflect the high-FOV low-resolution image stream toward the ICG  27050 . As a first light beam associated with the high-FOV low-resolution image stream is incident on a first surface  27050 - 1  of the ICG  27050 , a portion of the first light beam is diffracted into the eyepiece  27008  in a transmission mode (e.g., a first order transmission), which may subsequently propagate through the eyepiece  27008  and be out-coupled toward an eye of a user. 
     The display system  27000  can further include a second reflector  27040  positioned downstream from the foveal tracker  27006  along the second optical path. The second reflector  27040  can be configured to reflect the low-FOV high-resolution image stream toward the ICG  27050 . As a second light beam associated with the low-FOV high-resolution image stream is incident on a second surface  27050 - 2  of the ICG  27050  opposite to the first surface  27050 - 1 , a portion of the second light beam is diffracted into the eyepiece  27008  in a reflective mode (e.g., a first order reflection), which may subsequently propagate through the eyepiece  27008  and be out-coupled toward the eye of the user. 
       FIG.  49    illustrates schematically a display system  28000  for projecting image streams to an eye of a user according to some embodiments. The display system  28000  is similar to the display system  27000 , except that it does not include an ICG. Instead, the display system  28000  includes a first in-coupling prism  28030  (in place of the first reflector  27030  in the display system  27000 ) for coupling the high-FOV low-resolution image stream into the eyepiece  27008 , and a second in-coupling prism  28040  (in place of the second reflector  27040  in the display system  27000 ) for coupling the low-FOV high-resolution image stream into the eyepiece  27008 . The index of refraction of the first in-coupling prism  28030  and the index of refraction of the second in-coupling prism  28040  can be suitably selected with respect to the index of refraction of the eyepiece  27008 , so that a fraction of the power contained in a first light beam associated with the high-FOV low-resolution image stream and a fraction of the power contained in a second light beam associated with the low-FOV high-resolution image stream are coupled into the eyepiece  27008  by the first in-coupling prism  28030  and the second in-coupling prism  28040 , respectively. 
     IX. High Field of View and High Resolution Foveated Display Using Overlapping Optical Paths 
     In some embodiments, a display system may be configured so that the high-FOV low-resolution image stream and the low-FOV high-resolution image stream are provided to an eyepiece without utilizing a PBS to separate a composite image stream into two image streams that propagate in different directions. Rather, the high-FOV low-resolution image stream and the low-FOV high-resolution image stream may take substantially the same path from an image source to the eyepiece, which may obviate the PBS. This may have advantages for providing a compact form factor for the display system. 
       FIG.  50    illustrates schematically a display system  50000  for projecting image streams to an eye of a user. The display system  50000  may include an image source  50002  configured to provide modulated light containing image information. In some embodiments, the image source  50002  may provide a first image stream that is used to present high-FOV low-resolution imagery and a second image stream that is used to present low-FOV high-resolution image stream in a time-multiplexed manner, such as by interleaving frames from the first image stream with frames of the second stream. 
     The display system  50000  may also include variable optics  50004 . In some embodiments, the variable optics  50004  may provide a different angular magnification for light rays  50030  associated with the high-FOV low-resolution image stream than for light rays  50020  associated with the low-FOV high-resolution image stream, thereby enabling projection of the high-FOV low-resolution image stream out of the waveguide  50010  to provide a wider FOV than that projected by the low-FOV high-resolution image stream. It will be appreciated that the range of angles at which in-coupled light is incident on the ICG  50006  is preferably preserved upon the out-coupling of that light from the waveguide  50010 . Thus, in-coupled light incident on the ICG  50006  at a wide range of angles also propagates away from the waveguide  50010  at a wide range of angles upon being out-coupled, thereby providing a high FOV and more angular magnification. Conversely, light incident on the ICG  50006  at a comparatively narrow range of angles also propagates away from the waveguide  50010  at a narrow range of angles upon being out-coupled, thereby providing a low FOV and low angular magnification. 
     Additionally, to select the appropriate level of angular magnification, variable optics  50004  may alter light associated with the high-FOV low-resolution image stream so that it has a different optical property then light associated with the low-FOV high-resolution image stream. Preferably, the function of the variable optics  50004  and the properties of light of each image stream are matched such that changing the relevant property of the light changes the optical power and focal length provided by the variable optics  50004 . For example, the high-FOV low-resolution image stream may have a first polarization and the low-FOV low-resolution image stream may have a second polarization. Preferably, the variable optics  50004  is configured to provide different optical power and different focal lengths for different polarizations of light propagating through it, such that the desired optical power may be selected by providing light of a particular, associated polarization. The first polarization may be a right hand circular polarization (RHCP), a left hand circular polarization (LFCP), S-polarization, P-polarization, another polarization type, or un-polarized. The second polarization may be a right hand circular polarization (RHCP), a left hand circular polarization (LFCP), S-polarization, P-polarization, another polarization type, or un-polarized, so long as it is different from the first polarization. In some preferred embodiments, the first polarization is one of a right hand circular polarization (RHCP) and a left hand circular polarization (LFCP), and the second polarization is the other of the left hand circular polarization (LFCP) and right hand circular polarization (RHCP). 
     In some embodiments, the operation of the variable optics  50004  may be electronically programmed to be synchronized with the frame rates of the high-FOV low-resolution image stream and the low-FOV high-resolution image stream in the time-division multiplexing. In some embodiments, the image frames of the high-FOV stream are given their desired polarization and angular magnification to couple to waveguide  50010  via ICG  50006  while interleaved frames of the low-FOV stream are given their desired magnification and polarization to initially pass through ICG  50006 , be passed to mirror  50008 , be targeted to the user&#39;s fixation point, and then be coupled to waveguide  50010  via ICG  50006 . 
     The display system  50000  also includes an eyepiece  50010  and a polarization-sensitive in-coupling grating (ICG)  50006  coupled to the eyepiece  50010 . The eyepiece  50010  may be a waveguide, e.g., a plate, configured to propagate light therein, e.g., by total internal reflection. The polarization-sensitive ICG  50006  may be a polarization-sensitive diffractive optical element configured to diffract a portion of the light incident thereon into the eyepiece  50010 . In some embodiments, the ICG  50006  may be polarization-sensitive in that incident light having a particular polarization is preferentially diffracted into the eyepiece  50010 , while incident light of at least one other polarization passes through the ICG  50006 . Light that passes through the ICG  50006  without coupling into the eye piece  50010  may be directed towards mirror  50008 , which may be a MEMS mirror, and which may be configured to switch the polarization of incident light. As a first example, the polarization-sensitive ICG  50006  may couple light having a right-hand circular polarization (RHCP) into the waveguide, while passing light having a left-hand circular polarization (LHCP) through towards mirror  50008 . As a second example, polarization-sensitive ICG  50006  may couple light having a LHCP into the waveguide, while passing light having a RHCP through towards mirror  50008 . 
     In at least some embodiments, light reflected off of mirror  50008  may be directed towards ICG  50006 . Additionally, the reflection of the light off mirror  50008  may alter the polarization of the light (e.g., flip the polarization of the light from RHCP to LHCP and vice versa) such that the reflected light has the desired polarization to be diffracted by ICG  50006  and coupled into eye piece  50010 . As an example, if ICG  50006  is configured to couple light having a RHCP into eye piece  50010 , then light associated with the high FOV stream may be given a RHCP by variable optics  50004  and then coupled into eye piece  50010 . In such an example, light associated with the low FOV stream may be given a LHCP by variable optics  50004 , such that the LHCP light may then pass through ICG  50006  without coupling into eyepiece  50001  and instead may be directed towards mirror  50008 . Reflection of the LHCP light off of the mirror  50008  may flip the polarization of the light to RHCP. Then, when the now-RHCP light hits ICG  50006 , it may be coupled by ICG  50006  into eye piece  50010 . Similar examples apply when ICG  50006  is configured to couple LHCP into eye piece  50010 . 
     As disclosed herein, mirror  50008  may be a movable mirror, e.g., a scanning mirror, and may function as a fovea tracker. As also discussed herein, the mirror  50008  may be controlled and moved/tilted based on the determined fixation position of the user&#39;s eye. The tilting of the mirror  50008  may cause the reflected light to in-couple into the waveguide  500010  at different locations, thereby causing light to also out-couple at different locations corresponding to the location of the fovea of the user&#39;s eye. 
     With continued reference to  FIG.  50   , the light source  50002  may produce a high-FOV low-resolution (HFLR) image stream and a low-FOV high-resolution (LFHR) image stream in a time-multiplexed manner. Additionally, the variable optics  50004  may alter the HFLR image stream to have a particular polarization (such as RHCP) (and associated angular magnification) so that the HFLR image stream is coupled into waveguide  50010  by polarization-sensitive ICG  50006 . The variable optics may alter the LFHR image stream to have a different polarization (such as LHCP) and associated angular magnification. As a result, the LFHR image stream passes through polarization-sensitive ICG  50006 , reflects off of mirror  50008  (flipping the polarization to RHCP and targeting the LFHR images to a user&#39;s fixation position), and is then coupled into waveguide  50010  by ICG  50006 . 
     Optionally at least one device for switching the polarization state of the light maybe inserted in the optical path between the image source  50002  and the ICG  50006 . 
       FIG.  51    illustrates an example of an implementation of variable optics  50004 . As shown in  FIG.  51   , variable optics  50004  may be formed from polarizer  50012 , switchable quarter wave plate (QWP)  50013 , lens  50014 , diffractive waveplate lens  50015 , diffractive waveplate lens  50016 , and lens  500017 . This is merely one possible implementation of variable optics  50004 . 
     The polarizer  50012  may be configured to convert the high-FOV low-resolution image stream and the low-FOV high-resolution image stream from light source  50002  from an unpolarized state into a polarized state, such as S-polarized and P-polarized, or RHCP and LHCP polarized. 
     The switchable QWP  50013  may be configured to convert the polarized light from polarizer  50012  into either (1) a right-hand circular polarization (RHCP) or (2) a left-hand circular polarization (LHCP). 
     After exiting the QWP  50013 , the light may be incident on lens  50014  and diffractive waveplate lens  50015 . The diffractive waveplate lens  50015  may be a geometric phase lens including patternwise aligned liquid crystal material. Diffractive waveplate lens  50015  may have a positive optical power (e.g., be a positive lens) for circularly polarized light that has a handedness (RH or LH) that matches their handedness and may have a negative optical power (e.g., be a negative lens) for circularly polarized light of opposite handedness. Diffractive waveplate lens  50015  may also have the property that it reverses the handedness of circularly polarized light. Thus, if diffractive waveplate lens  50015  is right-handed and receives RHCP light from lens  500014 , the diffractive waveplate lens  50015  would act as a positive lens and the light would be left-handed after passing through diffractive waveplate lens  50015 . 
     After exiting the diffractive waveplate lens  50015 , the light will be incident on diffractive waveplate lens  50016  and then lens  50017 . Diffractive waveplate lens  50016  may operate in a manner similar to that of diffractive waveplate lens  50015 . Additionally, the handedness of diffractive waveplate lens  50016  may match that of diffractive waveplate lens  50015 , at least in some embodiments. With such an arrangement, the optical power of the diffractive waveplate lens  50016  will be opposite that of diffractive waveplate lens  50015 . Thus, in an example in which the switchable QWP  50013  provides light with a polarization matching diffractive waveplate lens  50015 , lens  50015  will have a positive optical power and will also reverse the handedness of the light. Then, when the subsequent diffractive waveplate lens  50016  receives the light, lens  50015  will have a negative optical power, as it receives the light after its handedness was reversed. 
     With an arrangement of the type shown in  FIG.  51   , the variable optics  50004  may provide a first angular magnification when the switchable QWP  50013  provides light matching the handedness of diffractive waveplate lens  50015  (e.g., such that lens  50015  provides a positive optical power, while lens  50016  provides a negative optical power) and may provide a second angular magnification when the switchable QWP  50013  provides light of opposite handedness (e.g., such that lens  50015  provides a negative optical power, while lens  50016  provides a positive optical power). In other embodiments, the handedness of the two diffractive waveplate lens  50015  and  50016  may be different. 
     With reference now to  FIGS.  52 A- 52 B , additional details regarding example ICG configurations are provided. For example, it will be appreciated that polarization sensitive ICG&#39;s may preferentially direct light in a particular lateral direction depending upon which side of the ICG the light is incident. For example, with reference to  FIG.  52 A , light incident on ICG  50006  from below is redirected to the left of the page. However, light incident on ICG  50006  from above would be undesirably directed towards the right of the page, away from the area of the waveguide from which light is out coupled to a viewer. In some embodiments, in order to in-couple light such that it propagates in the desired direction, different ICG&#39;s may be used for light incident from different directions or sides of the waveguide  50010 . 
     For example, in some embodiments, the display system may be configured so that the high-FOV low-resolution image stream and the low-FOV high-resolution image stream are coupled into waveguide  50010  (which may be an eyepiece) using a pair of a polarization-sensitive in-coupling gratings (ICG)  50006  and  50040 . Such an arrangement may be beneficial where, e.g., light that strikes an ICG from below (in the perspective of  FIGS.  50 - 53 B ) is coupled into the waveguide  50010  in a desired lateral direction (to the left), while light that strikes the ICG from above is coupled into the waveguide  50010  in the opposite direction (to the right). More details about in-coupling gratings (ICG) gratings are described in U.S. patent application Ser. No. 15/902,927, the contents of which are hereby expressly and fully incorporated by reference in their entirety, as though set forth in full. 
       FIGS.  52 A- 52 B  illustrate schematically a display system  52000  for projecting image streams to an eye of a user according to some embodiments of the present invention, which may include two ICGs  50006  and  50040 . In some embodiments, ICGs  50006  and  50040  may both be configured to couple light of the same polarization-type into waveguide  50010 . As an example, ICGs  50006  and  50040  may each couple light having a left-hand circular polarization (LHCP) into waveguide  50010 , while passing light having a right-hand circular polarization (RHCP). Alternatively, the polarizations may be swapped. 
     As shown in  FIG.  52 A , optical elements such as those shown in  FIGS.  50 - 51    may provide a high FOV low resolution image stream  50030  having a left-handed circular polarization (LHCP). The light  50030  may be incident upon ICG  50006 . Since the light  50030  is LHCP and the ICG  50006  is configured to couple LHCP light into waveguide  50010 , the light is coupled by ICG  50006  into the waveguide  50010 . 
     As shown in  FIG.  52 B , optical elements such as those shown in  FIGS.  50 - 51    may provide a low FOV high resolution image stream  50020  (which may be interleaved with the image stream of  FIG.  52 A  in a time-multiplexed manner) having a right-handed circular polarization (RHCP). The light  50020  may be incident upon ICG  50006 . However, since the light  50020  is RHCP and the ICG  50006  is configured to couple only LHCP light into waveguide  50010 , the light  50020  passes through ICG  50006 . ICG  50040  may, similarly, be configured to couple only LHCP light into waveguide  50010 , thus the light may also pass through ICG  50040 . After passing through both ICGs, the light  50020  may be incident on movable mirror  50008 , which may be in a particular orientation based upon a user&#39;s fixation point (as discussed herein in various sections). After reflecting off of mirror  50008 , the polarization of the light  50020  may be flipped, so the light is now LHCP. Then, the light  50020  may be incident on ICG  50040 , which may couple the now-LHCP light  50020  into the waveguide  50010 . 
     In some embodiments, the display system may be configured so that the high-FOV low-resolution image stream and the low-FOV high-resolution image stream are formed by light having the same polarization. As a result, both image streams may be in coupled by the same ICG, upon being incident on the same side of that ICG. 
       FIGS.  53 A- 53 B  illustrate schematically a display system  53000  for projecting image streams to an eye of a user according to some embodiments of the present invention, which may include a single ICG  50006  and a switchable reflector  50042 . The switchable reflector  50042  may be a liquid-crystal based planar device that switches between a substantially transparent state and a substantially reflective state at a sufficiently high rate; that is, the switching rate of the switchable reflector  50042  is preferably sufficiently high to allow coordination with interleaved frames of the high-FOV low-resolution image stream and the low-FOV high-resolution image stream. For example, the switchable reflector  50042  is preferably able to switch between reflective and transmissive states at at least the same rate as the high and low-FOV resolution image streams are switched. 
     As shown in  FIG.  53 A , the ICG  50006  may receive a high FOV low resolution image stream  50030  from optical elements such as those shown in  FIGS.  50 - 51   . As an example, the image stream may have a left-handed circular polarization (LHCP). The light of the image stream  50030  may be incident upon ICG  50006 . However, ICG  50006  may be configured to couple RHCP light and pass LHCP light. Thus, the LHCP light  50030  may pass through ICG  50006 . The light may then be incident on switchable reflector  50042 , which may be configured in its reflective state (while the system is projecting high FOV low resolution image stream  50030 ). Thus, the light of the image stream  50030  may reflect off of switchable reflector  50042 , thereby reversing the handedness of its polarization. After reflecting off of switchable reflector  50042 , the  50030  light may be incident again upon ICG  50006 , and ICG  50006  may couple the now-RHCP light  50030  into the waveguide  50010 . 
     As shown in  FIG.  53 B , optical elements such as those shown in  FIGS.  50 - 51    may provide a low FOV high resolution image stream  50020  having a left-handed circular polarization (LHCP). This arrangement differs slightly, in that the polarization of the low FOV image stream  50020  matches the polarization of the high FOV image stream  50030 . Such an arrangement may be achieved using a modification of the variable optics  50004  shown in  FIGS.  50 - 51   . As an example, an additional polarizer, e.g., a switchable polarizer, and may be provided between lens  50017  and ICG  50006 . 
     Returning to the low FOV high-resolution LHCP light  50020  in  FIG.  53 B , the light  50020  is incident upon ICG  50006 . However, ICG  50006  is configured to couple RHCP into waveguide  50010 . Thus, the light  50020  passes through ICG  50006 . The light  50020  is next incident upon the switchable reflector  50042 , which may be configured to be in its transparent state (while the system is projecting low FOV high resolution light  50020 ). Thus the light may pass through switchable reflector  50042  and be incident upon mirror  50008  and, optionally, be targeted by mirror  50008  on a user&#39;s fixation point (as discussed herein in various sections). After reflecting off of mirror  50008 , the polarization of the light  50020  may be flipped, so the light is now RHCP. Then, the light  50020  may be incident on ICG  50006 , which may couple the now-RHCP light  50020  into the waveguide  50010 . It will be appreciated that the mirror  50008  may be configured to provide fovea tracking and/or may be sufficiently spaced from the ICG  50006  to account for the different focal length of the wearable optics  50004  ( FIGS.  50 - 51   ), to provide a focused image. 
     The various aspects, embodiments, implementations or features of the described embodiments can be used separately or in any combination. Various aspects of the described embodiments can be implemented by software, hardware or a combination of hardware and software. The described embodiments can also be embodied as computer readable code on a computer readable medium for controlling manufacturing operations or as computer readable code on a computer readable medium for controlling a manufacturing line. The computer readable medium is any data storage device that can store data, which can thereafter be read by a computer system. Examples of the computer readable medium include read-only memory, random-access memory, CD-ROMs, HDDs, DVDs, magnetic tape, and optical data storage devices. The computer readable medium can also be distributed over network-coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of specific embodiments are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the described embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings. 
     It will also be appreciated that each of the processes, methods, and algorithms described herein and/or depicted in the figures may be embodied in, and fully or partially automated by, code modules executed by one or more physical computing systems, hardware computer processors, application-specific circuitry, and/or electronic hardware configured to execute specific and particular computer instructions. For example, computing systems may include general purpose computers (e.g., servers) programmed with specific computer instructions or special purpose computers, special purpose circuitry, and so forth. A code module may be compiled and linked into an executable program, installed in a dynamic link library, or may be written in an interpreted programming language. In some embodiments, particular operations and methods may be performed by circuitry that is specific to a given function. 
     Further, certain embodiments of the functionality of the present disclosure are sufficiently mathematically, computationally, or technically complex that application-specific hardware or one or more physical computing devices (utilizing appropriate specialized executable instructions) may be necessary to perform the functionality, for example, due to the volume or complexity of the calculations involved or to provide results substantially in real-time. For example, a video may include many frames, with each frame having millions of pixels, and specifically programmed computer hardware is necessary to process the video data to provide a desired image processing task or application in a commercially reasonable amount of time. 
     Code modules or any type of data may be stored on any type of non-transitory computer-readable medium, such as physical computer storage including hard drives, solid state memory, random access memory (RAM), read only memory (ROM), optical disc, volatile or non-volatile storage, combinations of the same and/or the like. In some embodiments, the non-transitory computer-readable medium may be part of one or more of the local processing and data module ( 140 ), the remote processing module ( 150 ), and remote data repository ( 160 ). The methods and modules (or data) may also be transmitted as generated data signals (e.g., as part of a carrier wave or other analog or digital propagated signal) on a variety of computer-readable transmission mediums, including wireless-based and wired/cable-based mediums, and may take a variety of forms (e.g., as part of a single or multiplexed analog signal, or as multiple discrete digital packets or frames). The results of the disclosed processes or process steps may be stored, persistently or otherwise, in any type of non-transitory, tangible computer storage or may be communicated via a computer-readable transmission medium. 
     Any processes, blocks, states, steps, or functionalities in flow diagrams described herein and/or depicted in the attached figures should be understood as potentially representing code modules, segments, or portions of code which include one or more executable instructions for implementing specific functions (e.g., logical or arithmetical) or steps in the process. The various processes, blocks, states, steps, or functionalities may be combined, rearranged, added to, deleted from, modified, or otherwise changed from the illustrative examples provided herein. In some embodiments, additional or different computing systems or code modules may perform some or all of the functionalities described herein. The methods and processes described herein are also not limited to any particular sequence, and the blocks, steps, or states relating thereto may be performed in other sequences that are appropriate, for example, in serial, in parallel, or in some other manner. Tasks or events may be added to or removed from the disclosed example embodiments. Moreover, the separation of various system components in the embodiments described herein is for illustrative purposes and should not be understood as requiring such separation in all embodiments. It should be understood that the described program components, methods, and systems may generally be integrated together in a single computer product or packaged into multiple computer products. 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense. 
     Indeed, it will be appreciated that the systems and methods of the disclosure each have several innovative aspects, no single one of which is solely responsible or required for the desirable attributes disclosed herein. The various features and processes described above may be used independently of one another, or may be combined in various ways. All possible combinations and subcombinations are intended to fall within the scope of this disclosure. 
     Certain features that are described in this specification in the context of separate embodiments also may be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment also may be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination may in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. No single feature or group of features is necessary or indispensable to each and every embodiment. 
     It will be appreciated that conditional language used herein, such as, among others, “can,” “could,” “might,” “may,” “e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular embodiment. The terms “comprising,” “including,” “having,” and the like are synonymous and are used inclusively, in an open-ended fashion, and do not exclude additional elements, features, acts, operations, and so forth. Also, the term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elements in the list. In addition, the articles “a,” “an,” and “the” as used in this application and the appended claims are to be construed to mean “one or more” or “at least one” unless specified otherwise. Similarly, while operations may be depicted in the drawings in a particular order, it is to be recognized that such operations need not be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Further, the drawings may schematically depict one more example processes in the form of a flowchart. However, other operations that are not depicted may be incorporated in the example methods and processes that are schematically illustrated. For example, one or more additional operations may be performed before, after, simultaneously, or between any of the illustrated operations. Additionally, the operations may be rearranged or reordered in other embodiments. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems may generally be integrated together in a single software product or packaged into multiple software products. Additionally, other embodiments are within the scope of the following claims. In some cases, the actions recited in the claims may be performed in a different order and still achieve desirable results. 
     Accordingly, the claims are not intended to be limited to the embodiments shown herein, but are to be accorded the widest scope consistent with this disclosure, the principles and the novel features disclosed herein.