Patent Publication Number: US-2023152597-A1

Title: Display systems and methods for determining registration between a display and eyes of a user

Description:
PRIORITY CLAIM 
     This application is a continuation of U.S. patent application Ser. No. 17/255,315, entitled “DISPLAY SYSTEMS AND METHODS FOR DETERMINING REGISTRATION BETWEEN A DISPLAY AND EYES OF A USER”, filed Dec. 22, 2020, which is a 371 of international PCT App. PCT/US2019/043096, entitled “DISPLAY SYSTEMS AND METHODS FOR DETERMINING REGISTRATION BETWEEN A DISPLAY AND EYES OF A USER”, filed Jul. 23, 2019, which claims priority to: U.S. Patent Prov. App. 62/702866, entitled “DISPLAY SYSTEMS AND METHODS FOR DETERMINING REGISTRATION BETWEEN A DISPLAY AND EYES OF A USER” and filed on Jul. 24, 2018, which is incorporated herein by reference in its entirety. 
     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. Patent Publication No. 2016/0270656; U.S. Patent Publication No. 2015/0178939, published Jun. 25, 2015; U.S. Patent Publication No. 2015/0016777; U.S. patent application Ser. No. 15/274,823; U.S. patent application Ser. No. 15/296,869; U.S. patent application Ser. No. 15/717,747, filed Sep. 27, 2017; U.S. patent application Ser. No. 15/497,726, filed Apr. 26, 2017; U.S. Patent Publication No. 2017/0053165, published Feb. 23, 2017; U.S. Patent Publication No. 2017/0053166, published Feb. 23, 2017; U.S. application Ser. No. 15/341,760, filed on Nov. 2, 2016, published on May 4, 2017 as U.S. Publication No. 2017/0122725; U.S. application Ser. No. 15/341,822, filed on Nov. 2, 2016, published on May 4, 2017 as U.S. Publication No. 2017/0124928; U.S. Provisional Patent Application No. 62/618,559, filed Jan. 17, 2018; U.S. Provisional Patent Application No. 62/642,761, filed Mar. 14, 2018; and U.S. Provisional Patent Application No. 62/644,321, filed Mar. 16, 2018. 
    
    
     FIELD 
     The present disclosure relates to display systems, including virtual reality and augmented reality display systems, and, more particularly, to systems and methods for evaluating fit of a display on a user. 
     BACKGROUND 
     Modern computing and display technologies have facilitated the development of systems for so called “virtual reality”, “augmented reality”, or “mixed reality” experiences, wherein 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 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”, related to merging real and virtual worlds to produce new environments where physical and virtual objects co-exist and interact in real time. As it turns out, the human visual perception system is very complex, and producing a VR, AR, or MR technology that facilitates a comfortable, natural-feeling, rich presentation of virtual image elements amongst other virtual or real-world imagery elements is challenging. Systems and methods disclosed herein address various challenges related to VR, AR and MR technology. 
     SUMMARY 
     In some embodiments, a display system is configured to project light to an eye of a user to display virtual image content. The display system comprises: a frame configured to be supported on a head of the user, a head-mounted display disposed on the frame, one or more eye-tracking cameras configured to image an eye of the user, and processing electronics in communication with the head-mounted display and the one or more eye-tracking cameras. The display is configured to project light into the user&#39;s eye to display virtual image content with different amounts of wavefront divergence to present virtual image content appearing to be located at different depths at different periods of time. The processing electronics is configured to: determine whether the head-mounted display is properly registered to the eye of the user by determining whether imaged features of the eye are within a predetermined range of vertical positions relative to the head-mounted display; and provide feedback to the user if the head-mounted display is not properly adjusted to fit the user. 
     In some other embodiments, a method is provided for evaluating registration of virtual image content from a head-mounted display system by a user&#39;s eye. The method comprises imaging the eye, determining whether imaged features of the eye are within a predetermined range of vertical positions relative to a head-mounted display, and providing a notification based on a determined position of the imaged features. The notification indicates at least that the head-mounted display and the eye are not properly registered. 
     Additional examples of embodiments are enumerated below. 
     Example 1. A display system configured to project light to an eye of a user to display virtual image content, the display system comprising:
         a frame configured to be supported on a head of the user;   a head-mounted display disposed on the frame, the display configured to project light into the user&#39;s eye to display virtual image content with different amounts of wavefront divergence to present virtual image content appearing to be located at different depths at different periods of time;   one or more eye-tracking cameras configured to image an eye of the user; and   processing electronics in communication with the head-mounted display and the one or more eye-tracking cameras, the processing electronics configured to:
           determine whether the head-mounted display is properly registered to the eye of the user by determining whether imaged features of the eye are within a predetermined range of vertical positions relative to the head-mounted display; and   provide feedback to the user if the head-mounted display is not properly adjusted to fit the user.   
               

     Example 2. The display system of Example 1, wherein the one or more eye-tracking cameras are configured to image a left eye of the user and a right eye of the user,
         wherein the processing electronics are further configured to determine a left eye tracking confidence score that indicates a confidence level in a position of the left eye of the user and to determine a right eye tracking confidence score that indicates a confidence level in a position of the right eye of the user, and   wherein, when one of the confidence scores is greater than the other, the processing electronics are further configured to determine whether the head-mounted display is properly registered based on the left eye or right eye of the user associated with the greater confidence score.       

     Example 3. The display system of Example 1, wherein the one or more eye-tracking cameras are configured to image a left eye of the user and a right eye of the user and
         wherein, when the left eye and right eye of the user are vertically offset from each other but by less than a first predetermined threshold, the processing electronics are further configured to determine whether the head-mounted display is properly registered based on a position of the left eye and right eye of the user that is furthest from a desired vertical position.       

     Example 4. The display system of Example 3, wherein, when the left eye and right eye of the user are vertically offset from each other by less than a second predetermined threshold that is less than the first predetermined threshold, the processing electronics are further configured to determine whether the head-mounted display is properly registered based on an average position of the left eye and right eye of the user. 
     Example 5. The display system of Example 4, wherein, when the left eye and right eye of the userare vertically offset from each other by more than the first predetermined threshold, the processing electronics are further configured to determine whether the head-mounted display is properly registered based on the average position of the left eye and right eye of the user. 
     Example 6. The display system of Example 1, further comprising at least one interchangeable fit piece removably mounted to the frame and configured to adjust a fit of the frame. 
     Example 7. The display system of Example 6, wherein the interchangeable fit piece comprises an interchangeable nose bridge configured to adjust the fit of the frame between the frame and a nose bridge of the user. 
     Example 8. The display system of Example 6, wherein the interchangeable fit piece comprises an interchangeable forehead pad configured to adjust the fit of the frame between the frame and a forehead of the user. 
     Example 9. The display system of any of Example 6, wherein the interchangeable fit piece comprises an interchangeable back pad configured to adjust the fit of the frame between the frame and a back of the head of the user. 
     Example 10. The display system of any of Example 1, wherein the providing feedback to the user if the head-mounted display is not properly adjusted to fit the user comprises providing a suggestion to the user to swap out a currently-installed interchangeable fit piece for another interchangeable fit piece. 
     Example 11. A method for evaluating registration of virtual image content from a head-mounted display system by a user&#39;s eye, the method comprising:
         imaging the eye;   determining whether imaged features of the eye are within a predetermined range of vertical positions relative to a head-mounted display; and   providing a notification based on a determined position of the imaged features, where the notification indicates at least that the head-mounted display and the eye are not properly registered.       

     Example 12. The method of Example 11, wherein determining whether the imaged features of the eye are within a predetermined range of vertical positions comprises determining positions of glints of the eye. 
     Example 13. The method of Example 12, further comprising determining a position of a pupil of the eye based upon the glints of the eye. 
     Example 14. The method of Example 11, wherein the head-mounted display system is configured to project light into the eye to display virtual image content in the field of view of the user, and wherein providing the notification comprises displaying the notification as virtual image content. 
     Example 15. The method of Example 11, further comprising automatically tracking a pupil of the eye over time and notifying the user when a center of rotation of the eye moves outside of the predetermined range of vertical positions. 
     Example 16. The method of Example 11, further comprising changing from a first field of view of the head-mounted display system to a second field of view of the head-mounted display system when the position of the eye is outside a display registration volume,
         wherein the head-mounted display system comprises at least one display having the first field of view when the position of the eye is inside the display registration volume, wherein the at least one display has the second field of view when the position of the eye is outside the display registration volume, and wherein the second field of view is smaller than the first field of view.       

     Example 17. The method of Example 11, wherein the head-mounted display system comprises at least one interchangeable fit piece, wherein providing the notification comprises indicating that the wearable system is not properly fitted to the user and suggesting or instructing the user to replace a currently-installed interchangeable fit piece with an alternative interchangeable fit piece. 
     Example 18. The method of Example 17, wherein the at least one interchangeable fit piece comprises at least one fit piece selected from the group consisting of: a nose bridge pad, a forehead pad, and a back pad that goes between the wearable system and a back of a user&#39;s head. 
     Example 19. The method of Example 18, wherein the at least one interchangeable fit piece comprises at least one interchangeable nose bridge pad, and further comprising determining that the head-mounted display is too low with respect to the eye, and wherein providing the notification to the user further comprises prompting the user to install a larger nose bridge pad. 
     Example 20. The method of Example 11, further comprising:
         identifying a plurality of pixels of a display of the head-mounted display system that the user is expected to perceive as dimmed as a result of the first position of the eye being outside a display registration volume; and   boosting brightness of the plurality of pixels of the display relative to other pixels in the display to mitigate the expected dimming.       

     Details of one or more implementations of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages will become apparent from the description, the drawings, and the claims. Neither this summary nor the following detailed description purports to define or limit the scope of the inventive subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    depicts an illustration of a mixed reality scenario with certain virtual reality objects, and certain physical objects viewed by a person. 
         FIG.  2    schematically illustrates an example of a wearable system. 
         FIG.  3    schematically illustrates example components of a wearable system. 
         FIG.  4    schematically illustrates an example of a waveguide stack of a wearable device for outputting image information to a user. 
         FIG.  5    schematically illustrates an example of an eye and an example coordinate system for determining an eye pose of an eye. 
         FIG.  6    is a schematic diagram of a wearable system that includes an eye tracking system. 
         FIG.  7 A  is a block diagram of a wearable system that may include an eye tracking system. 
         FIG.  7 B  is a block diagram of a render controller in a wearable system. 
         FIG.  7 C  is a block diagram of a registration observer in a head-mounted display system. 
         FIG.  8 A  is a schematic diagram of an eye showing the eye&#39;s corneal sphere. 
         FIG.  8 B  illustrates an example corneal glint detected by an eye-tracking camera. 
         FIGS.  8 C- 8 E  illustrate example stages of locating a user&#39;s corneal center with an eye tracking module in a wearable system. 
         FIGS.  9 A- 9 C  illustrate an example normalization of the coordinate system of eye tracking images. 
         FIGS.  9 D- 9 G  illustrate example stages of locating a user&#39;s pupil center with an eye tracking module in a wearable system. 
         FIG.  10    illustrates an example of an eye including the eye&#39;s optical and visual axes and the eye&#39;s center of rotation. 
         FIG.  11    is a process flow diagram of an example of a method for using eye tracking in rendering content and providing feedback on registration in a wearable device. 
         FIGS.  12 A and  12 B  illustrate a nominal position of a display element relative to a user&#39;s eye and illustrate a coordinate system for describing the positions of the display element and the user&#39;s eye relative to one another. 
         FIGS.  13 A and  13 B  illustrate nominal positioning and positioning tolerances of a display element relative to a user&#39;s eye in a head-mounted display system. 
         FIGS.  13 C and  13 D  illustrate a display registration volume and a user&#39;s eye viewing content from a display. 
         FIG.  14    illustrates an example of the perceived dimming of a display for various positions of a user&#39;s eye relative to the display. 
         FIGS.  15 A and  15 B  are exploded perspective views of a head-mounted display system having interchangeable pieces such as back pads, forehead pads, and nose bridge pads to adjust fit of a head-mounted display of the display system for different users. 
         FIG.  16    is a process flow diagram of an example of a method for observing registration and providing feedback on registration with a head-mounted display system. 
         FIGS.  17 A- 17 H  illustrate views of light fields projected by a display and how the intersections of the light fields may partly define a display registration volume. 
         FIG.  18    illustrates a top-down view of light fields projected by a display and how the intersections of the light fields may partly define a display registration volume. 
         FIG.  19    illustrates an example of an eye including glints and delineated regions for measuring the position of a pupil of the eye relative to a head-mounted display system. 
         FIG.  20    is an example of a process flow diagram for observing registration and providing feedback on registration with a head-mounted display system. 
         FIG.  21    is an illustrative screenshot showing fit indicators that may be provided to a wearer of a head-mounted display system. 
         FIGS.  22 A,  22 B, and  22 C  illustrate process flow diagrams of an example of further details of a method for observing registration and providing feedback on registration with a head-mounted display system. 
     
    
    
     Throughout the drawings, reference numbers may be re-used to indicate correspondence between referenced elements. The drawings are provided to illustrate example embodiments described herein and are not intended to limit the scope of the disclosure. 
     DETAILED DESCRIPTION 
     The display portion of a display system may include a head-mounted display (HMD) which may display a three-dimensional (3D) virtual object such that the object appears to be located within the user&#39;s ambient environment. As a result, the 3D virtual object may be perceived by the user in a similar manner as real world objects. 
     The HMD may display images by outputting spatially modulated light to the user, with the light corresponding to the virtual object. The spatially modulated light containing image information may be referred to as image light. To be perceived by the user, the image light travels from the HMD to an eye of the user, propagates through the pupil, and impinges on the eye&#39;s retina. It will be appreciated that if all or a portion of the image light for an image does not enter the pupil of the eye and/or does not impinge on the eye&#39;s retina, then the viewer would not see the image or the quality of the image may be degraded. As used herein, registration relates to the relative positioning of the display and the user&#39;s eyes. For example, a display may be said to be properly registered when the user&#39;s eyes and the display are positioned relative to one another for a desired amount of image light to enter the eye. A registration observer (e.g., a computer program) in the display device may be programmed to monitor whether the display is properly registered or positioned for the eye to receive the image light from the display. 
     In order to properly display content to users, e.g., by having the user&#39;s eyes positioned to receive image light, the user&#39;s eyes may need to be situated within a particular region or volume of space relative to the HMD. This volume may be referred to as the display registration volume. If the user&#39;s eyes are outside the display registration volume, display quality may be degraded (e.g., there may be dimming and/or displayed content that does not reach the users eyes). Various factors may combine to determining the positions of the user&#39;s eyes relative to the HMD and thus whether the user&#39;s eyes are situated within the desired display registration volume. As an example, anatomical variations between users may mean that the head-mounted display fits some users in a manner that places their eyes outside the display registration volume. As another example, the HMD may not be rigidly affixed to a user&#39;s head and may shift on the user&#39;s head over time, particularly when the user is moving around. As particular examples, the HMD may slip down the user&#39;s nose or tilt relative to a line (the interocular axis) between the user&#39;s eyes and, as a result, the HMD may not be able to provide desired virtual content (e.g., without some undesirable degradation) due to the shift of the display relative to the user&#39;s eyes. 
     Various systems and techniques described herein are at least in part directed to solving problems related to proper registration of a display to allow the viewer to view image content as desired. In some embodiments, a head-mounted display system may be configured to determine the position of an eye of the user. The display system may then determine whether the position of that eye is within a display registration volume of the head-mounted display system. Determining the position of the eye may include determining the position of a representative pointer volume associated with the eye e.g., the center of rotation of the eye. Determining whether the position of the eye is within the display registration volume may include determining whether the center of rotation of the eye is within the display registration volume. As discussed herein, the center of rotation of the eye may be determined using an inward-facing imaging system configured to image the eye. In addition, in some embodiments, the display registration volume is an imaginary volume associated with proper fit of the head-mounted display system relative to the user&#39;s eye. For example, the display registration volume may be a volume defined by a projection from the surface of the head-mounted display system outputting image light. More specifically, the display registration volume may be a three-dimensional geometric shape that tapers from a base to an apex. The shape of the display registration volume&#39;s base may be defined at least in part by the geometry of the display, and the depth of the display registration volume (i.e., the distance from base to apex on the z-axis) may be at least in part defined by the field of view (FOV) of the display. For example, a round or circular display may yield a conical display registration volume, and a polygonal display may yield a pyramidal display registration volume. As an additional example, a display with a larger FOV may yield a display registration volume having a smaller depth than a display with a smaller FOV. In some embodiments, the display registration volume may have the general shape of a truncated cone or pyramid. For example, the display registration volume may have the general shape of a frustum, e.g., a frustum of a pyramid such as a rectangular pyramid. 
     In some embodiments, an inward-facing imaging system of the head-mounted display system may acquire images of the user&#39;s face, including their eyes. The inward-facing imaging system may be an eye-tracking system, which may be mounted on a frame of the head-mounted display. The head-mounted display system may analyze the images to determine the relative position of the user&#39;s eyes and the HMD, and whether the position of each of the user&#39;s eyes falls within the display registration volume for that eye. Based on this information, the head-mounted display system may notify the user to adjust the fit of the HMD. For example, the notification may inform the user that the device has slipped and needs adjustment or a suggestion to make an adjustment of the HMD. In some embodiments, the head-mounted display system may take steps to mitigate any display degradation caused by misalignment of the HMD to the user, such as by boosting brightness in areas that would otherwise be dimmed by misalignment or by moving virtual content. Accordingly, such embodiments of the HMD may assist users with properly fitting the HMD and mitigating issues caused by improper fit of the HMD, such as when the HMD slips, moves, or tilts relative to the user&#39;s head. 
     Advantageously, the analysis of registration may be performed automatically utilizing images acquired from the inward-facing imaging system and information regarding the display registration volume stored or accessible by the display system. As a result, the fit of the HMD may be corrected upon first using the HMD, and optionally also during the course of continued usage of the HMD to ensure a high level of image quality in the use of the head-mounted display system. 
     Accordingly, a variety of implementations of systems and methods for observing registration of a head-mounted display system and taking action in response to the observed registration are provided herein. For example, the display system may be configured to observe registration by determining a center of rotation of a user&#39;s eyes, determining boundaries or location of a display system&#39;s registration volume, and determining whether the center of rotation is within that registration volume. It will be appreciated that the registration volume may be calculated by the display system and/or may be provided as predetermined information accessible by the display system. In some embodiments, in response to the observed registration, the display system may provide feedback to the user regarding whether and/or how registration may be improved. 
     As another example, the display system may be configured to observe registration and provide user feedback by estimating whether the center of rotation is within the registration volume by imaging the eye, but without specifically calculating the position of the center of rotation relative to the registration volume. Rather, the display system may be configured to image the eye and, based on those images, determine deviations of various features of the eyes from the desired orientations of those features. The display system may then make a determination of what particular adjustments may be made to the fit of the display system. The adjustments may be correlated with associated deviations of the various features of the eyes from their desired orientations. The desired orientations may be the orientations of those features when the centers of rotation of the eyes are within the registration volume. The adjustments may be adjustments correlated with addressing particular deviations and may include changing physical parts of the display system so that the display system sits on the user&#39;s head such that the centers of rotation of the eyes may be assumed to be in the desired registration volume in some embodiments. 
     Reference will now be made to the drawings, in which like reference numerals refer to like parts throughout. Unless indicated otherwise, the drawings are schematic and not necessarily drawn to scale. 
     Examples of 3D Displays of Wearable Systems 
     A wearable system (also referred to herein as a head-mounted display system or as an augmented reality (AR) system) may be configured to present 2D or 3D virtual images to a user. The images may be still images, frames of a video, or a video, in combination or the like. At least a portion of the wearable system may be implemented on a wearable device that may present a VR, AR, or MR environment, alone or in combination, for user interaction. The wearable device may be used interchangeably as an AR device (ARD). Further, for the purpose of the present disclosure, the term “AR” is used interchangeably with the term “MR”. 
       FIG.  1    depicts an illustration of a mixed reality scenario with certain virtual reality objects, and certain physical objects viewed by a person. In  FIG.  1   , an MR scene  100  is depicted wherein a user of an MR technology sees a real-world park-like setting  110  featuring people, trees, buildings in the background, and a concrete platform  120 . In addition to these items, the user of the MR technology also perceives that he “sees” a robot statue  130  standing upon the real-world platform  120 , and a cartoon-like avatar character  140  flying by which seems to be a personification of a bumble bee, even though these elements do not exist in the real world. 
     In order for the 3D display to produce a true sensation of depth, and more specifically, a simulated sensation of surface depth, it may be desirable for each point in the display&#39;s visual field to generate an accommodative response corresponding to its virtual depth. If the accommodative response to a display point does not correspond to the virtual depth of that point, as determined by the binocular depth cues of convergence and stereopsis, the human eye may experience an accommodation conflict, resulting in unstable imaging, harmful eye strain, headaches, and, in the absence of accommodation information, almost a complete lack of surface depth. 
     VR, AR, and MR experiences may be provided by display systems having displays in which images corresponding to a plurality of depth planes are provided to a viewer. The images may be different for each depth plane (e.g., provide slightly different presentations of a scene or object) and may be separately focused by the viewer&#39;s eyes, thereby helping to provide the user with depth cues based on the accommodation of the eye required to bring into focus different image features for the scene located on different depth plane or based on observing different image features on different depth planes being out of focus. As discussed elsewhere herein, such depth cues provide credible perceptions of depth. 
       FIG.  2    illustrates an example of wearable system  200  which may be configured to provide an AR/VR/MR scene. The wearable system  200  may also be referred to as the AR system  200 . The wearable system  200  includes a display  220 , and various mechanical and electronic modules and systems to support the functioning of display  220 . The display  220  may be coupled to a frame  230 , which is wearable by a user, wearer, or viewer  210 . The display  220  may be positioned in front of the eyes of the user  210 . The display  220  may present AR/VR/MR content to a user. Because the display  220  may be worn on the head of the user  210 , it may also be referred to as a head-mounted display (HMD) and the wearable system  200 , comprising the display  220 , may also be referred to as a head-mounted display system. 
     In some embodiments, a speaker  240  is coupled to the frame  230  and positioned adjacent the ear canal of the user (in some embodiments, another speaker, not shown, is positioned adjacent the other ear canal of the user to provide for stereo/shapeable sound control). The display  220  may include an audio sensor (e.g., a microphone)  232  for detecting an audio stream from the environment and capture ambient sound. In some embodiments, one or more other audio sensors, not shown, are positioned to provide stereo sound reception. Stereo sound reception may be used to determine the location of a sound source. The wearable system  200  may perform voice or speech recognition on the audio stream. 
     The wearable system  200  may include an outward-facing imaging system  464  (shown in  FIG.  4   ) which observes the world in the environment around the user. The wearable system  200  may also include an inward-facing imaging system  462  (shown in  FIG.  4   ) which may track the eye movements of the user. The inward-facing imaging system may track either one eye&#39;s movements or both eyes&#39; movements. The inward-facing imaging system  462  may be attached to the frame  230  and may be in electrical communication with the processing modules  260  or  270 , which may process image information acquired by the inward-facing imaging system to determine, e.g., the pupil diameters or orientations of the eyes, eye movements or eye pose of the user  210 . The inward-facing imaging system  462  may include one or more cameras. For example, at least one camera may be used to image each eye. The images acquired by the cameras may be used to determine pupil size or eye pose for each eye separately, thereby allowing presentation of image information to each eye to be dynamically tailored to that eye. 
     As an example, the wearable system  200  may use the outward-facing imaging system  464  or the inward-facing imaging system  462  to acquire images of a pose of the user. The images may be still images, frames of a video, or a video. 
     The display  220  may be operatively coupled  250 , such as by a wired lead or wireless connectivity, to a local data processing module  260  which may be mounted in a variety of configurations, such as fixedly attached to the frame  230 , fixedly attached to a helmet or hat worn by the user, embedded in headphones, or otherwise removably attached to the user  210  (e.g., in a backpack-style configuration, in a belt-coupling style configuration). 
     The local processing and data module  260  may comprise a hardware processor, as well as digital memory, such as non-volatile memory (e.g., flash memory), both of which may be utilized to assist in the processing, caching, and storage of data. The data may include data a) captured from sensors (which may be, e.g., operatively coupled to the frame  230  or otherwise attached to the user  210 ), such as image capture devices (e.g., cameras in the inward-facing imaging system or the outward-facing imaging system), audio sensors (e.g., microphones), inertial measurement units (IMUs), accelerometers, compasses, global positioning system (GPS) units, radio devices, or gyroscopes; or b) acquired or processed using remote processing module  270  or remote data repository  280 , possibly for passage to the display  220  after such processing or retrieval. The local processing and data module  260  may be operatively coupled by communication links  262  or  264 , such as via wired or wireless communication links, to the remote processing module  270  or remote data repository  280  such that these remote modules are available as resources to the local processing and data module  260 . In addition, remote processing module  280  and remote data repository  280  may be operatively coupled to each other. 
     In some embodiments, the remote processing module  270  may comprise one or more processors configured to analyze and process data or image information. In some embodiments, the remote data repository  280  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, all data is stored and all computations are performed in the local processing and data module, allowing fully autonomous use from a remote module. 
     Example Components of A Wearable System 
       FIG.  3    schematically illustrates example components of a wearable system.  FIG.  3    shows a wearable system  200  which may include a display  220  and a frame  230 . A blown-up view  202  schematically illustrates various components of the wearable system  200 . In certain implements, one or more of the components illustrated in  FIG.  3    may be part of the display  220 . The various components alone or in combination may collect a variety of data (such as e.g., audio or visual data) associated with the user of the wearable system  200  or the user&#39;s environment. It should be appreciated that other embodiments may have additional or fewer components depending on the application for which the wearable system is used. Nevertheless,  FIG.  3    provides a basic idea of some of the various components and types of data that may be collected, analyzed, and stored through the wearable system. 
       FIG.  3    shows an example wearable system  200  which may include the display  220 . The display  220  may comprise a display lens  226  that may be mounted to a user&#39;s head or a housing or frame  230 , which corresponds to the frame  230 . The display lens  226  may comprise one or more transparent mirrors positioned by the housing  230  in front of the user&#39;s eyes  302 ,  304  and may be configured to bounce projected light  338  into the eyes  302 ,  304  and facilitate beam shaping, while also allowing for transmission of at least some light from the local environment. The wavefront of the projected light beam  338  may be bent or focused to coincide with a desired focal distance of the projected light. As illustrated, two wide-field-of-view machine vision cameras  316  (also referred to as world cameras) may be coupled to the housing  230  to image the environment around the user. These cameras  316  may be dual capture visible light/non-visible (e.g., infrared) light cameras. The cameras  316  may be part of the outward-facing imaging system  464  shown in  FIG.  4   . Image acquired by the world cameras  316  may be processed by the pose processor  336 . For example, the pose processor  336  may implement one or more object recognizers  708  (e.g., shown in  FIG.  7   ) to identify a pose of a user or another person in the user&#39;s environment or to identify a physical object in the user&#39;s environment. 
     With continued reference to  FIG.  3   , a pair of scanned-laser shaped-wavefront (e.g., for depth) light projector modules with display mirrors and optics configured to project light  338  into the eyes  302 ,  304  are shown. The depicted view also shows two miniature infrared cameras  324  paired with infrared light sources  326  (such as light emitting diodes “LED”s), which are configured to be able to track the eyes  302 ,  304  of the user to support rendering and user input. The cameras  324  may be part of the inward-facing imaging system  462  shown in  FIG.  4   . The wearable system  200  may further feature a sensor assembly  339 , which may comprise X, Y, and Z axis accelerometer capability as well as a magnetic compass and X, Y, and Z axis gyro capability, preferably providing data at a relatively high frequency, such as 200 Hz. The sensor assembly  339  may be part of the IMU described with reference to  FIG.  2 A  The depicted system  200  may also comprise a head pose processor  336 , such as an ASIC (application specific integrated circuit), FPGA (field programmable gate array), or ARM processor (advanced reduced-instruction-set machine), which may be configured to calculate real or near-real time user head pose from wide field of view image information output from the capture devices  316 . The head pose processor  336  may be a hardware processor and may be implemented as part of the local processing and data module  260  shown in  FIG.  2 A . 
     The wearable system may also include one or more depth sensors  234 . The depth sensor  234  may be configured to measure the distance between an object in an environment to a wearable device. The depth sensor  234  may include a laser scanner (e.g., a lidar), an ultrasonic depth sensor, or a depth sensing camera. In certain implementations, where the cameras  316  have depth sensing ability, the cameras  316  may also be considered as depth sensors  234 . 
     Also shown is a processor  332  configured to execute digital or analog processing to derive pose from the gyro, compass, or accelerometer data from the sensor assembly  339 . The processor  332  may be part of the local processing and data module  260  shown in  FIG.  2   . The wearable system  200  as shown in  FIG.  3    may also include a position system such as, e.g., a GPS  337  (global positioning system) to assist with pose and positioning analyses. In addition, the GPS may further provide remotely-based (e.g., cloud-based) information about the user&#39;s environment. This information may be used for recognizing objects or information in user&#39;s environment. 
     The wearable system may combine data acquired by the GPS  337  and a remote computing system (such as, e.g., the remote processing module  270 , another user&#39;s ARD, etc.) which may provide more information about the user&#39;s environment. As one example, the wearable system may determine the user&#39;s location based on GPS data and retrieve a world map (e.g., by communicating with a remote processing module  270 ) including virtual objects associated with the user&#39;s location. As another example, the wearable system  200  may monitor the environment using the world cameras  316  (which may be part of the outward-facing imaging system  464  shown in  FIG.  4   ). Based on the images acquired by the world cameras  316 , the wearable system  200  may detect objects in the environment (e.g., by using one or more object recognizers  708  shown in  FIG.  7   ). The wearable system may further use data acquired by the GPS  337  to interpret the characters. 
     The wearable system  200  may also comprise a rendering engine  334  which may be configured to provide rendering information that is local to the user to facilitate operation of the scanners and imaging into the eyes of the user, for the user&#39;s view of the world. The rendering engine  334  may be implemented by a hardware processor (such as, e.g., a central processing unit or a graphics processing unit). In some embodiments, the rendering engine is part of the local processing and data module  260 . The rendering engine  334  may be communicatively coupled (e.g., via wired or wireless links) to other components of the wearable system  200 . For example, the rendering engine  334 , may be coupled to the eye cameras  324  via communication link  274 , and be coupled to a projecting subsystem  318  (which may project light into user&#39;s eyes  302 ,  304  via a scanned laser arrangement in a manner similar to a retinal scanning display) via the communication link  272 . The rendering engine  334  may also be in communication with other processing units such as, e.g., the sensor pose processor  332  and the image pose processor  336  via links  276  and  294  respectively. 
     The cameras  324  (e.g., mini infrared cameras) may be utilized to track the eye pose to support rendering and user input. Some example eye poses may include where the user is looking or at what depth he or she is focusing (which may be estimated with eye vergence). The GPS  337 , gyros, compass, and accelerometers  339  may be utilized to provide coarse or fast pose estimates. One or more of the cameras  316  may acquire images and pose, which in conjunction with data from an associated cloud computing resource, may be utilized to map the local environment and share user views with others. 
     The example components depicted in  FIG.  3    are for illustration purposes only. Multiple sensors and other functional modules are shown together for ease of illustration and description. Some embodiments may include only one or a subset of these sensors or modules. Further, the locations of these components are not limited to the positions depicted in  FIG.  3   . Some components may be mounted to or housed within other components, such as a belt-mounted component, a hand-held component, or a helmet component. As one example, the image pose processor  336 , sensor pose processor  332 , and rendering engine  334  may be positioned in a beltpack and configured to communicate with other components of the wearable system via wireless communication, such as ultra-wideband, Wi-Fi, Bluetooth, etc., or via wired communication. The depicted housing  230  preferably is head-mountable and wearable by the user. However, some components of the wearable system  200  may be worn to other portions of the user&#39;s body. For example, the speaker  240  may be inserted into the ears of a user to provide sound to the user. 
     Regarding the projection of light  338  into the eyes  302 ,  304  of the user, in some embodiment, the cameras  324  may be utilized to measure where the centers of a user&#39;s eyes are geometrically verged to, which, in general, coincides with a position of focus, or “depth of focus”, of the eyes. A  3 -dimensional surface of all points the eyes verge to may be referred to as the “horopter”. The focal distance may take on a finite number of depths, or may be infinitely varying. Light projected from the vergence distance appears to be focused to the subject eye  302 ,  304 , while light in front of or behind the vergence distance is blurred. Examples of wearable devices and other display systems of the present disclosure are also described in U.S. Patent Publication No. 2016/0270656, which is incorporated by reference herein in its entirety. 
     The human visual system is complicated and providing a realistic perception of depth is challenging. Viewers of an object may perceive the object as being three-dimensional due to a combination of vergence and accommodation. Vergence movements (e.g., rolling movements of the pupils 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 focusing (or “accommodation”) of the lenses of the eyes. Under normal conditions, changing the focus of the lenses of the eyes, or accommodating 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 accommodation, under normal conditions. Display systems that provide a better match between accommodation and vergence may form more realistic and comfortable simulations of three-dimensional imagery. 
     Further spatially coherent light with a beam diameter of less than about 0.7 millimeters may be correctly resolved by the human eye regardless of where the eye focuses. Thus, to create an illusion of proper focal depth, the eye vergence may be tracked with the cameras  324 , and the rendering engine  334  and projection subsystem  318  may be utilized to render all objects on or close to the horopter in focus, and all other objects at varying degrees of defocus (e.g., using intentionally-created blurring). Preferably, the system  220  renders to the user at a frame rate of about 60 frames per second or greater. As described above, preferably, the cameras  324  may be utilized for eye tracking, and software may be configured to pick up not only vergence geometry but also focus location cues to serve as user inputs. Preferably, such a display system is configured with brightness and contrast suitable for day or night use. 
     In some embodiments, the display system preferably has latency of less than about 20 milliseconds for visual object alignment, less than about 0.1 degree of angular alignment, and about 1 arc minute of resolution, which, without being limited by theory, is believed to be approximately the limit of the human eye. The display system  220  may be integrated with a localization system, which may involve GPS elements, optical tracking, compass, accelerometers, or other data sources, to assist with position and pose determination; localization information may be utilized to facilitate accurate rendering in the user&#39;s view of the pertinent world (e.g., such information would facilitate the glasses to know where they are with respect to the real world). 
     In some embodiments, the wearable system  200  is configured to display one or more virtual images based on the accommodation of the user&#39;s eyes. Unlike prior 3D display approaches that force the user to focus where the images are being projected, in some embodiments, the wearable system is configured to automatically vary the focus of projected virtual content to allow for a more comfortable viewing of one or more images presented to the user. For example, if the user&#39;s eyes have a current focus of 1 m, the image may be projected to coincide with the user&#39;s focus. If the user shifts focus to 3 m, the image is projected to coincide with the new focus. Thus, rather than forcing the user to a predetermined focus, the wearable system  200  of some embodiments allows the user&#39;s eye to a function in a more natural manner. 
     Such a wearable system  200  may eliminate or reduce the incidences of eye strain, headaches, and other physiological symptoms typically observed with respect to virtual reality devices. To achieve this, various embodiments of the wearable system  200  are configured to project virtual images at varying focal distances, through one or more variable focus elements (VFEs). In one or more embodiments, 3D perception may be achieved through a multi-plane focus system that projects images at fixed focal planes away from the user. Other embodiments employ variable plane focus, wherein the focal plane is moved back and forth in the z-direction to coincide with the user&#39;s present state of focus. 
     In both the multi-plane focus systems and variable plane focus systems, wearable system  200  may employ eye tracking to determine a vergence of the user&#39;s eyes, determine the user&#39;s current focus, and project the virtual image at the determined focus. In other embodiments, wearable system  200  comprises a light modulator that variably projects, through a fiber scanner, or other light generating source, light beams of varying focus in a raster pattern across the retina. Thus, the ability of the display of the wearable system  200  to project images at varying focal distances not only eases accommodation for the user to view objects in 3D, but may also be used to compensate for user ocular anomalies, as further described in U.S. Patent Publication No. 2016/0270656, which is incorporated by reference herein in its entirety. In some other embodiments, a spatial light modulator may project the images to the user through various optical components. For example, as described further below, the spatial light modulator may project the images onto one or more waveguides, which then transmit the images to the user. 
     Waveguide Stack Assembly 
       FIG.  4    illustrates an example of a waveguide stack for outputting image information to a user. A wearable system  400  includes a stack of waveguides, or stacked waveguide assembly  480  that may be utilized to provide three-dimensional perception to the eye/brain using a plurality of waveguides  432   b,    434   b,    436   b,    438   b,    4400   b.  In some embodiments, the wearable system  400  may correspond to wearable system  200  of  FIG.  2   , with  FIG.  4    schematically showing some parts of that wearable system  200  in greater detail. For example, in some embodiments, the waveguide assembly  480  may be integrated into the display  220  of  FIG.  2   . 
     With continued reference to  FIG.  4   , the waveguide assembly  480  may also include a plurality of features  458 ,  456 ,  454 ,  452  between the waveguides. In some embodiments, the features  458 ,  456 ,  454 ,  452  may be lenses. In other embodiments, the features  458 ,  456 ,  454 ,  452  may not be lenses. Rather, they may simply be spacers (e.g., cladding layers or structures for forming air gaps). 
     The waveguides  432   b,    434   b,    436   b,    438   b,    440   b  or the plurality of lenses  458 ,  456 ,  454 ,  452  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  420 ,  422 ,  424 ,  426 ,  428  may be utilized to inject image information into the waveguides  440   b,    438   b,    436   b,    434   b,    432   b,  each of which may be configured to distribute incoming light across each respective waveguide, for output toward the eye  410 . Light exits an output surface of the image injection devices  420 ,  422 ,  424 ,  426 ,  428  and is injected into a corresponding input edge of the waveguides  440   b,    438   b,    436   b,    434   b,    432   b.  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  410  at particular angles (and amounts of divergence) corresponding to the depth plane associated with a particular waveguide. 
     In some embodiments, the image injection devices  420 ,  422 ,  424 ,  426 ,  428  are discrete displays that each produce image information for injection into a corresponding waveguide  440   b,    438   b,    436   b,    434   b,    432   b,  respectively. In some other embodiments, the image injection devices  420 ,  422 ,  424 ,  426 ,  428  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  420 ,  422 ,  424 ,  426 ,  428 . 
     A controller  460  controls the operation of the stacked waveguide assembly  480  and the image injection devices  420 ,  422 ,  424 ,  426 ,  428 . The controller  460  includes programming (e.g., instructions in a non-transitory computer-readable medium) that regulates the timing and provision of image information to the waveguides  440   b,    438   b,    436   b,    434   b,    432   b.  In some embodiments, the controller  460  may be a single integral device, or a distributed system connected by wired or wireless communication channels. The controller  460  may be part of the processing modules  260  or  270  (illustrated in  FIG.  2   ) in some embodiments. 
     The waveguides  440   b,    438   b,    436   b,    434   b,    432   b  may be configured to propagate light within each respective waveguide by total internal reflection (TIR). The waveguides  440   b,    438   b,    436   b,    434   b,    432   b  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  440   b,    438   b,    436   b,    434   b,    432   b  may each include light extracting optical elements  440   a,    438   a,    436   a,    434   a,    432   a  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  410 . Extracted light may also be referred to as outcoupled light, and light extracting optical elements may also be referred to as outcoupling optical elements. An extracted beam of light is outputted by the waveguide at locations at which the light propagating in the waveguide strikes a light redirecting element. The light extracting optical elements ( 440   a,    438   a,    436   a,    434   a,    432   a ) may, for example, be reflective or diffractive optical features. While illustrated disposed at the bottom major surfaces of the waveguides  440   b,    438   b,    436   b,    434   b,    432   b  for ease of description and drawing clarity, in some embodiments, the light extracting optical elements  440   a,    438   a,    436   a,    434   a,    432   a  may be disposed at the top or bottom major surfaces, or may be disposed directly in the volume of the waveguides  440   b,    438   b,    436   b,    434   b,    432   b.  In some embodiments, the light extracting optical elements  440   a,    438   a,    436   a,    434   a,    432   a  may be formed in a layer of material that is attached to a transparent substrate to form the waveguides  440   b,    438   b,    436   b,    434   b,    432   b.  In some other embodiments, the waveguides  440   b,    438   b,    436   b,    434   b,    432   b  may be a monolithic piece of material and the light extracting optical elements  440   a,    438   a,    436   a,    434   a,    432   a  may be formed on a surface or in the interior of that piece of material. 
     With continued reference to  FIG.  4   , as discussed herein, each waveguide  440   b,    438   b,    436   b,    434   b,    432   b  is configured to output light to form an image corresponding to a particular depth plane. For example, the waveguide  432   b  nearest the eye may be configured to deliver collimated light, as injected into such waveguide  432   b,  to the eye  410 . The collimated light may be representative of the optical infinity focal plane. The next waveguide up  434   b  may be configured to send out collimated light which passes through the first lens  452  (e.g., a negative lens) before it may reach the eye  410 . First lens  452  may be configured to create a slight convex wavefront curvature so that the eye/brain interprets light coming from that next waveguide up  434   b  as coming from a first focal plane closer inward toward the eye  410  from optical infinity. Similarly, the third up waveguide  436   b  passes its output light through both the first lens  452  and second lens  454  before reaching the eye  410 . The combined optical power of the first and second lenses  452  and  454  may be configured to create another incremental amount of wavefront curvature so that the eye/brain interprets light coming from the third waveguide  436   b  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  434   b.    
     The other waveguide layers (e.g., waveguides  438   b,    440   b ) and lenses (e.g., lenses  456 ,  458 ) are similarly configured, with the highest waveguide  440   b  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  458 ,  456 ,  454 ,  452  when viewing/interpreting light coming from the world  470  on the other side of the stacked waveguide assembly  480 , a compensating lens layer  430  may be disposed at the top of the stack to compensate for the aggregate power of the lens stack  458 ,  456 ,  454 ,  452  below. (Compensating lens layer  430  and the stacked waveguide assembly  480  as a whole may be configured such that light coming from the world  470  is conveyed to the eye  410  at substantially the same level of divergence (or collimation) as the light had when it was initially received by the stacked waveguide assembly  480 .) Such a configuration provides as many perceived focal planes as there are available waveguide/lens pairings. Both the light extracting optical elements of the waveguides and the focusing aspects of the lenses may be static (e.g., not dynamic or electro-active). In some alternative embodiments, either or both may be dynamic using electro-active features. 
     With continued reference to  FIG.  4   , the light extracting optical elements  440   a,    438   a,    436   a,    434   a,    432   a  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 light extracting optical elements, which output light with a different amount of divergence depending on the associated depth plane. In some embodiments, as discussed herein, the light extracting optical elements  440   a,    438   a,    436   a,    434   a,    432   a  may be volumetric or surface features, which may be configured to output light at specific angles. For example, the light extracting optical elements  440   a,    438   a,    436   a,    434   a,    432   a  may be volume holograms, surface holograms, and/or diffraction gratings. Light extracting optical elements, such as diffraction gratings, are described in U.S. Patent Publication No. 2015/0178939, published Jun. 25, 2015, which is incorporated by reference herein in its entirety. 
     In some embodiments, the light extracting optical elements  440   a,    438   a,    436   a,    434   a,    432   a  are diffractive features that form a diffraction pattern, or “diffractive optical element” (also referred to herein as a “DOE”). Preferably, the DOE has a relatively low diffraction efficiency so that only a portion of the light of the beam is deflected away toward the eye  410  with each intersection of the DOE, while the rest continues to move through a waveguide via total internal reflection. The light carrying the image information may thus be 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  304  for this particular collimated beam bouncing around within a waveguide. 
     In some embodiments, one or more DOEs may be switchable between “on” state in which they actively diffract, and “off” state 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, the number and distribution of depth planes or depth of field may be varied dynamically based on the pupil sizes or orientations of the eyes of the viewer. Depth of field may change inversely with a viewer&#39;s pupil size. As a result, as the sizes of the pupils of the viewer&#39;s eyes decrease, the depth of field increases such that one plane that is not discernible because the location of that plane is beyond the depth of focus of the eye may become discernible and appear more in focus with reduction of pupil size and commensurate with the increase in depth of field. Likewise, the number of spaced apart depth planes used to present different images to the viewer may be decreased with the decreased pupil size. For example, a viewer may not be able to clearly perceive the details of both a first depth plane and a second depth plane at one pupil size without adjusting the accommodation of the eye away from one depth plane and to the other depth plane. These two depth planes may, however, be sufficiently in focus at the same time to the user at another pupil size without changing accommodation. 
     In some embodiments, the display system may vary the number of waveguides receiving image information based upon determinations of pupil size or orientation, or upon receiving electrical signals indicative of particular pupil size or orientation. For example, if the user&#39;s eyes are unable to distinguish between two depth planes associated with two waveguides, then the controller  460  (which may be an embodiment of the local processing and data module  260 ) may be configured or programmed to cease providing image information to one of these waveguides. Advantageously, this may reduce the processing burden on the system, thereby increasing the responsiveness of the system. In embodiments in which the DOEs for a waveguide are switchable between the on and off states, the DOEs may be switched to the off state when the waveguide does receive image information. 
     In some embodiments, it may be desirable to have an exit beam meet the condition of having a diameter that is less than the diameter of the eye of a viewer. However, meeting this condition may be challenging in view of the variability in size of the viewer&#39;s pupils. In some embodiments, this condition is met over a wide range of pupil sizes by varying the size of the exit beam in response to determinations of the size of the viewer&#39;s pupil. For example, as the pupil size decreases, the size of the exit beam may also decrease. In some embodiments, the exit beam size may be varied using a variable aperture. 
     The wearable system  400  may include an outward-facing imaging system  464  (e.g., a digital camera) that images a portion of the world  470 . This portion of the world  470  may be referred to as the field of view (FOV) of a world camera and the imaging system  464  is sometimes referred to as an FOV camera. The FOV of the world camera may or may not be the same as the FOV of a viewer  210  which encompasses a portion of the world  470  the viewer  210  perceives at a given time. For example, in some situations, the FOV of the world camera may be larger than the viewer  210  of the viewer  210  of the wearable system  400 . The entire region available for viewing or imaging by a viewer may be referred to as the field of regard (FOR). The FOR may include 4π steradians of solid angle surrounding the wearable system  400  because the wearer may move his body, head, or eyes to perceive substantially any direction in space. In other contexts, the wearer&#39;s movements may be more constricted, and accordingly the wearer&#39;s FOR may subtend a smaller solid angle. Images obtained from the outward-facing imaging system  464  may be used to track gestures made by the user (e.g., hand or finger gestures), detect objects in the world  470  in front of the user, and so forth. 
     The wearable system  400  may include an audio sensor  232 , e.g., a microphone, to capture ambient sound. As described above, in some embodiments, one or more other audio sensors may be positioned to provide stereo sound reception useful to the determination of location of a speech source. The audio sensor  232  may comprise a directional microphone, as another example, which may also provide such useful directional information as to where the audio source is located. The wearable system  400  may use information from both the outward-facing imaging system  464  and the audio sensor  230  in locating a source of speech, or to determine an active speaker at a particular moment in time, etc. For example, the wearable system  400  may use the voice recognition alone or in combination with a reflected image of the speaker (e.g., as seen in a mirror) to determine the identity of the speaker. As another example, the wearable system  400  may determine a position of the speaker in an environment based on sound acquired from directional microphones. The wearable system  400  may parse the sound coming from the speaker&#39;s position with speech recognition algorithms to determine the content of the speech and use voice recognition techniques to determine the identity (e.g., name or other demographic information) of the speaker. 
     The wearable system  400  may also include an inward-facing imaging system  466  (e.g., a digital camera), which observes the movements of the user, such as the eye movements and the facial movements. The inward-facing imaging system  466  may be used to capture images of the eye  410  to determine the size and/or orientation of the pupil of the eye  304 . The inward-facing imaging system  466  may be used to obtain images for use in determining the direction the user is looking (e.g., eye pose) or for biometric identification of the user (e.g., via iris identification). In some embodiments, at least one camera may be utilized for each eye, to separately determine the pupil size or eye pose of each eye independently, thereby allowing the presentation of image information to each eye to be dynamically tailored to that eye. In some other embodiments, the pupil diameter or orientation of only a single eye  410  (e.g., using only a single camera per pair of eyes) is determined and assumed to be similar for both eyes of the user. The images obtained by the inward-facing imaging system  466  may be analyzed to determine the user&#39;s eye pose or mood, which may be used by the wearable system  400  to decide which audio or visual content should be presented to the user. The wearable system  400  may also determine head pose (e.g., head position or head orientation) using sensors such as IMUs, accelerometers, gyroscopes, etc. 
     The wearable system  400  may include a user input device  466  by which the user may input commands to the controller  460  to interact with the wearable system  400 . For example, the user input device  466  may include a trackpad, a touchscreen, a joystick, a multiple degree-of-freedom (DOF) controller, a capacitive sensing device, a game controller, a keyboard, a mouse, a directional pad (D-pad), a wand, a haptic device, a totem (e.g., functioning as a virtual user input device), and so forth. A multi-DOF controller may sense user input in some or all possible translations (e.g., left/right, forward/backward, or up/down) or rotations (e.g., yaw, pitch, or roll) of the controller. A multi-DOF controller which supports the translation movements may be referred to as a 3 DOF while a multi-DOF controller which supports the translations and rotations may be referred to as 6 DOF. In some cases, the user may use a finger (e.g., a thumb) to press or swipe on a touch-sensitive input device to provide input to the wearable system  400  (e.g., to provide user input to a user interface provided by the wearable system  400 ). The user input device  466  may be held by the user&#39;s hand during the use of the wearable system  400 . The user input device  466  may be in wired or wireless communication with the wearable system  400 . 
     Other Components of the Wearable System 
     In many implementations, the wearable system may include other components in addition or in alternative to the components of the wearable system described above. The wearable system may, for example, include one or more haptic devices or components. The haptic devices or components may be operable to provide a tactile sensation to a user. For example, the haptic devices or components may provide a tactile sensation of pressure or texture when touching virtual content (e.g., virtual objects, virtual tools, other virtual constructs). The tactile sensation may replicate a feel of a physical object which a virtual object represents, or may replicate a feel of an imagined object or character (e.g., a dragon) which the virtual content represents. In some implementations, haptic devices or components may be worn by the user (e.g., a user wearable glove). In some implementations, haptic devices or components may be held by the user. 
     The wearable system may, for example, include one or more physical objects which are manipulable by the user to allow input or interaction with the wearable system. These physical objects may be referred to herein as totems. Some totems may take the form of inanimate objects, such as for example, a piece of metal or plastic, a wall, a surface of table. In certain implementations, the totems may not actually have any physical input structures (e.g., keys, triggers, joystick, trackball, rocker switch). Instead, the totem may simply provide a physical surface, and the wearable system may render a user interface so as to appear to a user to be on one or more surfaces of the totem. For example, the wearable system may render an image of a computer keyboard and trackpad to appear to reside on one or more surfaces of a totem. For example, the wearable system may render a virtual computer keyboard and virtual trackpad to appear on a surface of a thin rectangular plate of aluminum which serves as a totem. The rectangular plate does not itself have any physical keys or trackpad or sensors. However, the wearable system may detect user manipulation or interaction or touches with the rectangular plate as selections or inputs made via the virtual keyboard or virtual trackpad. The user input device  466  (shown in  FIG.  4   ) may be an embodiment of a totem, which may include a trackpad, a touchpad, a trigger, a joystick, a trackball, a rocker or virtual switch, a mouse, a keyboard, a multi-degree-of-freedom controller, or another physical input device. A user may use the totem, alone or in combination with poses, to interact with the wearable system or other users. 
     Examples of haptic devices and totems usable with the wearable devices, HMD, and display systems of the present disclosure are described in U.S. Patent Publication No. 2015/0016777, which is incorporated by reference herein in its entirety. 
     Example of an Eye Image 
       FIG.  5    illustrates an image of an eye  500  with eyelids  504 , sclera  508  (the “white” of the eye), iris  512 , and pupil  516 . Curve  516   a  shows the pupillary boundary between the pupil  516  and the iris  512 , and curve  512   a  shows the limbic boundary between the iris  512  and the sclera  508 . The eyelids  504  include an upper eyelid  504   a  and a lower eyelid  504   b.  The eye  500  is illustrated in a natural resting pose (e.g., in which the user&#39;s face and gaze are both oriented as they would be toward a distant object directly ahead of the user). The natural resting pose of the eye  500  may be indicated by a natural resting direction  520 , which is a direction orthogonal to the surface of the eye  500  when in the natural resting pose (e.g., directly out of the plane for the eye  500  shown in  FIG.  5   ) and in this example, centered within the pupil  516 . 
     As the eye  500  moves to look toward different objects, the eye pose will change relative to the natural resting direction  520 . The current eye pose may be determined with reference to an eye pose direction  524 , which is a direction orthogonal to the surface of the eye (and centered in within the pupil  516 ) but oriented toward the object at which the eye is currently directed. With reference to an example coordinate system shown in  FIG.  5 A , the pose of the eye  500  may be expressed as two angular parameters indicating an azimuthal deflection and a zenithal deflection of the eye pose direction  524  of the eye, both relative to the natural resting direction  520  of the eye. For purposes of illustration, these angular parameters may be represented as θ (azimuthal deflection, determined from a fiducial azimuth) and ϕ (zenithal deflection, sometimes also referred to as a polar deflection). In some implementations, angular roll of the eye around the eye pose direction  524  may be included in the determination of eye pose, and angular roll may be included in the following analysis. In other implementations, other techniques for determining the eye pose may be used, for example, a pitch, yaw, and optionally roll system. 
     An eye image may be obtained from a video using any appropriate process, for example, using a video processing algorithm that may extract an image from one or more sequential frames. The pose of the eye may be determined from the eye image using a variety of eye-tracking techniques. For example, an eye pose may be determined by considering the lensing effects of the cornea on light sources that are provided. Any suitable eye tracking technique may be used for determining eye pose in the eyelid shape estimation techniques described herein. 
     Example of an Eye Tracking System 
       FIG.  6    illustrates a schematic diagram of a wearable, or a head-mounted, display system  600  that includes an eye tracking system. The head-mounted display system  600  may, in at least some embodiments, include components located in a head-mounted unit  602  and components located in a non-head-mounted unit  604 . Non-head mounted unit  604  may be, as examples, a belt-mounted component, a hand-held component, a component in a backpack, a remote component, etc. Incorporating some of the components of the head-mounted display system  600  in non-head-mounted unit  604  may help to reduce the size, weight, complexity, and cost of the head-mounted unit  602 . In some implementations, some or all of the functionality described as being performed by one or more components of head-mounted unit  602  and/or non-head mounted  604  may be provided by way of one or more components included elsewhere in the head-mounted display system  600 . For example, some or all of the functionality described below in association with a CPU  612  of head-mounted unit  602  may be provided by way of a CPU  616  of non-head mounted unit  604 , and vice versa. In some examples, some or all of such functionality may be provided by way of peripheral devices of head-mounted display system  600 . Furthermore, in some implementations, some or all of such functionality may be provided by way of one or more cloud computing devices or other remotely-located computing devices in a manner similar to that which has been described above with reference to  FIG.  2   . 
     As shown in  FIG.  6   , head-mounted display system  600  may include an eye tracking system including a camera  324  that captures images of a user&#39;s eye  610 . If desired, the eye tracking system may also include light sources  326   a  and  326   b  (such as light emitting diodes “LED”s). The light sources  326   a  and  326   b  may generate glints (i.e., reflections off of the user&#39;s eyes that appear in images of the eye captured by camera  324 ). The positions of the light sources  326   a  and  326   b  relative to the camera  324  may be known and, as a consequence, the positions of the glints within images captured by camera  324  may be used in tracking the user&#39;s eyes (as will be discussed in more detail below in connection with  FIGS.  7 - 11   ). In at least one embodiment, there may be one light source  326  and one camera  324  associated with a single one of the user&#39;s eyes  610 . In another embodiment, there may be one light source  326  and one camera  324  associated with each of a user&#39;s eyes.  610 . In yet other embodiments, there may be one or more cameras  324  and one or more light sources  326  associated with one or each of a user&#39;s eyes  610 . As a specific example, there may be two light sources  326   a  and  326   b  and one or more cameras  324  associated with each of a user&#39;s eyes  610 . As another example, there may be three or more light sources such as light sources  326   a  and  326   b  and one or more cameras  324  associated with each of a user&#39;s eyes  610 . 
     Eye tracking module  614  may receive images from eye-tracking camera(s)  324  and may analyze the images to extract various pieces of information. As examples, the eye tracking module  614  may detect the user&#39;s eye poses, a three-dimensional position of the user&#39;s eye relative to the eye-tracking camera  324  (and to the head-mounted unit  602 ), the direction one or both of the user&#39;s eyes  610  are focused on, the user&#39;s vergence depth (i.e., the depth from the user at which the user is focusing on), the positions of the user&#39;s pupils, the positions of the user&#39;s cornea and cornea sphere, the center of rotation of each of the user&#39;s eyes, and the center of perspective of each of the user&#39;s eyes. The eye tracking module  614  may extract such information using techniques described below in connection with  FIGS.  7 - 11   . As shown in  FIG.  6   , eye tracking module  614  may be a software module implemented using a CPU  612  in a head-mounted unit  602 . 
     Data from eye tracking module  614  may be provided to other components in the wearable system. As example, such data may be transmitted to components in a non-head-mounted unit  604  such as CPU  616  including software modules for a light-field render controller  618  and a registration observer  620 , which may be configured to evaluate whether the display of the head-mounted display system  600  is properly registered with the eyes of the user. 
     Render controller  618  may use information from eye tracking module  614  to adjust images displayed to the user by render engine  622  (e.g., a render engine that may be a software module in GPU  621  and that may provide images to display  220 ). As an example, the render controller  618  may adjust images displayed to the user based on the user&#39;s center of rotation or center of perspective. In particular, the render controller  618  may use information on the user&#39;s center of perspective to simulate a render camera (i.e., to simulate collecting images from the user&#39;s perspective) and may adjust images displayed to the user based on the simulated render camera. 
     A “render camera,” which is sometimes also referred to as a “pinhole perspective camera” (or simply “perspective camera”) or “virtual pinhole camera” (or simply “virtual camera”), is a simulated camera for use in rendering virtual image content possibly from a database of objects in a virtual world. The objects may have locations and orientations relative to the user or wearer and possibly relative to real objects in the environment surrounding the user or wearer. In other words, the render camera may represent a perspective within render space from which the user or wearer is to view 3D virtual contents of the render space (e.g., virtual objects). The render camera may be managed by a render engine to render virtual images based on the database of virtual objects to be presented to the eye. The virtual images may be rendered as if taken from the perspective the user or wearer. For example, the virtual images may be rendered as if captured by a pinhole camera (corresponding to the “render camera”) having a specific set of intrinsic parameters (e.g., focal length, camera pixel size, principal point coordinates, skew/distortion parameters, etc.), and a specific set of extrinsic parameters (e.g., translational components and rotational components relative to the virtual world). The virtual images are taken from the perspective of such a camera having a position and orientation of the render camera (e.g., extrinsic parameters of the render camera). It follows that the system may define and/or adjust intrinsic and extrinsic render camera parameters. For example, the system may define a particular set of extrinsic render camera parameters such that virtual images may be rendered as if captured from the perspective of a camera having a specific location with respect to the user&#39;s or wearer&#39;s eye so as to provide images that appear to be from the perspective of the user or wearer. The system may later dynamically adjust extrinsic render camera parameters on-the-fly so as to maintain registration with the specific location. Similarly, intrinsic render camera parameters may be defined and dynamically adjusted over time. In some implementations, the images are rendered as if captured from the perspective of a camera having an aperture (e.g., pinhole) at a specific location with respect to the user&#39;s or wearer&#39;s eye (such as the center of perspective or center of rotation, or elsewhere). 
     In some embodiments, the system may create or dynamically reposition and/or reorient one render camera for the user&#39;s left eye, and another render camera for the user&#39;s right eye, as the user&#39;s eyes are physically separated from one another and thus consistently positioned at different locations. It follows that, in at least some implementations, virtual content rendered from the perspective of a render camera associated with the viewer&#39;s left eye may be presented to the user through an eyepiece on the left side of a head-mounted display (e.g., head-mounted unit  602 ), and that virtual content rendered from the perspective of a render camera associated with the user&#39;s right eye may be presented to the user through an eyepiece on the right side of such a head-mounted display. Further details discussing the creation, adjustment, and use of render 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. 
     In some examples, one or more modules (or components) of the system  600  (e.g., light-field render controller  618 , render engine  622 , etc.) may determine the position and orientation of the render camera within render space based on the position and orientation of the user&#39;s head and eyes (e.g., as determined based on head pose and eye tracking data, respectively). That is, the system  600  may effectively map the position and orientation of the user&#39;s head and eyes to particular locations and angular positions within a 3D virtual environment, place and orient render cameras at the particular locations and angular positions within the 3D virtual environment, and render virtual content for the user as it would be captured by the render 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. As an example, the render controller  618  may adjust the depths at which images are displayed by selecting which depth plane (or depth planes) are utilized at any given time to display the images. In some implementations, such a depth plane switch may be carried out through an adjustment of one or more intrinsic render camera parameters. 
     Registration observer  620  may use information from eye tracking module  614  to identify whether the head-mounted unit  602  is properly positioned on a user&#39;s head. As an example, the eye tracking module  614  may provide eye location information, such as the positions of the centers of rotation of the user&#39;s eyes, indicative of the three-dimensional position of the user&#39;s eyes relative to camera  324  and head-mounted unit  602  and the eye tracking module  614  may use the location information to determine if display  220  is properly aligned in the user&#39;s field of view, or if the head-mounted unit  602  (or headset) has slipped or is otherwise misaligned with the user&#39;s eyes. As examples, the registration observer  620  may be able to determine if the head-mounted unit  602  has slipped down the user&#39;s nose bridge, thus moving display  220  away and down from the user&#39;s eyes (which may be undesirable), if the head-mounted unit  602  has been moved up the user&#39;s nose bridge, thus moving display  220  closer and up from the user&#39;s eyes, if the head-mounted unit  602  has been shifted left or right relative the user&#39;s nose bridge, if the head-mounted unit  602  has been lifted above the user&#39;s nose bridge, or if the head-mounted unit  602  has been moved in these or other ways away from a desired position or range of positions. In general, registration observer  620  may be able to determine if head-mounted unit  602 , in general, and displays  220 , in particular, are properly positioned in front of the user&#39;s eyes. In other words, the registration observer  620  may determine if a left display in display system  220  is appropriately aligned with the user&#39;s left eye and a right display in display system  220  is appropriately aligned with the user&#39;s right eye. The registration observer  620  may determine if the head-mounted unit  602  is properly positioned by determining if the head-mounted unit  602  is positioned and oriented within a desired range of positions and/or orientations relative to the user&#39;s eyes. 
     In at least some embodiments, registration observer  620  may generate user feedback in the form of alerts, messages, or other content. Such feedback may be provided to the user to inform the user of any misalignment of the head-mounted unit  602 , along with optional feedback on how to correct the misalignment (such as a suggestion to adjust the head-mounted unit  602  in a particular manner). 
     Example registration observation and feedback techniques, which may be utilized by registration observer  620 , are described in U.S. patent application Ser. No. 15/717,747, filed Sep. 27, 2017 (Attorney Docket No. MLEAP.052A2), which is incorporated by reference herein in its entirety. 
     Example of an Eye Tracking Module 
     A detailed block diagram of an example eye tracking module  614  is shown in  FIG.  7 A . As shown in  FIG.  7 A , eye tracking module  614  may include a variety of different submodules, may provide a variety of different outputs, and may utilize a variety of available data in tracking the user&#39;s eyes. As examples, eye tracking module  614  may utilize available data including eye tracking extrinsics and intrinsics, such as the geometric arrangements of the eye-tracking camera  324  relative to the light sources  326  and the head-mounted-unit  602 ; assumed eye dimensions  704  such as a typical distance of approximately 4.7 mm between a user&#39;s center of cornea curvature and the average center of rotation of the user&#39;s eye or typical distances between a user&#39;s center of rotation and center of perspective; and per-user calibration data  706  such as a particular user&#39;s interpupillary distance. Additional examples of extrinsics, intrinsics, and other information that may be employed by the eye tracking module  614  are described in U.S. patent application Ser. No. 15/497,726, filed Apr. 26, 2017 (Attorney Docket No. MLEAP.023A7), which is incorporated by reference herein in its entirety. 
     Image preprocessing module  710  may receive images from an eye camera such as eye camera  324  and may perform one or more preprocessing (i.e., conditioning) operations on the received images. As examples, image preprocessing module  710  may apply a Gaussian blur to the images, may down sample the images to a lower resolution, may applying an unsharp mask, may apply an edge sharpening algorithm, or may apply other suitable filters that assist with the later detection, localization, and labelling of glints, a pupil, or other features in the images from eye camera  324 . The image preprocessing module  710  may apply a low-pass filter or a morphological filter such as an open filter, which may remove high-frequency noise such as from the pupillary boundary  516   a  (see  FIG.  5   ), thereby removing noise that may hinder pupil and glint determination. The image preprocessing module  710  may output preprocessed images to the pupil identification module  712  and to the glint detection and labeling module  714 . 
     Pupil identification module  712  may receive preprocessed images from the image preprocessing module  710  and may identify regions of those images that include the user&#39;s pupil. The pupil identification module  712  may, in some embodiments, determine the coordinates of the position, or coordinates, of the center, or centroid, of the user&#39;s pupil in the eye tracking images from camera  324 . In at least some embodiments, pupil identification module  712  may identify contours in eye tracking images (e.g., contours of pupil iris boundary), identify contour moments (i.e., centers of mass), apply a starburst pupil detection and/or a canny edge detection algorithm, reject outliers based on intensity values, identify sub-pixel boundary points, correct for eye-camera distortion (i.e., distortion in images captured by eye camera  324 ), apply a random sample consensus (RANSAC) iterative algorithm to fit an ellipse to boundaries in the eye tracking images, apply a tracking filter to the images, and identify sub-pixel image coordinates of the user&#39;s pupil centroid. The pupil identification module  712  may output pupil identification data, which may indicate which regions of the preprocessing images module  712  identified as showing the user&#39;s pupil, to glint detection and labeling module  714 . The pupil identification module  712  may provide the 2D coordinates of the user&#39;s pupil (i.e., the 2D coordinates of the centroid of the user&#39;s pupil) within each eye tracking image to glint detection module  714 . In at least some embodiments, pupil identification module  712  may also provide pupil identification data of the same sort to coordinate system normalization module  718 . 
     Pupil detection techniques, which may be utilized by pupil identification module  712 , are described in U.S. Patent Publication No. 2017/0053165, published Feb. 23, 2017 and in U.S. Patent Publication No. 2017/0053166, published Feb. 23, 2017, each of which is incorporated by reference herein in its entirety. 
     Glint detection and labeling module  714  may receive preprocessed images from module  710  and pupil identification data from module  712 . Glint detection module  714  may use this data to detect and/or identify glints (i.e., reflections off of the user&#39;s eye of the light from light sources  326 ) within regions of the preprocessed images that show the user&#39;s pupil. As an example, the glint detection module  714  may search for bright regions within the eye tracking image, sometimes referred to herein as “blobs” or local intensity maxima, that are in the vicinity of the user&#39;s pupil. In at least some embodiments, the glint detection module  714  may rescale (e.g., enlarge) the pupil ellipse to encompass additional glints. The glint detection module  714  may filter glints by size and/or by intensity. The glint detection module  714  may also determine the 2D positions of each of the glints within the eye tracking image. In at least some examples, the glint detection module  714  may determine the 2D positions of the glints relative to the user&#39;s pupil, which may also be referred to as the pupil-glint vectors. Glint detection and labeling module  714  may label the glints and output the preprocessing images with labeled glints to the 3D cornea center estimation module  716 . Glint detection and labeling module  714  may also pass along data such as preprocessed images from module  710  and pupil identification data from module  712 . 
     Pupil and glint detection, as performed by modules such as modules  712  and  714 , may use any suitable techniques. As examples, edge detection may be applied to the eye image to identify glints and pupils. Edge detection may be applied by various edge detectors, edge detection algorithms, or filters. For example, a Canny Edge detector may be applied to the image to detect edges such as in lines of the image. Edges may include points located along a line that correspond to the local maximum derivative. For example, the pupillary boundary  516   a  (see  FIG.  5   ) may be located using a Canny edge detector. With the location of the pupil determined, various image processing techniques may be used to detect the “pose” of the pupil  116 . Determining an eye pose of an eye image may also be referred to as detecting an eye pose of the eye image. The pose may also be referred to as the gaze, pointing direction, or the orientation of the eye. For example, the pupil may be looking leftwards towards an object, and the pose of the pupil could be classified as a leftwards pose. Other methods may be used to detect the location of the pupil or glints. For example, a concentric ring may be located in an eye image using a Canny Edge detector. As another example, an integro-differential operator may be used to find the pupillary or limbus boundaries of the iris. For example, the Daugman integro-differential operator, the Hough transform, or other iris segmentation techniques may be used to return a curve that estimates the boundary of the pupil or the iris. 
     3D cornea center estimation module  716  may receive preprocessed images including detected glint data and pupil identification data from modules  710 ,  712 ,  714 . 3D cornea center estimation module  716  may use these data to estimate the 3D position of the user&#39;s cornea. In some embodiments, the 3D cornea center estimation module  716  may estimate the 3D position of an eye&#39;s center of cornea curvature or a user&#39;s corneal sphere, i.e., the center of an imaginary sphere having a surface portion generally coextensive with the user&#39;s cornea. The 3D cornea center estimation module  716  may provide data indicating the estimated 3D coordinates of the corneal sphere and/or user&#39;s cornea to the coordinate system normalization module  718 , the optical axis determination module  722 , and/or the light-field render controller  618 . Further details of the operation of the 3D cornea center estimation module  716  are provided herein in connection with  FIGS.  8 A- 8 E . Techniques for estimating the positions of eye features such as a cornea or corneal sphere, which may be utilized by 3D cornea center estimation module  716  and other modules in the wearable systems of the present disclosure are discussed in U.S. patent application Ser. No. 15/497,726, filed Apr. 26, 2017 (Attorney Docket No. MLEAP.023A7), which is incorporated by reference herein in its entirety. 
     Coordinate system normalization module  718  may optionally (as indicated by its dashed outline) be included in eye tracking module  614 . Coordinate system normalization module  718  may receive data indicating the estimated 3D coordinates of the center of the user&#39;s cornea (and/or the center of the user&#39;s corneal sphere) from the 3D cornea center estimation module  716  and may also receive data from other modules. Coordinate system normalization module  718  may normalize the eye camera coordinate system, which may help to compensate for slippages of the wearable device (e.g., slippages of the head-mounted component from its normal resting position on the user&#39;s head, which may be identified by registration observer  620 ). Coordinate system normalization module  718  may rotate the coordinate system to align the z-axis (i.e., the vergence depth axis) of the coordinate system with the cornea center (e.g., as indicated by the 3D cornea center estimation module  716 ) and may translate the camera center (i.e., the origin of the coordinate system) to a predetermined distance away from the cornea center such as 30 mm (i.e., module  718  may enlarge or shrink the eye tracking image depending on whether the eye camera  324  was determined to be nearer or further than the predetermined distance). With this normalization process, the eye tracking module  614  may be able to establish a consistent orientation and distance in the eye tracking data, relatively independent of variations of headset positioning on the user&#39;s head. Coordinate system normalization module  718  may provide 3D coordinates of the center of the cornea (and/or corneal sphere), pupil identification data, and preprocessed eye tracking images to the 3D pupil center locator module  720 . Further details of the operation of the coordinate system normalization module  718  are provided herein in connection with  FIGS.  9 A- 9 C . 
     3D pupil center locator module  720  may receive data, in the normalized or the unnormalized coordinate system, including the 3D coordinates of the center of the user&#39;s cornea (and/or corneal sphere), pupil location data, and preprocessed eye tracking images. 3D pupil center locator module  720  may analyze such data to determine the 3D coordinates of the center of the user&#39;s pupil in the normalized or unnormalized eye camera coordinate system. The 3D pupil center locator module  720  may determine the location of the user&#39;s pupil in three-dimensions based on the 2D position of the pupil centroid (as determined by module  712 ), the 3D position of the cornea center (as determined by module  716 ), assumed eye dimensions  704  such as the size of the a typical user&#39;s corneal sphere and the typical distance from the cornea center to the pupil center, and optical properties of eyes such as the index of refraction of the cornea (relative to the index of refraction of air) or any combination of these. Further details of the operation of the 3D pupil center locator module  720  are provided herein in connection with  FIGS.  9 D- 9 G . Techniques for estimating the positions of eye features such as a pupil, which may be utilized by 3D pupil center locator module  720  and other modules in the wearable systems of the present disclosure are discussed in U.S. patent application Ser. No. 15/497,726, filed Apr. 26, 2017 (Attorney Docket No. MLEAP.023A7), which is incorporated by reference herein in its entirety. 
     Optical axis determination module  722  may receive data from modules  716  and  720  indicating the 3D coordinates of the center of the user&#39;s cornea and the user&#39;s pupil. Based on such data, the optical axis determination module  722  may identify a vector from the position of the cornea center (i.e., from the center of the corneal sphere) to the center of the user&#39;s pupil, which may define the optical axis of the user&#39;s eye. Optical axis determination module  722  may provide outputs specifying the user&#39;s optical axis to modules  724 ,  728 ,  730 , and  732 , as examples. 
     Center of rotation (CoR) estimation module  724  may receive data from module  722  including parameters of the optical axis of the user&#39;s eye (i.e., data indicating the direction of the optical axis in a coordinate system with a known relation to the head-mounted unit  602 ). CoR estimation module  724  may estimate the center of rotation of a user&#39;s eye (i.e., the point around which the user&#39;s eye rotates when the user eye rotates left, right, up, and/or down). While eyes may not rotate perfectly around a singular point, assuming a singular point may be sufficient. In at least some embodiments, CoR estimation module  724  may estimate an eye&#39;s center of rotation by moving from the center of the pupil (identified by module  720 ) or the center of curvature of the cornea (as identified by module  716 ) toward the retina along the optical axis (identified by module  722 ) a particular distance. This particular distance may be an assumed eye dimension  704 . As one example, the particular distance between the center of curvature of the cornea and the CoR may be approximately 4.7 mm. This distance may be varied for a particular user based on any relevant data including the user&#39;s age, sex, vision prescription, other relevant characteristics, etc. 
     In at least some embodiments, the CoR estimation module  724  may refine its estimate of the center of rotation of each of the user&#39;s eyes over time. As an example, as time passes, the user will eventually rotate their eyes (to look somewhere else, at something closer, further, or sometime left, right, up, or down) causing a shift in the optical axis of each of their eyes. CoR estimation module  724  may then analyze two (or more) optical axes identified by module  722  and locate the 3D point of intersection of those optical axes. The CoR estimation module  724  may then determine the center of rotation lies at that 3D point of intersection. Such a technique may provide for an estimate of the center of rotation, with an accuracy that improves over time. Various techniques may be employed to increase the accuracy of the CoR estimation module  724  and the determined CoR positions of the left and right eyes. As an example, the CoR estimation module  724  may estimate the CoR by finding the average point of intersection of optical axes determined for various different eye poses over time. As additional examples, module  724  may filter or average estimated CoR positions over time, may calculate a moving average of estimated CoR positions over time, and/or may apply a Kalman filter and known dynamics of the eyes and eye tracking system to estimate the CoR positions over time. As a specific example, module  724  may calculate a weighted average of determined points of optical axes intersection and assumed CoR positions (such as 4.7 mm from an eye&#39;s center of cornea curvature), such that the determined CoR may slowly drift from an assumed CoR position (i.e., 4.7 mm behind an eye&#39;s center of cornea curvature) to a slightly different location within the user&#39;s eye over time as eye tracking data for the user is obtain and thereby enables per-user refinement of the CoR position. 
     Interpupillary distance (IPD) estimation module  726  may receive data from CoR estimation module  724  indicating the estimated 3D positions of the centers of rotation of the user&#39;s left and right eyes. IPD estimation module  726  may then estimate a user&#39;s IPD by measuring the 3D distance between the centers of rotation of the user&#39;s left and right eyes. In general, the distance between the estimated CoR of the user&#39;s left eye and the estimated CoR of the user&#39;s right eye may be roughly equal to the distance between the centers of a user&#39;s pupils, when the user is looking at optical infinity (i.e., the optical axes of the user&#39;s eyes are substantially parallel to one another), which is the typical definition of interpupillary distance (IPD). A user&#39;s IPD may be used by various components and modules in the wearable system. As example, a user&#39;s IPD may be provided to registration observer  620  and used in assessing how well the wearable device is aligned with the user&#39;s eyes (e.g., whether the left and right display lenses are properly spaced in accordance with the user&#39;s IPD). As another example, a user&#39;s IPD may be provided to vergence depth estimation module  728  and be used in determining a user&#39;s vergence depth. Module  726  may employ various techniques, such as those discussed in connection with CoR estimation module  724 , to increase the accuracy of the estimated IPD. As examples, IPD estimation module  724  may apply filtering, averaging over time, weighted averaging including assumed IPD distances, Kalman filters, etc. as part of estimating a user&#39;s IPD in an accurate manner. 
     Vergence depth estimation module  728  may receive data from various modules and submodules in the eye tracking module  614  (as shown in connection with  FIG.  7 A ). In particular, vergence depth estimation module  728  may employ data indicating estimated 3D positions of pupil centers (e.g., as provided by module  720  described above), one or more determined parameters of optical axes (e.g., as provided by module  722  described above), estimated 3D positions of centers of rotation (e.g., as provided by module  724  described above), estimated IPD (e.g., Euclidean distance(s) between estimated 3D positions of centers of rotations) (e.g., as provided by module  726  described above), and/or one or more determined parameters of optical and/or visual axes (e.g., as provided by module  722  and/or module  730  described below). Vergence depth estimation module  728  may detect or otherwise obtain a measure of a user&#39;s vergence depth, which may be the distance from the user at which the user&#39;s eyes are focused. As examples, when the user is looking at an object three feet in front of them, the user&#39;s left and right eyes have a vergence depth of three feet; and, while when the user is looking at a distant landscape (i.e., the optical axes of the user&#39;s eyes are substantially parallel to one another such that the distance between the centers of the user&#39;s pupils may be roughly equal to the distance between the centers of rotation of the user&#39;s left and right eyes), the user&#39;s left and right eyes have a vergence depth of infinity. In some implementations, the vergence depth estimation module  728  may utilize data indicating the estimated centers of the user&#39;s pupils (e.g., as provided by module  720 ) to determine the 3D distance between the estimated centers of the user&#39;s pupils. The vergence depth estimation module  728  may obtain a measure of vergence depth by comparing such a determined 3D distance between pupil centers to estimated IPD (e.g., Euclidean distance(s) between estimated 3D positions of centers of rotations) (e.g., as indicated by module  726  described above). In addition to the 3D distance between pupil centers and estimated IPD, the vergence depth estimation module  728  may utilize known, assumed, estimated, and/or determined geometries to calculate vergence depth. As an example, module  728  may combine 3D distance between pupil centers, estimated IPD, and 3D CoR positions in a trigonometric calculation to estimate (i.e., determine) a user&#39;s vergence depth. Indeed, an evaluation of such a determined 3D distance between pupil centers against estimated IPD may serve to indicate a measure of the user&#39;s current vergence depth relative to optical infinity. In some examples, the vergence depth estimation module  728  may simply receive or access data indicating an estimated 3D distance between the estimated centers of the user&#39;s pupils for purposes of obtaining such a measure of vergence depth. In some embodiments, the vergence depth estimation module  728  may estimate vergence depth by comparing a user&#39;s left and right optical axis. In particular, vergence depth estimation module  728  may estimate vergence depth by locating the distance from a user at which the user&#39;s left and right optical axes intersect (or where projections of the user&#39;s left and right optical axes on a plane such as a horizontal plane intersect). Module  728  may utilize a user&#39;s IPD in this calculation, by setting the zero depth to be the depth at which the user&#39;s left and right optical axes are separated by the user&#39;s IPD. In at least some embodiments, vergence depth estimation module  728  may determine vergence depth by triangulating eye tracking data together with known or derived spatial relationships. 
     In some embodiments, vergence depth estimation module  728  may estimate a user&#39;s vergence depth based on the intersection of the user&#39;s visual axes (instead of their optical axes), which may provide a more accurate indication of the distance at which the user is focused on. In at least some embodiments, eye tracking module  614  may include optical to visual axis mapping module  730 . As discussed in further detail in connection with  FIG.  10   , a user&#39;s optical and visual axis are generally not aligned. A visual axis is the axis along which a person is looking, while an optical axis is defined by the center of that person&#39;s lens and pupil, and may go through the center of the person&#39;s retina. In particular, a user&#39;s visual axis is generally defined by the location of the user&#39;s fovea, which may be offset from the center of a user&#39;s retina, thereby resulting in different optical and visual axis. In at least some of these embodiments, eye tracking module  614  may include optical to visual axis mapping module  730 . Optical to visual axis mapping module  730  may correct for the differences between a user&#39;s optical and visual axis and provide information on the user&#39;s visual axis to other components in the wearable system, such as vergence depth estimation module  728  and light-field render controller  618 . In some examples, module  730  may use assumed eye dimensions  704  including a typical offset of approximately 5.2° inwards (nasally, towards a user&#39;s nose) between an optical axis and a visual axis. In other words, module  730  may shift a user&#39;s left optical axis (nasally) rightwards by 5.2° towards the nose and a user&#39;s right optical axis (nasally) leftwards by 5.2° towards the nose in order to estimate the directions of the user&#39;s left and right optical axes. In other examples, module  730  may utilize per-user calibration data  706  in mapping optical axes (e.g., as indicated by module  722  described above) to visual axes. As additional examples, module  730  may shift a user&#39;s optical axes nasally by between 4.0° and 6.5°, by between 4.5° and 6.0°, by between 5.0° and 5.4°, etc., or any ranges formed by any of these values. In some arrangements, the module  730  may apply a shift based at least in part upon characteristics of a particular user such as their age, sex, vision prescription, or other relevant characteristics and/or may apply a shift based at least in part upon a calibration process for a particular user (i.e., to determine a particular user&#39;s optical-visual axis offset). In at least some embodiments, module  730  may also shift the origins of the left and right optical axes to correspond with the user&#39;s CoP (as determined by module  732 ) instead of the user&#39;s CoR. 
     Optional center of perspective (CoP) estimation module  732 , when provided, may estimate the location of the user&#39;s left and right centers of perspective (CoP). A CoP may be a useful location for the wearable system and, in at least some embodiments, is a position just in front of a pupil. In at least some embodiments, CoP estimation module  732  may estimate the locations of a user&#39;s left and right centers of perspective based on the 3D location of a user&#39;s pupil center, the 3D location of a user&#39;s center of cornea curvature, or such suitable data or any combination thereof. As an example, a user&#39;s CoP may be approximately 5.01 mm in front of the center of cornea curvature (i.e., 5.01 mm from the corneal sphere center in a direction that is towards the eye&#39;s cornea and that is along the optical axis) and may be approximately 2.97 mm behind the outer surface of a user&#39;s cornea, along the optical or visual axis. A user&#39;s center of perspective may be just in front of the center of their pupil. As examples, a user&#39;s CoP may be less than approximately 2.0 mm from the user&#39;s pupil, less than approximately 1.0 mm from the user&#39;s pupil, or less than approximately 0.5 mm from the user&#39;s pupil or any ranges between any of these values. As another example, the center of perspective may correspond to a location within the anterior chamber of the eye. As other examples, the CoP may be between 1.0 mm and 2.0 mm, about 1.0 mm, between 0.25 mm and 1.0 mm, between 0.5 mm and 1.0 mm, or between 0.25 mm and 0.5 mm. 
     The center of perspective described herein (as a potentially desirable position for a pinhole of a render camera and an anatomical position in a user&#39;s eye) may be a position that serves to reduce and/or eliminate undesired parallax shifts. In particular, the optical system of a user&#39;s eye is very roughly equivalent to theoretical system formed by a pinhole in front of a lens, projecting onto a screen, with the pinhole, lens, and screen roughly corresponding to a user&#39;s pupil/iris, lens, and retina, respectively. Moreover, it may be desirable for there to be little or no parallax shift when two point light sources (or objects) at different distances from the user&#39;s eye are rigidly rotated about the opening of the pinhole (e.g., rotated along radii of curvature equal to their respective distance from the opening of the pinhole). Thus, it would seem that the CoP should be located at the center of the pupil of an eye (and such a CoP may be used in some embodiments). However, the human eye includes, in addition to the lens and pinhole of the pupil, a cornea that imparts additional optical power to light propagating toward the retina). Thus, the anatomical equivalent of the pinhole in the theoretical system described in this paragraph may be a region of the user&#39;s eye positioned between the outer surface of the cornea of the user&#39;s eye and the center of the pupil or iris of the user&#39;s eye. For instance, the anatomical equivalent of the pinhole may correspond to a region within the anterior chamber of a user&#39;s eye. For various reasons discussed herein, it may be desired to set the CoP to such a position within the anterior chamber of the user&#39;s eye. 
     As discussed above, eye tracking module  614  may provide data, such as estimated 3D positions of left and right eye centers of rotation (CoR), vergence depth, left and right eye optical axis, 3D positions of a user&#39;s eye, 3D positions of a user&#39;s left and right centers of cornea curvature, 3D positions of a user&#39;s left and right pupil centers, 3D positions of a user&#39;s left and right center of perspective, a user&#39;s IPD, etc., to other components, such as light-field render controller  618  and registration observer  620 , in the wearable system. Eye tracking module  614  may also include other submodules that detect and generate data associated with other aspects of a user&#39;s eye. As examples, eye tracking module  614  may include a blink detection module that provides a flag or other alert whenever a user blinks and a saccade detection module that provides a flag or other alert whenever a user&#39;s eye saccades (i.e., quickly shifts focus to another point). 
     Example of a Render Controller 
     A detailed block diagram of an example light-field render controller  618  is shown in  FIG.  7 B . As shown in  FIGS.  6  and  7 B , render controller  618  may receive eye tracking information from eye tracking module  614  and may provide outputs to render engine  622 , which may generate images to be displayed for viewing by a user of the wearable system. As examples, render controller  618  may receive information regarding a vergence depth, left and right eye centers of rotation (and/or centers of perspective), and other eye data such as blink data, saccade data, etc. 
     Depth plane selection module  750  may receive vergence depth information and, based on such data, may cause render engine  622  to provide content to a user, with the content appearing to be located on a particular depth plane (i.e., at a particular accommodation or focal distance). As discussed in connection with  FIG.  4   , a wearable system may include a plurality of discrete depth planes formed by a plurality of waveguides, each conveying image information with a varying level of wavefront curvature. In some embodiments, a wearable system may include one or more variable depth planes, such as an optical element that conveys image information with a level of wavefront curvature that varies over time. In these and other embodiments, depth plane selection module  750  may cause render engine  622  to convey content to a user at a selected depth (i.e., cause render engine  622  to direct display  220  to switch depth planes), based in part of the user&#39;s vergence depth. In at least some embodiments, depth plane selection module  750  and render engine  622  may render content at different depths and also generate and/or provide depth plane selection data to display hardware such as display  220 . Display hardware such as display  220  may perform an electrical depth plane switching in response to depth plane selection data (which may be control signals) generated by and/or provided by modules such as depth plane selection module  750  and render engine  622 . 
     In general, it may be desirable for depth plane selection module  750  to select a depth plane matching the user&#39;s current vergence depth, such that the user is provided with accurate accommodation cues. However, it may also be desirable to switch depth planes in a discreet and unobtrusive manner. As examples, it may be desirable to avoid excessive switching between depth planes and/or it may be desire to switch depth planes at a time when the user is less likely to notice the switch, such as during a blink or eye saccade. 
     Hysteresis band crossing detection module  752  may help to avoid excessive switching between depth planes, particularly when a user&#39;s vergence depth fluctuates at the midpoint or transition point between two depth planes. In particular, module  752  may cause depth plane selection module  750  to exhibit hysteresis in its selection of depth planes. As an example, modules  752  may cause depth plane selection module  750  to switch from a first farther depth plane to a second closer depth plane only after a user&#39;s vergence depth passes a first threshold. Similarly, module  752  may cause depth plane selection module  750  (which may in turn direct displays such as display  220 ) to switch to the first farther depth plane only after the user&#39;s vergence depth passes a second threshold that is farther from the user than the first threshold. In the overlapping region between the first and second thresholds, module  750  may cause depth plane selection module  750  to maintain whichever depth plane is currently select as the selected depth plane, thus avoiding excessive switching between depth planes. 
     Ocular event detection module  750  may receive other eye data from the eye tracking module  614  of  FIG.  7 A  and may cause depth plane selection module  750  to delay some depth plane switches until an ocular event occurs. As an example, ocular event detection module  750  may cause depth plane selection module  750  to delay a planned depth plane switch until a user blink is detected; may receive data from a blink detection component in eye tracking module  614  that indicates when the user is currently blinking; and, in response, may cause depth plane selection module  750  to execute the planned depth plane switch during the blink event (such by causing module  750  to direct display  220  to execute the depth plane switch during the blink event). In at least some embodiments, the wearable system may be able to shift content onto a new depth plane during a blink event such that the user is unlikely to perceive the shift. As another example, ocular event detection module  750  may delay planned depth plane switches until an eye saccade is detected. As discussed in connection with eye blinks, such as an arrangement may facilitate the discretely shifting of depth planes. 
     If desired, depth plane selection module  750  may delay planned depth plane switches only for a limited period of time before executing the depth plane switch, even in the absence of an ocular event. Similarly, depth plane selection module  750  may execute a depth plane switch when the user&#39;s vergence depth is substantially outside of a currently-selected depth plane (i.e., when the user&#39;s vergence depth has exceeded a predetermined threshold beyond the regular threshold for a depth plane switch), even in the absence of an ocular event. These arrangements may help ensure that ocular event detection module  754  does not indefinitely delay depth plane switches and does not delay depth plane switches when a large accommodation error is present. 
     Render camera controller  758  may provide information to render engine  622  indicating where the user&#39;s left and right eyes are. Render engine  622  may then generate content by simulating cameras at the positions of the user&#39;s left and right eyes and generating content based on the perspectives of the simulated cameras. As discussed above, the render camera is a simulated camera for use in rendering virtual image content possibly from a database of objects in a virtual world. The objects may have locations and orientations relative to the user or wearer and possibly relative to real objects in the environment surrounding the user or wearer. The render camera may be included in a render engine to render virtual images based on the database of virtual objects to be presented to the eye. The virtual images may be rendered as if taken from the perspective the user or wearer. For example, the virtual images may be rendered as if captured by a camera (corresponding to the “render camera”) having an aperture, lens, and detector viewing the objects in the virtual world. The virtual images are taken from the perspective of such a camera having a position of the “render camera.” For example, the virtual images may be rendered as if captured from the perspective of a camera having a specific location with respect to the user&#39;s or wearer&#39;s eye so as to provide images that appear to be from the perspective of the user or wearer. In some implementations, the images are rendered as if captured from the perspective of a camera having an aperture at a specific location with respect to the user&#39;s or wearer&#39;s eye (such as the center of perspective or center of rotation as discussed herein, or elsewhere). 
     Render camera controller  758  may determine the positions of the left and right cameras based on the left and right eye centers of rotation (CoR), determined by CoR estimation module  724 , and/or based on the left and right eye centers of perspective (CoP), determined by CoP estimation module  732 . In some embodiments, render camera controller  758  may switch between the CoR and CoP locations based on various factors. As examples, the render camera controller  758  may, in various modes, register the render camera to the CoR locations at all times, register the render camera to the CoP locations at all times, toggle or discretely switch between registering the render camera to the CoR locations and registering the render camera to the CoP locations over time based on various factors, or dynamically register the render camera to any of a range of different positions along the optical (or visual) axis between the CoR and CoP locations over time based on various factors. The CoR and CoP positions may optionally pass through smoothing filter  756  (in any of the aforementioned modes for render camera positioning) which may average the CoR and CoP locations over time to reduce noise in these positions and prevent jitter in the render simulated render cameras. 
     In at least some embodiments, the render camera may be simulated as a pinhole camera with the pinhole disposed at the position of the estimated CoR or CoP identified by eye tracking module  614 . As the CoP is offset from the CoR, the location of the render camera and its pinhole both shift as the user&#39;s eye rotates, whenever the render camera&#39;s position is based on a user&#39;s CoP. In contrast, whenever the render camera&#39;s position is based on a user&#39;s CoR, the location of the render camera&#39;s pinhole does not move with eye rotations, although the render camera (which is behind the pinhole) may, in some embodiments, move with eye rotation. In other embodiments where the render camera&#39;s position is based on a user&#39;s CoR, the render camera may not move (i.e., rotate) with a user&#39;s eye. 
     Example of a Registration Observer 
     A block diagram of an example registration observer  620  is shown in  FIG.  7 C . As shown in  FIGS.  6 ,  7 A, and  7 C , registration observer  620  may receive eye tracking information from eye tracking module  614  ( FIGS.  6  and  7 A ). As examples, registration observer  620  may receive information on a user&#39;s left and right eye centers of rotation (e.g., the three-dimensional positions of the user&#39;s left and right eye centers of rotations, which may be on a common coordinate system or have a common frame of reference with the head-mounted display system  600 ). As other examples, registration observer  620  may receive display extrinsics, fit tolerances, and an eye-tracking valid indicator. The display extrinsics may include information on the display (e.g., display  200  of  FIG.  2   ) such as the field of view of the display, the size of one or more display surfaces, and the positions of the display surfaces relative to the head-mounted display system  600 . The fit tolerances may include information on display registration volumes, which may indicate how far the user&#39;s left and right eyes may move from nominal positions before display performance is impacted. In addition, the fit tolerances may indicate the amount of display performance impact that is expected as a function of the positions of the user&#39;s eyes. 
     As shown in  FIG.  7 C , registration observer  620  may include a 3D positional fit module  770 . The positional fit module  770  may obtain and analyze various pieces of data including, as examples, a left eye center of rotation 3D position (e.g., CoR Left), a right eye center of rotation 3D position (e.g., CoR Right), display extrinsics, and fit tolerances. The 3D positional fit module  770  may determine how far the user&#39;s left and right eyes are from the respective left and right eye nominal positions (e.g., may calculate 3D left error and 3D right error) and may provide the error distances (e.g., 3D left error and 3D right error) to device 3D fit module  772 . 
     3D positional fit module  770  may also compare the error distances to the display extrinsics and the fit tolerances to determine if the users eye are within a nominal volume, a partially-degraded volume (e.g., a volume in which the performance of display  220  is partially degraded), or in a fully degraded or nearly fully degraded volume (e.g., a volume in which display  220  is substantially unable to provide content to the user&#39;s eyes). In at least some embodiments, 3D positional fit module  770  or 3D fit module  772  may provide an output qualitatively describing the fit of the HMD on the user, such as the Quality of Fit output shown in  FIG.  7 C . As an example, module  770  may provide an output indicating whether the current fit of the HMD on the user is good, marginal, or failed. A good fit may correspond to a fit that enables the user to view at least a certain percentage of the image (such as 90%), a marginal fit may enable the user to view at least a lower percentage of the image (such as 80%), while a failed fit may be a fit in which only an even lower percentage of the image is visible to the user. 
     As another example, the 3D positional fit module  770  and/or device 3D fit module  772  may calculate a visible area metric, which may be a percentage of the overall area (or pixels) of images display by display  220  that are visible to the user. Modules  770  and  772  may calculate the visible area metric by evaluating the positions of the user&#39;s left and right eyes (e.g., which may be based on the centers of rotation of the user&#39;s eyes) relative to display  220  and using one or more models (e.g., a mathematical or geometric model), one or more look-up tables, or other techniques or combinations of these and other techniques to determine what percentage of the images are visible to the user as a function of the positions of the user&#39;s eyes. Additionally, modules  770  and  772  may determine which regions or portions of the images display by display  220  are expected to be visible to the user as a function of the positions of the user&#39;s eyes. 
     Registration observer  620  may also include a device 3D fit module  772 . Module  772  may receive data from 3D positional fit module  770  and may also receive an eye tracking valid indicator, which may be provided by eye tracking module  614  and may indicate whether the eye tracking system is currently tracking the positions of the user&#39;s eyes or if eye tracking data is unavailable or in an error condition (e.g., determined to be no reliable). Device 3D fit module  772  may, if desired, modify quality of fit data received from 3D positional fit module  770  depending on the state of the eye tracking valid data. For example, if the data from the eye tracking system is indicated to not be available or to have an error, the device 3D fit module  772  may provide a notification that there is an error and/or not provide output to the user regarding fit quality or fit errors. 
     In at least some embodiments, registration observer  620  may provide feedback to users on the quality of fit as well as details of the nature and magnitude of the error. As examples, the head-mounted display system may provide feedback to the user during calibration or fitting processes (e.g., as part of a setup procedure) and may provide feedback during operation (e.g., if the fit degrades due to slippage, the registration observer  620  may prompt the user to readjust the head-mounted display system). In some embodiments, the registration analysis may be performed automatically (e.g., during use of the head-mounted display system) and the feedback may be provided without user input. These are merely illustrative examples. 
     Example of Locating a User&#39;s Cornea with an Eye Tracking System 
       FIG.  8 A  is a schematic diagram of an eye showing the eye&#39;s corneal sphere. As shown in  FIG.  8 A , a user&#39;s eye  810  may have a cornea  812 , a pupil  822 , and a lens  820 . The cornea  812  may have an approximately spherical shape, shown by corneal sphere  814 . Corneal sphere  814  may have a center point  816 , also referred to as a corneal center, and a radius  818 . The semispherical cornea of a user&#39;s eye may curve around the corneal center  816 . 
       FIGS.  8 B- 8 E  illustrate an example of locating a user&#39;s corneal center  816  using 3D cornea center estimation module  716  and eye tracking module  614 . 
     As shown in  FIG.  8 B , 3D cornea center estimation module  716  may receive an eye tracking image  852  that includes a corneal glint  854 . The 3D cornea center estimation module  716  may then simulate, in an eye camera coordinate system  850 , the known 3D positions of the eye camera  324  and light source  326  (which may be based on data in eye tracking extrinsics &amp; intrinsics database  702 , assumed eye dimensions database  704 , and/or per-user calibration data  706 ) in order to cast a ray  856  in the eye camera coordinate system. In at least some embodiments, the eye camera coordinate system  850  may have its origin at the 3D position of the eye-tracking camera  324 . 
     In  FIG.  8 C , 3D cornea center estimation module  716  simulates a corneal sphere  814   a  (which may be based on assumed eye dimensions from database  704 ) and corneal curvature center  816   a  at a first position. The 3D cornea center estimation module  716  may then check to see whether the corneal sphere  814   a  would properly reflect light from the light source  326  to the glint position  854 . As shown in  FIG.  8 C , the first position is not a match as the ray  860   a  does not intersect light source  326 . 
     Similarly in  FIG.  8 D , 3D cornea center estimation module  716  simulates a corneal sphere  814   b  and corneal curvature center  816   b  at a second position. The 3D cornea center estimation module  716  then checks to see whether the corneal sphere  814   b  properly reflects light from the light source  326  to the glint position  854 . As shown in  FIG.  8 D , the second position is also not a match. 
     As shown in  FIG.  8 E , the 3D cornea center estimation module  716  eventually is able to determine the correct position of the corneal sphere is corneal sphere  814   c  and corneal curvature center  816   c.  The 3D cornea center estimation module  716  confirms the illustrated position is correct by checking that light from source  326  will properly reflect off of the corneal sphere and be imaged by camera  324  at the correct location of glint  854  on image  852 . With this arrangement and with the known 3D positions of the light source  326 , the camera  324 , and the optical properties of the camera (focal length, etc.), the 3D cornea center estimation module  716  may determine the 3D location of the cornea&#39;s center of curvature  816  (relative to the wearable system). 
     The processes described herein in connection with at least  FIGS.  8 C- 8 E  may effectively be an iterative, repetitious, or optimization process to identify the 3D position of the user&#39;s cornea center. As such, any of a plurality of techniques (e.g., iterative, optimization techniques, etc.) may be used to efficiently and quickly prune or reduce the search space of possible positions. Moreover, in some embodiments, the system may include two, three, four, or more light sources such as light source  326  and some of all of these light sources may be disposed at different positions, resulting in multiple glints such as glint  854  located at different positions on image  852  and multiple rays such as ray  856  having different origins and directions. Such embodiments may enhance the accuracy of the 3D cornea center estimation module  716 , as the module  716  may seek to identify a cornea position that results in some or all of the glints &amp; rays being properly reflected between their respective light sources and their respective positions on image  852 . In other words and in these embodiments, the positions of some or all of the light sources may be relied upon in the 3D cornea position determination (e.g., iterative, optimization techniques, etc.) processes of  FIGS.  8 B- 8 E . 
     Example of Normalizing the Coordinate System of Eye Tracking Images 
       FIGS.  9 A- 9 C  illustrate an example normalization of the coordinate system of eye tracking images, by a component in the wearable system such as coordinate system normalization module  718  of  FIG.  7 A . Normalizing the coordinate system of eye tracking images relative to a user&#39;s pupil location may compensate for slippage of the wearable system relative to a user&#39;s face (i.e., headset slippage) and such normalization may establish a consistent orientation and distance between eye tracking images and a user&#39;s eyes. 
     As shown in  FIG.  9 A , coordinate system normalization module  718  may receive estimated 3D coordinates  900  of a user&#39;s center of corneal rotation and may receive un-normalized eye tracking images such as image  852 . Eye tracking image  852  and coordinates  900  may be in an un-normalized coordinate system  850  that is based on the location of eye-tracking camera  324 , as an example. 
     As a first normalization step, coordinate system normalization module  718  may rotate coordinate system  850  into rotated coordinate system  902 , such that the z-axis (i.e., the vergence depth axis) of the coordinate system may be aligned with a vector between the origin of the coordinate system and cornea center of curvature coordinates  900 , as shown in  FIG.  9 B . In particular, coordinate system normalization module  718  may rotate eye tracking image  850  into rotated eye tracking image  904 , until the coordinates  900  of the user&#39;s corneal center of curvature are normal to the plane of the rotated image  904 . 
     As a second normalization step, coordinate system normalization module  718  may translate rotated coordinate system  902  into normalized coordinate system  910 , such that cornea center of curvature coordinates  900  are a standard, normalized distance  906  from the origin of normalized coordinate system  910 , as shown in  FIG.  9 C . In particular, coordinate system normalization module  718  may translate rotated eye tracking image  904  into normalized eye tracking image  912 . In at least some embodiments, the standard, normalized distance  906  may be approximately 30 millimeters. If desired, the second normalization step may be performed prior to the first normalization step. 
     Example of Locating a User&#39;s Pupil Centroid with an Eye Tracking System 
       FIGS.  9 D- 9 G  illustrate an example of locating a user&#39;s pupil center (i.e., the center of a user&#39;s pupil  822  as shown in  FIG.  8 A ) using 3D pupil center locator module  720  and eye tracking module  614 . 
     As shown in  FIG.  9 D , 3D pupil center locator module  720  may receive a normalized eye tracking image  912  that includes a pupil centroid  913  (i.e., a center of a user&#39;s pupil as identified by pupil identification module  712 ). The 3D pupil center locator module  720  may then simulate the normalized 3D position  910  of eye camera  324  to cast a ray  914  in the normalized coordinate system  910 , through the pupil centroid  913 . 
     In  FIG.  9 E , 3D pupil center locator module  720  may simulate a corneal sphere such as corneal sphere  901  having center of curvature  900  based on data from 3D cornea center estimation module  716  (and as discussed in more detail in connection with  FIGS.  8 B- 8 E ). As an example, the corneal sphere  901  may be positioned in the normalized coordinate system  910  based on the location of the center of curvature  816   c  identified in connection with  FIG.  8 E  and based on the normalization processes of  FIGS.  9 A- 9 C . Additionally, 3D pupil center locator module  720  may identify a first intersection  916  between ray  914  (i.e., a ray between the origin of normalized coordinate system  910  and the normalized location of a user&#39;s pupil) and the simulated cornea, as shown in  FIG.  9 E . 
     As shown in  FIG.  9 F , 3D pupil center locator module  720  may determine pupil sphere  918  based on corneal sphere  901 . Pupil sphere  918  may share a common center of curvature with corneal sphere  901 , but have a small radius. 3D pupil center locator module  720  may determine a distance between cornea center  900  and pupil sphere  918  (i.e., a radius of pupil sphere  918 ) based on a distance between the corneal center and the pupil center. In some embodiments, the distance between a pupil center and a corneal center of curvature may be determined from assumed eye dimensions  704  of  FIG.  7 A , from eye tracking extrinsics and intrinsics database  702 , and/or from per-user calibration data  706 . In other embodiments, the distance between a pupil center and a corneal center of curvature may be determined from per-user calibration data  706  of  FIG.  7 A . 
     As shown in  FIG.  9 G , 3D pupil center locator module  720  may locate the 3D coordinates of a user&#39;s pupil center based on variety of inputs. As examples, the 3D pupil center locator module  720  may utilize the 3D coordinates and radius of the pupil sphere  918 , the 3D coordinates of the intersection  916  between a simulated cornea sphere  901  and a ray  914  associated with a pupil centroid  913  in a normalized eye tracking image  912 , information on the index of refraction of a cornea, and other relevant information such as the index of refraction of air (which may be stored in eye tracking extrinsics &amp; intrinsics database  702 ) to determine the 3D coordinates of the center of a user&#39;s pupil. In particular, the 3D pupil center locator module  720  may, in simulation, bend ray  916  into refracted ray  922  based on refraction difference between air (at a first index of refraction of approximately 1.00) and corneal material (at a second index of refraction of approximately 1.38). After taking into account refraction caused by the cornea, 3D pupil center locator module  720  may determine the 3D coordinates of the first intersection  920  between refracted ray  922  and pupil sphere  918 . 3D pupil center locator module  720  may determine that a user&#39;s pupil center  920  is located at approximately the first intersection  920  between refracted ray  922  and pupil sphere  918 . With this arrangement, the 3D pupil center locator module  720  may determine the 3D location of the pupil center  920  (relative to the wearable system), in the normalized coordinate system  910 . If desired, the wearable system may un-normalize the coordinates of the pupil center  920  into the original eye camera coordinate system  850 . The pupil center  920  may be used together with the corneal curvature center  900  to determine, among other things, a user&#39;s optical axis using optical axis determination module  722  and a user&#39;s vergence depth by vergence depth estimation module  728 . 
     Example of Differences Between Optical and Visual Axes 
     As discussed in connection with optical to visual mapping module  730  of  FIG.  7 A , a user&#39;s optical and visual axes are generally not aligned, due in part to a user&#39;s visual axis being defined by their fovea and that foveae are not generally in the center of a person&#39;s retina. Thus, when a person desires to concentrate on a particular object, the person aligns their visual axis with that object to ensure that light from the object falls on their fovea while their optical axis (defined by the center of their pupil and center of curvature of their cornea) is actually slightly offset from that object.  FIG.  10    is an example of an eye  1000  illustrating the eye&#39;s optical axis  1002 , the eye&#39;s visual axis  1004 , and the offset between these axes. Additionally,  FIG.  10    illustrates the eye&#39;s pupil center  1006 , the eye&#39;s center of cornea curvature  1008 , and the eye&#39;s average center of rotation (CoR)  1010 . In at least some populations, the eye&#39;s center of cornea curvature  1008  may lie approximately 4.7 mm in front, as indicated by dimension  1012 , of the eye&#39;s average center of rotation (CoR)  1010 . Additionally, the eye&#39;s center of perspective  1014  may lie approximately 5.01 mm in front of the eye&#39;s center of cornea curvature  1008 , about 2.97 mm behind the outer surface  1016  of the user&#39;s cornea, and/or just in front of the user&#39;s pupil center  1006  (e.g., corresponding to a location within the anterior chamber of eye  1000 ). As additional examples, dimension  1012  may between 3.0 mm and 7.0 mm, between 4.0 and 6.0 mm, between 4.5 and 5.0 mm, or between 4.6 and 4.8 mm or any ranges between any values and any values in any of these ranges. The eye&#39;s center of perspective (CoP)  1014  may be a useful location for the wearable system as, in at least some embodiments, registering a render camera at the CoP may help to reduce or eliminate parallax artifacts. 
       FIG.  10    also illustrates such a within a human eye  1000  with which the pinhole of a render camera may be aligned. As shown in  FIG.  10   , the pinhole of a render camera may be registered with a location  1014  along the optical axis  1002  or visual axis  1004  of the human eye  1000  closer to the outer surface of the cornea than both (a) the center of the pupil or iris  1006  and (b) the center of cornea curvature  1008  of the human eye  1000 . For example, as shown in  FIG.  10   , the pinhole of a render camera may be registered with a location  1014  along the optical axis  1002  of the human eye  1000  that is about 2.97 millimeters rearward from the outer surface of the cornea  1016  and about 5.01 millimeters forward from the center of cornea curvature  1008 . The location  1014  of the pinhole of the render camera and/or the anatomical region of the human eye  1000  to which the location  1014  corresponds may be seen as representing the center of perspective of the human eye  1000 . The optical axis  1002  of the human eye  1000  as shown in  FIG.  10    represents the most direct line through the center of cornea curvature  1008  and the center of the pupil or iris  1006 . The visual axis  1004  of the human eye  1000  differs from the optical axis  1002 , as it represents a line extending from the fovea of the human eye  1000  to the center of the pupil or iris  1006 . 
     Example Processes of Rendering Content and Checking Registration based on Eye Tracking 
       FIG.  11    is a process flow diagram of an example method  1100  for using eye tracking in rendering content and providing feedback on registration in a wearable device. The method  1100  may be performed by the wearable system described herein. Embodiments of the method  1100  may be used by the wearable system to render content and provide feedback on registration (i.e., fit of the wearable device to the user) based on data from an eye tracking system. 
     At block  1110 , the wearable system may capture images of a user&#39;s eye or eyes. The wearable system may capture eye images using one or more eye cameras  324 , as shown at least in the example of  FIG.  3   . If desired, the wearable system may also include one or more light sources  326  configured to shine IR light on a user&#39;s eyes and produce corresponding glints in the eye images captured by eye cameras  324 . As discussed herein, the glints may be used by an eye tracking module  614  to derive various pieces of information about a user&#39;s eye including where the eye is looking. 
     At block  1120 , the wearable system may detect glints and pupils in the eye images captured in block  1110 . As an example, block  1120  may include processing the eye images by glint detection &amp; labeling module  714  to identify the two-dimensional positions of glints in the eye images and processing the eye images by pupil identification module  712  to identify the two-dimensional positions of pupils in the eye images. 
     At block  1130 , the wearable system may estimate the three-dimensional positions of a user&#39;s left and right corneas relative to the wearable system. As an example, the wearable system may estimate the positions of the center of curvature of a user&#39;s left and right corneas as well as the distances between those centers of curvature and the user&#39;s left and right corneas. Block  1130  may involve 3D cornea center estimation module  716  identifying the position of the centers of curvature as described herein at least in connection with  FIGS.  7 A and  8 A- 8 E . 
     At block  1140 , the wearable system may estimate the three-dimensional positions of a user&#39;s left and right pupil centers relative to the wearable system. As an example, the wearable system and 3D pupil center locator module  720  in particular, may estimate the positions of the user&#39;s left and right pupil centers as described at least in connection with  FIGS.  7 A and  9 D- 9 G , as part of block  1140 . 
     At block  1150 , the wearable system may estimate the three-dimensional positions of a user&#39;s left and right centers or rotation (CoR) relative to the wearable system. As an example, the wearable system and CoR estimation module  724  in particular, may estimate the positions of the CoR for the user&#39;s left and right eyes as described at least in connection with  FIGS.  7 A and  10   . As a particular example, the wearable system may find the CoR of an eye by walking back along the optical axis from the center of curvature of a cornea towards the retina. 
     At block  1160 , the wearable system may estimate a user&#39;s IPD, vergence depth, center of perspective (CoP), optical axis, visual axis, and other desired attributes from eye tracking data. As examples, IPD estimation module  726  may estimate a user&#39;s IPD by comparing the 3D positions of the left and right CoRs, vergence depth estimation module  728  may estimate a user&#39;s depth by finding an intersection (or near intersection) of the left and right optical axes or an intersection of the left and right visual axes, optical axis determination module  722  may identify the left and right optical axes over time, optical to visual axis mapping module  730  may identify the left and right visual axes over time, and the CoP estimation module  732  may identify the left and right centers of perspective, as part of block  1160 . 
     At block  1170 , the wearable system may render content and may, optionally, provide feedback on registration (i.e., fit of the wearable system to the user&#39;s head) based in part on the eye tracking data identified in blocks  1120 - 1160 . As an example, the wearable system may identify a suitable location for a render camera and then generate content for a user based on the render camera&#39;s location, as discussed in connection with light-field render controller  618 ,  FIG.  7 B , and render engine  622 . As another example, the wearable system may determine if it is properly fitted to the user, or has slipped from its proper location relative to the user, and may provide optional feedback to the user indicating whether the fit of the device needs adjustment, as discussed in connection with registration observer  620  and as discussed in connection with block  1608  of  FIG.  16   . In some embodiments, the wearable system may adjust rendered content based on improper or less than ideal registration in an attempt to reduce, minimize or compensate for the effects of improper or mis-registration, as discussed in connection with block  1610  of  FIG.  16   . 
     Overview of Device Registration 
     In order for the wearable system  200  described herein to output images of high perceived image quality, the display  220  of the wearable system  200  ( FIG.  2   ) is preferably properly fitted to a user (e.g., positioned and oriented with respect to the user&#39;s head such that the inputs and outputs of system  200  interface appropriately with corresponding portions of the user&#39;s head and such that the device is stable and comfortable to wear and use). As an example, for display  220  to provide visual content to a user&#39;s eyes, the display  220  is preferably situated in front of the user&#39;s eyes and, depending on the relevant properties of the display  220 , the user&#39;s eyes are preferably situated in a particular volume (see, e.g., the further discussion associated with  FIGS.  13 A and  13 B ). As additional examples, the speaker  240  is preferably situated near, on, or in the user&#39;s ears to provide high-quality audio content to the user, audio sensor (e.g., a microphone)  232  is preferably situated in a particular area to receive sound from the user, and inward-facing imaging system  462  (which may include one or more cameras  324  and one or more infrared light sources  326 ) is preferably properly situated in a position and orientation to obtain clear, unobstructed images of a user&#39;s eyes (which may be part of an eye tracking system). These are merely examples of various reasons why wearable system  200  are preferably properly fitted to users. 
     In order to ensure the wearable system  200  is properly registered to a user, the wearable system  200  may include a registration observer such as registration observer  620  of  FIG.  6   . In some embodiments, the properly registered wearable system  200  includes a display that is positioned so that one or more eyes of the user are able to receive sufficient image light to see substantially the entirety of the field of view provided by the display  220  of the wearable display system  200 . For example, a properly registered display may allow an image to be seen across about 80% or more, about 85% or more, about 90% or more, or about 95% or more of the field of view of the display with a brightness uniformity of 80% or more, about 85% or more, about 90% or more, or about 95% or more. It will be appreciated that the brightness uniformity may be equal to 100% times the minimum luminance divided by the maximum luminance across the entirety of the field of view of the display (100%×L min /L max ), when the display is displaying the same content throughout the field of view. 
     The registration observer  620  may determine how the wearable system  200  is fitted on the user (e.g., if the display  220  of the wearable system  200  is positioned on the user properly) using various sensors. As an example, the registration observer  620  may use an inward-facing imaging system  462 , which may include an eye tracking system, to determine how relevant parts of the wearable system  200  are spatially oriented with respect to the user and, in particular, the user&#39;s eyes, ears, mouth, or other parts that interface with the wearable system  200 . 
     The registration observer  620  may assist with a calibration process, such an initial or subsequent configuration or setup of the wearable system  200  for a particular user. As an example, registration observer  620  may provide feedback to a user during configuration or setup of the wearable system  200  for that particular user. Additionally or alternatively, the registration observer  620  may continuously, or intermittently, monitor registration of the wearable system  200  on a user to check for continued proper registration during use and may provide user feedback on the fly. Registration observer  620  may provide user feedback, either as part of a configuration process or as part of registration monitoring during use, that indicates when the wearable system  200  is properly registered and when the wearable system  200  is not properly registered. The registration observer  620  may also provide particular recommendations for how the user may correct any misregistration and achieve proper registration. As examples, the registration observer  620  may recommend the user to push the wearable device back up after detecting slippage of the wearable device (such as down the user&#39;s nasal bridge), may recommend that the user adjust some adjustable component of the wearable device (e.g., as described herein in connection with  FIGS.  15 A and  15 B ), etc. 
     Example of a Registration Coordinate System 
       FIGS.  12 A- 12 B  illustrate an example eye position coordinate system, which may be used for defining three-dimensional positions of a user&#39;s left and right eyes relative to the display of the wearable system described herein. As examples, the coordinate system may include axis x, y, and z. Axis z of the coordinate system may correspond to depth, such the distance between the plane a user&#39;s eyes lie in and the plane that display  220  lies in (e.g., the direction normal to the plane of the front of a user&#39;s face). Axis x of the coordinate system may correspond to a left-right direction, such as the distance between the users left and right eyes. Axis y of the coordinate system may correspond to an up-down direction, which may be a vertical direction when the user is upright. 
       FIG.  12 A  illustrates a side view of a user&#39;s eye  1200  and a display surface  1202  (which may be a part of display  220  of  FIG.  2   ), while  FIG.  12 B  illustrates a top down view of the user&#39;s eye  1200  and the display surface  1202 . Display surface  1202  may be located in front of the user&#39;s eyes and may output image light to the user&#39;s eyes. As an example, display surface  1202  may comprise one or more out-coupling light elements, active or pixel display elements, and may be part of a stack of waveguides, such as stacked waveguide assembly  480  of  FIG.  4   . In some embodiments, the display surface  1202  may be planar. In some other embodiments, the display surface  1202  may have other topologies (e.g., be curved). It will be appreciated that the display surface  1202  may be a physical surface of the display, or simply a plane or other imaginary surface from which image light is understood to propagate from the display  220  to the user&#39;s eyes. 
     As shown in  FIG.  12 A , the user&#39;s eye  1200  may have an actual position  1204  offset from a nominal position  1206  and the display surface  1202  may be at position  1214 .  FIG.  12 A  also illustrates the corneal apex  1212  of the user&#39;s eye  1200 . The user&#39;s line of sight (e.g., their optical and/or visual axis) may be substantially along the line between the actual position  1204  and the corneal apex  1212 . As shown in  FIGS.  12 A and  12 B , the actual position  1204  may be offset from the nominal position  1206  by an z-offset  1210 , a y-offset  1208 , and an x-offset  1209 . The nominal position  1206  may represent a preferred position (sometimes referred to as a design position, which may be generally centered within a desired volume) for the user&#39;s eye  1200  with respect to the display surface  1202 . As the user&#39;s eye  1200  moves away from the nominal position  1206 , the performance of display surface  1202  may be degraded, as discussed herein in connection with  FIG.  14    for example. 
     It will be appreciated that a point or volume associated with the user&#39;s eye  1200  may be used to represent the position of the user&#39;s eye in analyses of registration herein. The representative point or volume may be any point or volume associated with the eye  1200 , and preferably is consistently used. For example, the point or volume may be on or in the eye  1200 , or may be disposed away from the eye  1200 . In some embodiments, the point or volume is the center of rotation of the eye  1200 . The center of rotation may be determined as described herein and may have advantages for simplifying the registration analyses, since it is roughly symmetrically disposed on the various axes within the eye  1200  and allows a single display registration volume aligned with the optical axis to be utilized for the analyses. 
       FIG.  12 A  also illustrates that the display surface  1202  may be centered below the user&#39;s horizon (as seen along the y-axis when the user is looking straight ahead, with their optical axis parallel to the ground) and may be tilted (with respect to the y-axis). In particular, the display surface  1202  may be disposed somewhat below the user&#39;s horizon such that the user would have to look downward, at approximately the angle  1216 , to look at the center of the display surface  1202 , when the eye  1200  is at position  1206 . This may facilitate a more natural and comfortable interaction with the display surface  1202 , particularly when viewing content rendered at shorter depths (or distances from the user), as users may be more comfortable viewing content below their horizon than above their horizon. Additionally, display surface  1202  may be tilted, such as at angle  1218  (with respect to the y-axis) such that, when the user is looking at the center of the display surface  1202  (e.g., looking slightly below the user&#39;s horizon), the display surface  1202  is generally perpendicular to the user&#39;s line of sight. In at least some embodiments, the display surface  1202  may also be shifted left or right (e.g., along the x-axis) relative to the nominal position of the user&#39;s eye. As an example, a left-eye display surface may be shifted right-wards and a right-eye display surface may be shifted left-wards (e.g., display surfaces  1202  may be shifted towards each other) such that the user&#39;s lines of sight hits the centers of the display surfaces when focused at some distance less than infinity, which may increase user comfort during typical usage on the wearable device. 
     Example of a Display Registration Volume 
       FIGS.  13 A- 13 B  illustrate an example display registration volume  1302   a.  The display registration volume  1302   a  may represent the spatial volume in which the eye  1200  is positioned so as to receive image light from the display device. In some embodiments, a center of rotation of a user&#39;s eye is preferably located so that the eye registers, or receives, image information from the display device. In some embodiments, when the center of rotation of the user&#39;s eyes is located within the display registration volume  1302   a,  the user is able to see the entirety of the image outputted by the display device with high brightness uniformity. For example, as described herein, a properly registered display may allow an image to be seen across about 80% or more, about 85% or more, about 90% or more, or about 95% or more of the field of view of the display, with a brightness uniformity of 80% or more, about 85% or more, about 90% or more, or about 95% or more. In other words, a display with a “good” registration (as determined by module  772  of  FIG.  7 C , as an example) may have a brightness uniformity of 90% or more, a display with a “marginal” registration may have a brightness uniformity of 80% or more, and a display with a “failed” registration may have a brightness uniformity of less than 80%. 
     As also described herein, the center of rotation  1204  may serve as a convenient reference point for referring to and determining the three-dimensional position of the user&#39;s eyes. The center of rotation of each of a user&#39;s eyes may be determined using the techniques described herein, such as by walking back from the center of curvature of a cornea to the center of rotation (CoR) along the user&#39;s optical axis. However, in general, any desired reference point associated with a user&#39;s eye may be utilized in the processes and systems described herein. The display registration volume  1203  may represent the volume of space in which display surface  1202  is able to operate at near full potential (e.g., without significant degradation, of the type described in connection with  FIGS.  15 A and  15 B , of the performance of the display surface  1202 ). If the user&#39;s eye (e.g., the center of rotation  1204  of the user&#39;s eye) is not within the registration volume  1302   a,  the user may experience degraded performance and some or all of the content provided by display surface  1202  may be partially dimmed or completely invisible to the user. 
     As shown in  FIG.  13 A , the registration volume  1302   a  may have the shape of a frustum, which is the portion of a pyramid remaining after its upper portion has been cut off, typically by a plane parallel to its base. In other words, the registration volume  1302   a  may be larger along the x axis and the y axis (see, e.g.,  FIGS.  12 A and  12 B ) when the user&#39;s eye is closer to the display surface  1202  and may be smaller along the x and y axis when the user&#39;s eye is further from the display surface  1202 . A frustum is an example of a truncation in which the shearing plane (e.g., the line at which a portion of the original shape is cut off) is parallel to the base of the volume. In general, the registration volume such as volume  1302   a  may take the shape of a volume truncated in any manner, such as by one or more non-parallel shearing planes (e.g., such as shown in  FIG.  13 B ) or by one or more non-planar shearings. 
     The dimensions of the registration volume may depend on the specific implementation of display surface  1202  and other elements of the wearable system. As an example,  FIG.  13 B  illustrates that a registration volume  1302   b  that may be angled with respect to the display surface  1202 . In the example of  FIG.  13 B , the portion of the registration volume  1302   b  closest to display surface  1202  may be angled away from the display surface  1202 , such that as the user&#39;s eye moves vertically (in the y direction) at the front of the volume (the z position closest to the display surface  1202 ), the user&#39;s eye would need to move away from the display surface (along the z axis) to remain inside the registration volume  1302   b.  In some embodiments, the shape of the registration volume  1302   b  may be based on the capabilities of an eye tracking system, which may not be able to track the user&#39;s eyes outside the angled volume  1302   b  of  FIG.  13 B . 
     The dimensions and shape of the registration volume may also depend upon the properties of the various parts of display  220 , which may include display surface  1202 . As an example, display  220  may be a light field display with one or more waveguides (which may be stacked and which can provide multiple vergence cues to the user), in-coupling elements that receive light from an image injection device and couple the light into the waveguides, light distributing elements (sometimes referred to as orthogonal pupil expanders (OPE&#39;s)) disposed on the waveguide(s) that distribute light to out-coupling elements, and out-coupling elements (sometimes referred to as exit pupil expanders (EPE&#39;s)) that direct light towards a viewer&#39;s eye. In some embodiments, as noted herein, the display surface  1202  is a surface or portion of a surface from which light with image information is output from the display system to form images in the eye of the user. For example, the display surface  1202  may be the area on the waveguide surface defined by the out-coupling elements or EPE&#39;s, and the perimeter of the display surface  1202  is the perimeter of the area defined by the out-coupling elements or EPE&#39;s. Further examples and details of light field displays and the components of such displays are also described in connection with at least FIGS. 9A-9C U.S. Provisional Patent Application No. 62/642,761, filed Mar. 14, 2018, which is incorporated by reference herein in its entirety. 
     In some embodiments, the x dimensions of registration volume  1302   a  may span approximately 3.0 mm, 3.0 mm, 3.5 mm, 4.0 mm, 4.5 mm, 4.7 mm, 5.0 mm, 5.5 mm, or 6.0 mm; or may be less than 3.0 mm; or more than 6.0 mm along the back of the volume (e.g., at the largest distances along the z-axis from the display surface). Similarly, the y dimensions of the registration volume  1302   a  may span approximately 2.5 mm, 3.0 mm, 3.5 mm, 3.9 mm, 4.0 mm, 4.5 mm, 5.0 mm, 5.5 mm, or 6.0 mm; or may be less than 2.5 mm; or more than 6.0 mm along the back of the volume. At nominal x and y positions, the z dimensions of the registration volume  1302   a  may span approximately 7.0 mm, 7.5 mm, 8.0 mm, 8.5 mm, 9.0 mm, 9.5 mm, 10.0 mm, 10.5 mm, or 11.0 mm; or less than 7.0 mm; or more than 11.0 mm. The x and y dimensions may be larger at the front of the volume. As examples, the x and y dimensions of the registration volume at the front of the volume may be approximately 7.0 mm, 7.5 mm, 8.0 mm, 8.5 mm, 8.9 mm, 9.0 mm, 9.5 mm, 10.0 mm, 10.0 mm, 10.5 mm, 11.0 mm, 11.4 mm, 11.5 mm, 12.0 mm, or 12.5 mm; or less than 7.0 mm; or more than 12.5 mm. As specific examples, the dimensions of the registration volume may include a z-dimension of approximately 9 mm; an x-dimension of approximately 4.7 mm at the back of the volume and approximately 11.4 mm at the front of the volume; and a y-dimension of approximately 3.9 mm at the back of the volume and approximately 8.9 mm at the front of the volume. 
     In at least some embodiments, there may be multiple registration volumes, such as volumes  1302   b  and  1304 , each of which is associated with a different minimum level of display performance. As an example, volume  1304  of  FIG.  13 B  may be smaller than volume  1302  and may represent the volume in which the user perceives all of the content provided by display surface  1202  at 100% brightness uniformity, whereas the larger volume  1302   b  may represent the volume in which the user perceives at least 90% of the content provided by display surface  1202  at 100% brightness uniformity. 
       FIGS.  13 C and  13 D  illustrate an example display registration volume, configured to use the center of rotation of an eye as a reference point indicative of the position of the eye, relative to the eye of a user and a display surface. In particular,  FIG.  13 C  illustrates an example positioning of a display registration volume, such as registration volume  1302   b,  within a user&#39;s eye  1200 . In the example of  FIG.  13 C , the center of rotation  1204  of the eye  1200  is roughly centered within the registration volume  1302   b.  Additionally, the registration volume  1302   b  is illustrated with example dimensions of approximately 9 mm of depth and, at the mid-point of the depth axis, approximately 3.5 mm of width and 3 mm of height. As discussed herein, the dimensions of registration volume may vary and may be related to the properties of various components of the wearable system.  FIG.  13 C  also illustrates an eye structure  1340 , which may be the lens or pupil of eye  1200 . 
       FIG.  13 D  shows a larger context, in which the user&#39;s eye  1200  is generally positioned within registration volume  1302   b  and is looking through display surface  1202  at virtual content  1350 . As discussed herein, virtual content such as the virtual content  1350  may be provided to the user with vergence and accommodation cues associated with greater depths than the depth of the display surface  1202 . In other words, the virtual content  1350  may appear to the user with eye  1200  to be at a greater distance from the user than the display  1202 . Such an arrangement is illustrated in the example of  FIG.  13 D . 
     With continued reference to  FIG.  13 D , it will be appreciated that the display registration volume  1302   b  may be an imaginary volume having boundaries defined by a projection from the perimeter of the display surface  1202  to a point inside the eye  1200 . For example, the projection may define a pyramid and the display registration volume  1302   b  may be a frustum of that pyramid. Thus, the cross-sectional shape of the display registration volume  1302   b,  along planes facing the display surface  1202  on the optical axis, is similar to the shape made out by the perimeter of the display surface  1202 . For example, as illustrated, where the display surface  1202  is square, the cross-sectional shape of the display registration volume  1302   b  is also square. In addition, as also illustrated, where the center of the display surface  1202  is below the user&#39;s horizon, a frustum may also be slanted such that a center of the front of the display registration volume  1302   b  is also below the user&#39;s horizon. It will be appreciated that, in some embodiments, the relevant perimeter of the display surface  1202  is the perimeter of the area of the display over which image light or display content is outputted. 
     In some embodiments, the center of rotation  1204  of the eye is centered within the frustum that defines the display registration volume  1302   b.  It will be appreciated, however, that the nominal placement of center of rotation  1204  of the eye and/or the overall shape of the frustum may be determined empirically or selected using criteria other than projection from the display surface  1202  so that the display system is able to properly register the display and to provide accurate feedback regarding the quality of the registration and the levels of registration that may be acceptable even if not ideal. 
     Examples of a Display Performance at Various Registration Positions 
       FIG.  14    illustrates how the performance of display surface  1202  may vary with the position of the user&#39;s eye  1200 . As illustrated, light rays from the display surface  1202  may be directed to the eye at an angle, such that light rays from the edges of the display surface  1202  propagate inwards towards the eye  1200 . Thus, the cone  1202 ′ represents a cone of light outputted by the display surface  1202  to the eye  1200  to form an image. 
     Consequently, as the display surface  1202  shifts relative to eye  1200 , the exit pupils of pixels corresponding to a respective portion of the field of view do not reach the retina of eye  1200 , and the image appears to dim at those portions of the field of view. The positions  1204   a,    1204   b,    1204   c,  and  1204   d  of the center of rotation of the eye are effectively shifted relative to the idealized position  1204 ′ of the center of rotation; movement of the display surface  1202  relative to the eye  1200  may cause the center of rotation of the eye to possibly move outside of the display registration volumes  1302   a,    1304 ,  1302   b  ( FIGS.  13 A and  13 B ) for the display surface  1202 . As discussed herein, the display registration volumes may be tied to the display surface  1202 , e.g., the display registration volumes may be defined by projections from the display surface  1202 . Consequently, as the display surface  1202  moves relative to the eye  1200 , so do the display registration volumes  1302   a,    1302   b  ( FIGS.  13 A and  13 B ).  FIG.  14    illustrates various positions (e.g., positions  1204   a,    1204   b,    1204   c,  and  1204   d ) of the center of rotation of a user&#39;s eyes, the relative position of a display surface  1202 , and representations (e.g., representations  1400   a,    1400   b,    1400   c,  and  1400   d ) of how the content provided by display surface  1202  would be perceived by the user at each of the various positions. 
     In example  1400   a,  the center of rotation of the user&#39;s eye may be at position  1204   a,  which may be centered within a registration volume such as registration volume  1300   b  (e.g., a volume in which image quality is high due to the eye  1200  receiving on its retina nearly all of the image light outputted by the display surface  1202 ). Representation  1400   a  may represent the user&#39;s perception (or view) of the content provided by display surface  1202 , when the user&#39;s eye is at position  1204   a.  As shown by representation  1400   a,  the luminance for substantially all of the content across the display surface  1202  is uniform and may be at or near full brightness levels. 
     In example  1400   b,  the center of rotation of the user&#39;s eye may be at position  1204   b,  which may be outside a preferred display registration volume such as volume  1304  ( FIG.  13 B ) but inside a secondary registration volume such as volume  1302   b  (e.g., a volume in which display performance is only slightly degraded). Representation  1400   b  may represent the user&#39;s perception (or view) of the content provided by display surface  1202 , when the center of rotation of the user&#39;s eye is at position  1204   b.  As shown by representation  1400   b,  the portion  1402  of the image along the right side of the display surface  1202  may have a perceived reduced brightness (e.g., a 50% brightness) due to misregistration of the user&#39;s eye relative to the display surface  1202 . 
     In example  1400   c,  the center of rotation of the user&#39;s eye may be at position  1204   c,  which may be outside (or on the outside edge of) a second registration volume such as volume  1302   b  ( FIG.  13 B ). Representation  1400   c  may represent the user&#39;s perception (or view) of the content provided by display surface  1202 , when the center of rotation of the user&#39;s eye is at position  1204   c.  As shown by representation  1400   c,  a portion  1406  along the edge of the displayed image user&#39;s perception may appear completely (or nearly completely) dimmed and thus not seen by the user due to misregistration. In arrangements in which some pixels of the display are below a perceived luminance level, the display may provide a reduced field of view (e.g., the user may not be able to perceive the full field of view the display is otherwise capable of presenting). Additionally, there may be a band or portion  1404  of the image having progressively reduced brightness between the dark portion  1406  and the rest of the representation. 
     In example  1400   d,  the center of rotation of the user&#39;s eye may be at position  1204   d,  which may be well outside the desired registration volumes. Representation  1400   d  may represent the user&#39;s perception (or view) of the content provided by display surface  1202 , when the center of rotation of the user&#39;s eye is at position  1204   d.  As shown by representation  1400   d,  large portions  1410  of the image may appear completely (or nearly completely) dark to the user and a substantial portion  1408  of the image may appear dimmed, due to the significant misregistration. 
     Example of Interchangeable Fit Pieces for Wearable Systems 
       FIGS.  15 A and  15 B  show exploded perspective views of the wearable system  220 , which may include interchangeable fit pieces. In particular,  FIG.  15 A  illustrates how the wearable system  200  may include interchangeable back padding such as pads  1500   a,    1500   b,  and  1500   c;  while  FIG.  15 B  illustrates how the system  200  may include interchangeable forehead pads such as pad  1502  and interchangeable nose bridge pads such as pad  1504 . These interchangeable pads may be used to adjust the fit of the wearable system  200  for individual users, whom may have varying anatomical attributes (e.g., how the display  220  and frame  230  fit for various different users). As examples, users with relatively small heads may benefit from attaching relatively large back pads  1500   a,    1500   b,  and  1500   c  to the frame  230 , while users with relatively large heads may obtain better results (e.g., better optical performance and stability of the frame  300  on their head) by attaching relatively small back pads, or even omitting the back pads. Similarly, users with prominent noses and/or foreheads may benefit from smaller forehead pads  1502  and/or nose bridge pads  1504 ; while users with less prominent noses and/or foreheads may benefit from larger forehead pads  1502  and/or nose bridge pads  1504 . These are merely illustrative examples and, in general, determining the set of interchangeable pads that result in the best fit for any particular user may be complex. As described herein, the display system may display a notification to the user indicating that a different interchangeable fit piece may be desirable to provide proper registration of the display to the user. 
     Example Processes of Observing Device Registration 
       FIG.  16    is a process flow diagram of an example method  1600  for observing device registration and providing feedback on registration or compensation for misregistration in a wearable device. The method  1600  may be performed by the wearable systems described herein. Embodiments of the method  1600  may be used by the wearable system to provide feedback on registration (i.e., fit of the wearable device to the user) based on data from an eye tracking system and to adjust a display to attempt to compensate for fit errors (e.g., misregistration). 
     At block  1602 , the wearable system may obtain fit tolerances. The fit tolerances may include information associated with display registration volumes such as volumes  1302   a,    1302   b,  or  1304 . In particular, the fit tolerances may include information associated with nominal (e.g., normal) positions of the user&#39;s eyes relative to the wearable device and may include information associated with how variances from the nominal positions impact device performance. As one example, the fit tolerances may include information on a range of nominal positions for which the wearable device is able to interface with a user at least a certain desired amount of performance (e.g., with no more than 50% dimming on any pixel in a display). 
     At block  1604 , the wearable system may obtain registration data. The registration data may include spatial relationships between various components of the wearable system and associated portions of the user. As examples, the registration data may include one or more of the three-dimensional positions of a user&#39;s left eye relative to a left-eye display of the wearable system; 3D positions of the user&#39;s right eye relative to a right-eye display; and 3D positions of the user&#39;s ears relative to audio outputs (e.g., speakers, headphones, headsets, etc.) of the wearable system. The wearable system may obtain registration data using any suitable mechanisms. As an example, the wearable system may capture images of a user&#39;s eye or eyes using eye-tracking cameras  324  of the type shown in  FIG.  3    (or other cameras, which may or may not be inward-facing cameras) to determine the relative positions of the user&#39;s eyes and the wearable system. As other examples, the wearable system may include depth sensors, pressure sensors, temperature sensors, light sensors, audio sensors, or other sensors to measure or obtain registration data such as the position of the wearable device relative to a user. 
     At block  1606 , the wearable system may determine fit characteristics. As an example, the wearable system may determine whether the user&#39;s left eye lies within a left-eye registration volume (such as one of volumes  1302   a,    1302   b,  or  1304  for the left eye) and whether the user&#39;s right eye lies within a right-eye registration volume (such as one of volumes  1302   a,    1302   b,  or  1304  for the right eye). Block  1606  may also involve determining how far the user&#39;s eyes (or other body parts) are from their nominal positions. As an example, the wearable system, in block  1606 , may determine that at least one of the user&#39;s eyes is outside of its respective the display registration volume, by how much and in which direction the user&#39;s eyes are outside of their display registration volumes. Information on the direction and magnitude of the misregistration (e.g., the distance between the registration volumes or nominal positions and the actual positions of the user&#39;s eyes or other body part) may be beneficially utilized in blocks  1608  and  1610 . 
     At block  1608 , the wearable system may provide a user (or some other entity) with feedback on the fit characteristics determined in block  1608 . As an example, if the wearable system determines in block  1606  that the wearable device is too low relative to the user&#39;s eyes, the wearable system may provide the user, at block  1608 , with a notification suggesting that the user utilize an appropriate nose bridge pad  1504  (e.g., to add a nose bridge pad if none were previously attached or to swap out an existing nose bridge pad for a larger or taller nose bridge pad). Conversely, if the wearable device determines it is too high relative to the user&#39;s eyes, the system may provide a suggestion to the user to use a smaller nose bridge pad or remove the pad altogether (if designed to be wearable without a pad). As other examples, the wearable system may provide the user with feedback suggesting a change to forehead pads such as pad  1502 , back pads such as pads  1500   a - 1500   c,  a change to other adjustable components of the wearable system, a change to how the user is wearing the wearable system (e.g., instructions to move or rotate the system in a particular direction relative to the user). In general, user feedback may be generated based on the position(s) of the user&#39;s eye(s) relative to the display or other metrics such as the visible image portions identified by the system. As an example, when the system determines that the user&#39;s eye is above the registration volume, the system may recommend to the user that the user push the wearable device upwards along the bridge of their nose in order to correct the misregistration. 
     User feedback may be provided using any suitable device. As examples, user feedback may be provided via video presented by a display in the wearable device or an external display or via audio presented by the wearable device or by an external device. In various embodiments, the wearable device may provide an interactive guide for assisting the user is obtaining proper registration in a relatively intuitive manner. As an example, the wearable device could display two virtual targets, one representative of the position of the user&#39;s eyes and the other representative of the nominal registration position. Then, as the user moves the wearable device around and adjusts its fit, the user can perceive how their adjustments impact registration and the user can quickly and intuitively achieve proper registration. 
     In arrangements in which user feedback is provided by an output device, such as a display, that is part of the wearable device, the wearable device may provide the user feedback in a manner that ensures the user is able to perceive the feedback. Consider, as an example, representation  1400   d  of  FIG.  14   . In such an example, the wearable system may move user feedback into portion of the displayed image that is perceived by the user e.g., the left-half of the display, as opposed to the invisible right-half of the display in the example  1400   d  of  FIG.  14   . 
     In some embodiments, feedback of the type described herein may be provided to a sale associate in a retail environment and the feedback may be communicated over a network to the sale associate&#39;s computer or mobile device. 
     At block  1608 , the wearable system may adjust its outputs and inputs to compensate for uncorrected fit errors. In some embodiments, block  1608  may be performed only after a user has failed to correct fit errors in response to feedback. In other embodiments, block  1608  may be performed until a user corrects fit errors. In some embodiments, block  1608  may be performed whenever the user decides to continue using the wearable system with fit errors. In some embodiments, block  1608  may be omitted. 
     As examples, the wearable system may adjust its outputs and inputs in block  1608  by adjusting portions of a displayed image (e.g., to compensate for misregistration-induced dimming, of the type shown in  FIG.  14   ), by adjusting microphone inputs (e.g., boosting microphone gain when a user is too far from a microphone, or reducing microphone gain when the user is too close to the microphone), by adjusting speaker outputs (e.g., boosting or dimming speaker volume when a user is too close or too far, respectively, from a speaker in the wearable device), etc. As one particular examples, the wearable system may selectively boost the luminance of portions of the image, such as portions  1402 ,  1404 , or  1408  of  FIG.  14   , in an attempt to reduce misregistration-induced dimming. In some other embodiments, the wearable system may recognize that certain portions of the image, such as portions  1406  or  1410  of  FIG.  14   . are not visible to the user and may reduce light output in those regions to reduce energy consumption by the wearable system. For example, in configurations where different portions of the image may have dedicated, selectively-activated light sources or portions of a light source, the one or more light sources or portions of a light source associated with the unseen portions of the image may have their light output reduced or turned off. 
     Example of Identifying a Display Registration Volume 
       FIGS.  17 A- 17 H  illustrate views of light fields projected by a display and how the intersections of the light fields may partly define a display registration volume.  FIG.  18    illustrates a top-down view of overlapping light fields projected by a display and how the intersections of the light fields can partly define a display registration volume. As  FIGS.  17 A- 17 H and  18    illustrate, the size and shape of display registration volume can depend in part upon the geometry of the display (which can be display  220  of  FIG.  2   ) as well as the angles at which out-coupled light propagates out of the display (e.g., out of the waveguide the display). It will be appreciated that the angles at which the light is output may define the FOV of the display; larger angles relative to the normal provide a larger FOV. In some embodiments, the display surface may output angles large enough to provide a desired FOV. 
       FIGS.  17 A- 17 H and  18    illustrate display  220 , which can be a light field display including elements such as waveguide  1701 , in-coupling elements  1702 , orthogonal pupil expanders (OPE&#39;s)  1704 , and exit pupil expanders (EPE&#39;s)  1706  (which can form a display surface  1202 , which is also illustrated in various other Figures herein including  FIGS.  12 A- 14   ). As an example, the in-coupling elements  1702  can receive light from an image source and couple the light into waveguide  1701 . The waveguide  1701  can convey the light to OPE&#39;s  1704 , the OPEs  1704  may provide pupil expansion and direct the light to EPE&#39;s  1706 , and the EPE&#39;s  1706  (which can be provided on display surface  1202 ) provide further pupil expansion and convey the light to the user&#39;s eye(s). Further examples and details of light field displays and the components of such displays are also described in connection with at least FIGS. 9A-9C U.S. Provisional Patent Application No. 62/642,761, filed Mar. 14, 2018, which is incorporated by reference herein in its entirety. 
       FIG.  17 A  illustrates an example in which the display  220  is projecting light  1710  associated with virtual image content at optical infinity and the right-most region (e.g., right-most pixel) of the FOV of the display. In contrast,  FIG.  17 B  illustrates an example in which the display  220  is projecting light  1712  associated with an object at optical infinity and the left-most region (e.g., left-most pixel) of the FOV of the display.  FIG.  17 C  illustrates the overlapping region  1714  of the light  1710  of  FIG.  17 A  and the light  1712  of  FIG.  17 B . Region  1714  may be a horizontal registration volume. In particular, when the user&#39;s eye is disposed within region  1714  of  FIG.  17 C , the user is able to perceive (e.g., display  220  is able to provide the user with light from) objects at both the right-most region of the FOV (as in  FIG.  17 A ) and the left-most region of the FOV (as in  FIG.  17 B ). 
       FIGS.  17 D-F  illustrate examples similar to those of  FIGS.  17 A- 17 E , except in the vertical direction. In particular,  FIG.  17 D  illustrates an example in which the display  220  is projecting light  1716  associated with an object at optical infinity and the bottom-most region (e.g., bottom-most pixel) of the FOV of the display, while  FIG.  17 E  illustrates an example in which the display  220  is projecting light  1718  associated with an object at optical infinity and the top-most region (e.g., bottom-most pixel) of the FOV of the display. Similarly,  FIG.  17 F  illustrates the overlapping region  1720  of the light  1716  of  FIG.  17 D  and the light  1718  of  FIG.  17 E . Region  1720  may be a vertical registration volume. In particular, when the user&#39;s eye is disposed within region  1720  of  FIG.  17 F , the user is able to perceive (e.g., display  220  is able to provide the user with light from) objects at both the bottom-most region of the FOV (as in  FIG.  17 D ) and the top-most region of the FOV (as in  FIG.  17 E ). 
       FIGS.  17 G and  17 H  illustrate the intersection (as region  1722 ) of the regions  1714  of  FIG.  17 C  and the region  1720  of  FIG.  17 F . In particular,  FIG.  17 G  illustrates the region  1722  in which light from objects at the four corners of the FOV of display  220  overlaps.  FIG.  17 H  illustrates just the outline of region  1722 . As should be apparent, when the user&#39;s eye is disposed within region  1722 , the user is able to perceive (e.g., display  220  is able to provide the user with light from) objects anywhere within the FOV of the display. 
     In some embodiments, increasing the FOV of display  220  (horizontally, vertically, or a combination thereof) while holding other attributes (such as display size) constant may have the effect of shrinking the relevant registration volume (e.g., the horizontal volume  1714 , the vertical volume  1720 , or the combined registration volume  1722 ). Consider, as an example,  FIGS.  17 A-C  and the horizontal FOV and registration volume  1714 . An increase in the horizontal FOV of display  220  means light  1710  from objects on the right horizontal edge is projected by display surface  1202  (e.g., EPE&#39;s  1706 ) at a sharper angle (e.g., a greater angle from normal to display surface  1202 ). Similarly, light  1712  from objects on the left horizontal edge is projected at a sharper angle. Thus, in the perspective of  FIG.  17 C , the apex of the horizontal registration volume  1714  moves toward display surface  1202  with increases in horizontal FOV, thereby shrinking volume  1714 . Similar considerations may apply in some embodiments to the vertical FOV and the vertical registration volume, as well as the overall FOV and overall registration volume. 
       FIG.  18    shows a top-down view of display  220  including display surface  1202 , which may have a rectangular shape and a particular FOV, as well as light rays produced by the display. In general, the registration volume of the display  220  of  FIG.  18    may the volume  1802 , which appears triangular in the top-down perspective of  FIG.  18   . The volume  1802  may represent the volume where the various light fields formed by the light shown in  FIGS.  17 A- 17 G  overlap. If a user&#39;s eye is located outside of volume  1802  (e.g., in volume  1804 ), it may be seen that light of light fields from at least some portion of the display  220  would fail to reach the user&#39;s eye, resulting in partial or complete dimming of a portion of the FOV. 
     It should be noted that a side-view of the display and registration volume would have much the same appearance (at least for a rectangular display) as that shown in  FIG.  18   , although the illustrated dimension of display  220  would be the height of the display  220  rather than its width and the illustrated FOV would be the vertical FOV rather than the horizontal FOV shown in  FIG.  18   . Thus, the volume  1802  may actually have a somewhat pyramidal shape. In other embodiments, the display may have non-rectangular shapes such as a circular shape, an elliptical shape, a free-form shape, or any other desired shape. In such embodiments, the corresponding registration volume may be determined by projecting light fields at the relevant FOV and identifying where those light fields intersect (which may correspond to volume  1802 ) and where the light fields do not intersect (which may correspond to volume  1804 ). 
     As discussed herein, the “base” of the pyramid may be truncated (which may help to move the user&#39;s eyes away from the display such that the user&#39;s eyelashes don&#39;t impact the display when properly registered) and the “top” of the pyramid may also be truncated (which may be helpful in reducing the impacts of noise in the location determination of the user&#39;s eyes, which might otherwise rapidly move into and out of registration at the “top” of a pyramidal shaped registration volume). It will be appreciated that the “top” is proximate the apex of the volume  1802 , and the base is proximate the waveguide  1701 . When the user&#39;s eyes are located in regions  1804  outside of the registration volume  1802 , the user may perceive dimming of some or all of the pixels of display  220 , as discussed herein (see, e.g.  FIG.  14   ). 
     In general, the registration volume may be adjusted (e.g., truncated or otherwise reduced) in any number of ways for a variety of reasons. As an example, the registration volume may be truncated such that the volume has a minimum distance from display  220 , to prevent the user&#39;s eyelashes or lids from impacting the display  220 . As another example, the wearable system may have an eye tracking system, including elements such as cameras  324  and light sources  326  of  FIG.  6   , which may only track the user&#39;s eyes if they user&#39;s eyes are within an eye tracking volume, which may not overlap exactly with the display registration volume. 
     Example Processes of Observing Device Registration and Providing User Feedback to Improve Registration 
     In some embodiments, a wearable system may determine device registration and provide feedback on registration using eye tracking information such as the position of a user&#39;s eyes along the y-axis and z-axis (see, e.g.,  FIGS.  12 A and  12 B , which illustrate the y-axis as being a vertical direction or a direction of gravity when the user and the wearable system are upright, and the z-axis as being a horizontal direction extending along a user&#39;s line of sight, when they are looking forward). In some embodiments, the wearable system may be configured to track a position of a user&#39;s pupil along the y-axis and a center of rotation of the eye along the z-axis. The wearable system may determine if the user&#39;s pupils are vertically offset from desired positions (e.g., along the y-axis) or if the eye (e.g., the center of rotation of the eye) is too close or too far from the wearable system (e.g., along the z-axis) and may provide customized feedback to the user based on either of these offsets. The feedback may include instructions to modify various physical wearable system parts that impact the position of the display relative to the user. For example, the instructions may include prompting the user to install or remove various parts connected to the display  220  and the frame  230  ( FIG.  2   ). In some embodiments, the user may be prompted to install or remove a thicker forehead pad, a slimmer forehead pad, a taller nose pad, a shorter nose pad, another fit piece, another backpad, or some combination or sub-combination of such fit pieces so as to improve the fit of the device (e.g., to adjust the positions of the user&#39;s pupils towards the desired position). As another example, the wearable system may include physically adjustable or moveable parts and the instructions may include prompting the user to make adjustments to those parts (including prompting the user with specific instructions such as how to adjust those parts, by how much to adjust those parts, and/or in what direction or manner to adjust those parts). As a particular example, the wearable system may include a dynamic IPD adjustment mechanism that physically adjusts the separation between left and right displays of the wearable system and the wearable system may prompt the user to adjust the dynamic IPD adjustment mechanism when it determines that the user&#39;s IPD does not match a current setting of the mechanism. For example, the wearable system may have a slide mechanism joining the left and right displays, and the displays may be moved closer or farther apart by collapsing or extending the slide mechanism. 
     As shown in the example of  FIG.  19   , the wearable system may identify the locations of each of a user&#39;s pupils vertically within multiple vertical regions. As an example, the wearable system may determine whether each of a user&#39;s pupils is within one of regions T 1  or T 2  (e.g., the wearable system is too low relative to the user&#39;s pupil), one of the regions M 1  or M 2  (e.g., the wearable system is within a desired vertical range relative to the user&#39;s pupil), or one of the regions B 1  or B 2  (e.g., the wearable system is too high relative to the user&#39;s pupil). 
     The scale along the right-hand side of  FIG.  19    may be in units of pixels and may refer to an image captured by an eye tracking system. Thus, an eye pupil in the top 80 pixels of an eye tracking image may be said to be in region T 2 . Similarly, if the eye pupil is in the next 40 pixels it may be in region T 2  (from 80 to 120 pixels), if it is in the following 90 pixels it may be in region M 1  (from 120 to 210 pixels), if it is in the following 90 pixels it may be in region M 2  (from 210 to 300 pixels), if it is in the following 100 pixels it may be in region B 1  (from 300 to 400 pixels), and if it is the bottom pixels (beyond 400) it may be in region B 2 . It will be appreciated that the association of each region with particular pixels may vary depending on the resolution of the image captured by the eye tracking system, the location of an eye imaging camera relative to the display, and the like. Moreover, different numbers and sizes of regions may be utilized. Preferably, a sufficient number of regions is provided to distinguish between eye pupils at acceptable positions and eye pupils at positions that are considered too high or too low, or otherwise unacceptable or undesirable. 
     The wearable system may, in various embodiments, determine the vertical location of each of a user&#39;s pupils based on images captured by an eye tracking system such as inward-facing imaging system  462  of  FIG.  4    (which may include one or more cameras  324  and may also include one or more infrared light sources  326  of  FIG.  3   ). As shown in  FIG.  19   , an inward-facing imaging system may capture images of an eye that includes glints such as glints  1900   a,    1900   b,    1900   c,  and  1900   d.  The glints are reflections off of the user&#39;s eyes that appear in images of the eye captured by a cameras such as the camera  324 . Positions of light sources such as the light sources  326   a  and  326   b  relative to the camera  324  may be known and, as a consequence, the positions of the glints within images captured by the camera  324  may be used in tracking the user&#39;s eyes (as is discussed in more detail herein in connection with  FIGS.  7 - 11   ), including determining the vertical location of the user&#39;s pupils (e.g., which of the regions of  FIG.  19    the user&#39;s pupils are located in). For example, the glints  1900   a,    1900   b,    1900   c,  and  1900   d  may be utilized as reference points and the position of the user&#39;s pupil may be determined based on the location of the pupil relative to these glints, as disclosed herein. 
       FIG.  20    is an example of a process flow diagram for observing device registration and providing feedback on registration with a head-mounted wearable system. The diagram illustrates a method  2000  which may be performed by the wearable systems described herein. The method  2000  may be used by the wearable system to provide feedback on registration (position of the wearable device relative to the user) based on data from an eye tracking system. 
     At block  2002 , the wearable system may provide a user with initial fitting instructions. In some embodiments, the initial fitting instructions may guide the user on how to initially properly place the wearable system on their head. As an example, the wearable system may display a screen, an example of which is illustrated via the screenshot of  FIG.  21   , including visual aids (e.g., text, pictures, animations, videos, etc.) that assist the user in initially placing the system on their head. Additionally or alternatively, the wearable system may provide auditory aids (e.g., verbal instructions and/or feedback) for the same purposes. 
     As shown in the example of  FIG.  21   , the user may be directed to raise or lower the back of the wearable system until markings  2100  are visible in each of the corners of the display. It will be appreciated that raising or lowering the back of the wearable system changes the pitch of the display  220  ( FIG.  2   ) relative to the user&#39;s eyes. This changes the extent of the vertical dimension of the display&#39;s field-of-view (FOV) viewable by the user. The markings  2100  are preferably located at positions that demarcate the vertical extent of the desired FOV (also referred to as the vertical FOV) and otherwise may take any desired shape and have any desired size. Consequently, in some embodiments, the markings  2100  may simply take the form of horizontal bars at the top and bottom of the desired vertical FOV, and/or vertical bars at the left and right of the desired horizontal FOV. 
     In some embodiments, the markings  2100  may be located at the extreme corners of the wearable system&#39;s FOV, such that a user that is able to see all of the markings is able to see 100% of the wearable system&#39;s nominal vertical FOV with 100% pixel saturation. Pixel saturation may refer to the apparent brightness of pixels from a user&#39;s perspective. 100% pixel saturation may refer to situations in which the user is able to perceive a given pixel at 100% of its brightness. In situations in which the user&#39;s eye is not properly aligned with the wearable system&#39;s display, the user may perceive some pixels with diminished brightness and may be unable to perceive other pixels. Further discussion of pixel saturation is provided in connection with  FIG.  14   , which discusses how pixel saturation, or pixel brightness, can be degraded in the event of misregistration. In some embodiments, the markings  2100  may be located some distance away from the edges of the nominal vertical FOV. When the markings  2100  are placed away from the edges of the nominal vertical FOV, it may be easier for users to adjust the wearable system such that they can see all of the markings  2100 . Given the diversity of face geometry amongst different people, it may be difficult to provide a wearable system having a large vertical FOV where 100% of users can see 100% of the vertical FOV. If the markings  2100  are placed at the edge of the vertical FOV, some users may find it difficult or impossible to adjust the wearable device to see all of the markings, leading to user disengagement and/or a calibration that may not be completed. If the markings  2100  are placed to cover slightly less than the vertical FOV of the wearable system, most, if not all users, may be able to adjust the wearable system to see all of the markings. On the other hand, if the markings  2100  are placed to cover much smaller amounts of the vertical FOV, users may be able to see the markings  2100  even without properly fitting the wearable system, may decide to skip proper fitting, and may thereby be unable to view content displayed near the extremities of the vertical FOV. As examples, the markings  2100  may be placed to cover 95% of the nominal vertical FOV, 90% of the nominal vertical FOV, 85% of the nominal vertical FOV, or 80% of the nominal vertical FOV. In some embodiments, the markings  2100  cover 95% of the vertical FOV of the wearable system. Preferably, the amount of the vertical FOV that is covered allows the entirety of a user interface of the wearable system to be seen by the user. 
     It will be appreciated that changing the pitch of the wearble system may not significantly alter a horizontal extent of the FOV viewable by the user. As such, the horizontal separation between the markings  2100  may cover a different percentage of the horizontal FOV than the vertical FOV. For example, the horizontal separation between the markings  2100  may be selected such that the horizontal separation does not prevent the user from seeing the markings  2100  through the entire range of possible pitch adjustments. 
     With reference again to  FIG.  20   , at block  2004 , the wearable system may determine the vertical positions of the user&#39;s eyes and may also determine eye tracking confidence levels. The vertical positions of the user&#39;s eyes may be determined with reference to the regions of  FIG.  19   . In other words, the wearable system may determine the vertical positions of the user&#39;s eyes relative to the wearable system and may determine which region (e.g., T 2 , T 1 , M 1 , M 2 , B 1 , or B 2 ) the user&#39;s eyes, and/or portions thereof, are located in. The wearable system may also determine the z-axis positions of the user&#39;s eyes relative to the wearable system. In some embodiments, the z-axis position of the user&#39;s eyes may be understood to be the z-axis position of the center of rotation of the user&#39;s eyes, which may be determined as disclosed herein. The block  2004  may also involve determining if there is a Y-offset between the two eyes of a user. For example, the wearable system may determine if one of the user&#39;s eyes is located in a different one of the regions (T 2 , T 1 , M 1 , M 2 , B 1 , or B 2 ) than the other eye of the user. 
     With continued reference to block  2004 , eye tracking confidence levels may be an indication of the reliability of eye tracking data (e.g., what level of confidence the wearable system has that each eye is actually in the region the eye tracking data suggests it is in). The eye tracking confidence levels may be based on factors such as whether a clear image of the eye may be captured, whether glints may be adequately detected, and whether the pupil may be adequately detected. Particular examples of confidence factors include:
         whether the user is blinking;   how many glints are detected (e.g., detection of 3 or 4 glints may indicate good confidence, while detection of 2 glint may reduce a confidence score and detection of no or just one glint may result in a zero confidence score);   difficulty in detecting the pupil;   a pupil aspect ratio (high aspect ratios may indicate poor pupil detection);   whether the pupil is on an image boundary (which may incur a confidence penalty if the pupil touches the top or bottom edge of the eye tracking image); and   an eye movement factor (e.g., there may be a confidence penalty if the eye center recently moved, for example, moved since the last captured image).       

     In some embodiments, each of the aforementioned factors may be weighted equally. For example, confidence (C) may be given by the equation C=1.0−F f /10, where F f =the number of flagged factors. A higher numerical value for C indicates a higher confidence level and a lower numerical value for C indicates a lower confidence level. It will be appreciated that the various factors used to determine confidence may vary depending upon the calculation and methods used to conduct eye tracking. Consequently, in some embodiments, the confidence determination may utilize more or fewer factors than that listed above. In some embodiments, each of (or some of) the aforementioned factors may be weighted unequally. 
     It will be appreciated that eye tracking confidence levels are used in the method  2000  to provide a comparison between successively lower confidence levels (e.g., high, low, and bad) of the left and right eyes. In some embodiments, the confidence factors and their weighting and calculation may differ from the example above, so long as a relative comparison of high, low, or bad levels between the left and right eyes may be obtained. 
     At block  2006 , the wearable system may determine whether the current fit is satisfactory. As an example, the wearable system may determine if the user&#39;s eyes are within one of the desired vertical ranges (e.g., M 1  or M 2 ) of  FIG.  19    and within a desired range of z-axis positions. Additionally, the wearable system may determine whether fit should be deemed satisfactory at the block  2006  due to exhaustion of available fit adjustments. As an example, if prior recommended fit adjustment(s) have been made and no further helpful adjustments can be made, the wearable system may determine that the fit is acceptable, even when the user&#39;s eyes are outside of the desired vertical ranges and desired Z-positions. 
     If the current fit is determined to be unsatisfactory, the method may continue with block  2008 . At the block  2008 , the wearable system may generate and provide to a user one or more fit adjustment recommendations based on the pupil (eye) positions and eye tracking confidence levels determined at the block  2004 . The block  2008  may also involve generating and providing to a user one or more fit adjustment recommendations based on eye ball center positions, sometimes referred to as center of rotation (CoR) positions. As an example, the wearable system may determine that the wearable device is sitting too low on a user&#39;s face (e.g., along the y-axis of  FIGS.  12 A and  12 B ) and therefore generate a recommendation to install a taller nose pad. As another example, the wearable system may determine that the user&#39;s eye is “too close” to the wearable device. As a particular example and with reference to  FIGS.  12 A and  12 B , the wearable system may determine that the user&#39;s eye  1200  is too close to the display surface  1202 , along the z-axis, and may therefore provide a fit adjustment recommendation that includes switching to a thicker forehead pad. The recommendation to switch to a thicker forehead pad may be made regardless of the y-position or vertical offset. As another example, the wearable system may determine if the wearable device is sitting too far right or left on a user&#39;s face (e.g., along the x-axis of  FIGS.  12 A and  12 B ) and may then provide appropriate fit adjustment recommendations. Additional details and examples are discussed in connection with  FIGS.  22 A,  22 B, and  22 C . After providing the user with fit adjustment recommendations at the block  2008 , method  2000  may return to the block  2004 . In some embodiments, multiple iterations of fit checks at the block  2004  and fit adjustment recommendations at the block  2008  may be performed to achieve a desired fit. 
     Once the current fit is determined to be satisfactory at the block  2006 , the wearable system may end the fit process at block  2010 . As an example, the wearable system may display or otherwise provide a message to the user indicating that they have completed the fit process. 
       FIGS.  22 A,  22 B, and  22 C  are process flow diagrams of an example of details of a method for observing device registration and providing feedback on registration in a wearable device.  FIGS.  22 A,  22 B, and  22 C  illustrate different parts of a method  2200  that may be performed by the wearable systems described herein. Embodiments of the method  2200  may be used by the wearable system to provide feedback on registration (i.e., fit of the wearable device to the user) based on data from an eye tracking system. 
       FIGS.  22 A,  22 B, and  22 C  include various “off-page references” to simplify the flow diagram. As an example, the method splits at block  2208  into a two paths, one to off-page reference  1  and another to off-page reference  4 . These references on  FIG.  22 A  (similar references appear on  FIG.  22 B ) are to the corresponding off-page references on  FIG.  22 C . Thus, the off-page reference  1  from block  2208  should be understood to lead to the off-page reference  1  on  FIG.  22 C  and the attached block  2291 . Similarly, the off-page reference  4  from block  2208  should be understood to lead to the off-page reference  4  on  FIG.  22 C  and the attached block  2294 . 
     With reference to  FIG.  22 A , at block  2202 , the wearable system may provide a user with initial fitting instructions (e.g., how to tilt the device such that the markings  2100  are visible). Block  2202  corresponds to block  2002  of  FIG.  20   , and additional details of block  2202  are thus described in connection with block  2002  of  FIG.  20   . 
     At block  2204 , the wearable system may obtain eye positions and eye tracking confidence levels. In particular, the wearable system may determine the y-axis and z-axis positions of each of the user&#39;s eyes (the pupil and center of rotation of each eye, respectively) and may also determine confidence levels associated with the eye tracking data for each of the user&#39;s eyes (including a confidence level for the left eye and a confidence level for the right eye). Block  2204  corresponds to block  2004  of  FIG.  20   , and additional details of block  2204  are described in connection with block  2004  of  FIG.  20   . 
     At block  2206 , the method  2200  may split depending on the eye tracking confidence levels. When the eye tracking confidence levels are (1) bad for both eyes or are (2) bad for one eye and low for the other eye, the method  2200  may move to block  2208 . At block  2208 , the wearable system may determine if the user is already using a thicker forehead pad. In general, the wearable system may support installation of a plurality of different forehead pads of varying thicknesses. In some embodiments, the wearable system may support installation of a limited number of different forehead pads of varying thicknesses. As a particular example, the wearable system may support installation of two forehead pads, one being relatively thin (and which may be referred to herein as a thinner forehead pad) and one being relatively thick (and which may be referred to herein as a thicker forehead pad). Where three or more forehead pads of different thicknesses are available, the wearable system may determine if the user is already using a forehead pad of a particular thickness (e.g., the thickness forehead pad) at block  2208 . In some embodiments, the wearable system may determine that a user is already using the thicker forehead pad based on a prior recommendation to the user to install the thicker forehead pad. In some embodiments, the wearable system may ask the user if they are already using the thicker forehead pad. In some embodiments, the wearable system may include a sensor that detects the presence of the thicker forehead pad. When the user is already using a thicker forehead pad, the method  2200  may move to block  2294  (e.g., as indicated by the off-page reference  4  in  FIG.  22 A  coupled to the “YES” branch of block  2208  and by the off-page reference  4  in  FIG.  22 C  coupled to block  2294 ). When the user is not yet using a thicker forehead pad, the method  220  may move to block  2291  ( FIG.  22 C ). 
     When the eye tracking confidence levels are low or bad for one eye and high for the other eye, the method  2200  may move from block  2206  to block  2212 . At block  2212 , the wearable system may decide to continue based on the eye position of the eye having a high eye tracking confidence score. In particular, the method  2200  may utilize the eye position of eye having a high confidence score for the blocks shown on  FIG.  22 B . 
     When the eye tracking confidence levels are high for both eyes or low for both eyes, the method  2200  may move from block  2206  to block  2214 . At block  2214 , the wearable system may determine if there is a Y-offset between the two eyes of the user. For example, the wearable system may determine if one of the user&#39;s eyes is located in a given one of the vertical regions of  FIG.  19    (e.g., T 1 , T 2 , M 1 , M 2 , B 1 , or B 2 ), while the other eye is located in a different one of the vertical regions of  FIG.  19   . 
     If there is no Y-offset between the user&#39;s eyes, the method  2200  continues to block  2216 . At block  2216 , the wearable system may decide to continue based on the average position of the user&#39;s eyes (e.g., the system may average the position of the user&#39;s right eye with the position of the user&#39;s left eye). For example, after block  2216 , the method  2200  may utilize the average position for performing the blocks shown on  FIG.  22 B . 
     If there is a Y-offset between the user&#39;s eyes, the method  2200  continues at block  2218 . At block  2218 , the wearable system may determine if the Y-offset is just one region or is larger than one region. For example, the system may determine if the user&#39;s eyes are located in adjacent vertical regions (i.e., have just a one region offset) or non-adjacent vertical regions (i.e., have an offset greater than one region). As an example, a user&#39;s eyes may have a Y-offset of two if the user&#39;s left eye is located in T 2  and the user&#39;s right eye is located in M 1 . 
     If the Y-offset is greater than one region, the method  2200  continues at block  2216  (which is described above). If the Y-offset is just one region, the method  2200  continues at block  2220 . At block  2220 , the wearable system may continue based on the eye position of the more offset eye (e.g., the eye that is further away from the desired range of M 1  or M 2 ). In particular, the method  2200  may utilize the position for the more offset eye for the blocks shown on  FIG.  22 B . 
     At block  2230  ( FIG.  22 B ), the method  2200  splits based on the y-axis location of the pupil center. As previously discussed, block  2230  may utilize the location of either (1) the confident eye (as in block  2212 ), (2) the average of the two eyes (as in block  2216 ), or (3) the more offset eye (as in block  2220 ). 
     When the relevant eye position is in region T 2 , the method  2200  may continue at block  2232 . At block  2232 , the wearable system may determine if the Z-position of the relevant eye (e.g., the confident eye, the average of the eyes, or the more offset eye) is beyond (greater than) a threshold (e.g., if the eye is too far away from the wearable display) or if the eye tracking confidence levels are low for both eyes. As noted above, the Z-position for the method  2200  may be the position of the center of rotation of the relevant eye. If neither condition exists, the method  2200  continues at block  2292  (as indicated by off-page reference  2 ). If either condition exists, the method  2200  continues at block  2234 . At block  2234 , the wearable system may determine if the user is already using a thicker forehead pad. When the user is already using a thicker forehead pad, the method  2200  may move to block  2292 . When the user is not yet using a thicker forehead pad, the method  2200  may move to block  2291 . 
     When the relevant eye position is in region T 1 , the method  2200  may continue at block  2236 . At block  2236 , the wearable system may determine if the Z-position of the relevant eye (e.g., the confident eye, the average of the eyes, or the more offset eye) is beyond (greater than) a threshold (e.g., if the eye is too far away from the wearable display) or if the eye tracking confidence levels are low for both eyes. If neither condition exists, the method  2200  continues at block  2294  (as indicated by off-page reference  4 ). If either condition exists, the method  2200  continues at block  2238 . At block  2238 , the wearable system may determine if the user is already using a thicker forehead pad. When the user is already using a thicker forehead pad, the method  2200  may move to block  2294 . When the user is not yet using a thicker forehead pad, the method  2200  may move to block  2293 . 
     When the relevant eye position is in region B 1 , the method  2200  may continue at block  2240 . At block  2240 , the wearable system may determine if the Z-position of the relevant eye (e.g., the confident eye, the average of the eyes, or the more offset eye) is beyond a threshold (e.g., if the eye is too far away from the wearable display) or if the eye tracking confidence levels are low for both eyes. If neither condition exists, the method  2200  continues at block  2296  (as indicated by off-page reference  6 ). If either condition exists, the method  2200  continues at block  2242 . At block  2242 , the wearable system may determine if the user is already using a thicker forehead pad. When the user is already using a thicker forehead pad, the method  2200  may move to block  2296 . When the user is not yet using a thicker forehead pad, the method  2200  may move to block  2295 . 
     When the relevant eye position is in region B 2 , the method  2200  may continue at block  2244 . At block  2244 , the wearable system may determine if the Z-position of the relevant eye (e.g., the confident eye, the average of the eyes, or the more offset eye) is beyond a threshold (e.g., if the eye is too far away from the wearable display) or if the eye tracking confidence levels are low for both eyes. If neither condition exists, the method  2200  continues at block  2298  (as indicated by off-page reference  8 ). If either condition exists, method  2200  continues at block  2246 . At block  2246 , the wearable system may determine if the user is already using a thicker forehead pad. When the user is already using a thicker forehead pad, the method  2200  may move to block  2298 . When the user is not yet using a thicker forehead pad, the method  2200  may move to block  2297 . 
     When the relevant eye position is in region M 1  or region M 2 , the method  2200  may continue at block  2250 . At block  2250 , the wearable system may determine if the Z-position of the relevant eye (e.g., the confident eye, the average of the eyes, or the more offset eye) is beyond a threshold (e.g., if the eye is too far away from the wearable display) or if the eye tracking confidence levels are low for both eyes. If neither condition exists, the method  2200  continues at block  2252 . If either condition exists, the method  2200  continues at block  2254 . At block  2254 , the wearable system may determine if the user is already using a thicker forehead pad. When the user is already using a thicker forehead pad, the method  2200  may move to block  2252 . When the user is not yet using a thicker forehead pad, the method  2200  may move to block  2293 . 
     At block  2252 , the method  2200  may complete (e.g., may end the fitting process). If desired, the wearable system may provide feedback to the user indicating the fit process has completed. Optionally, the wearable system may provide the user with an indication of the quality of the fit (e.g., an indication of how successful the fitting process was at achieving a proper fit or improving fit). 
     At each of blocks  2291 - 2298 , the wearable system may provide the user with recommendations to improve the fit or position of the wearable system on the user. The recommendations may be based on the measured eye position (e.g., the vertical positions of the user&#39;s eyes as discussed in connection with  FIG.  19   ). In the example of  FIGS.  21 A- 21 C , the recommendations include replacing a forehead pad with a thicker forehead pad, replacing a nose pad with a nose pad that is one or two sizes taller or shorter. A taller nose pad may generally raise the display relative to the user&#39;s eyes (e.g., along the y-axis), but may also alter the z-axis positions of the user&#39;s eyes. A thicker forehead pad may generally move the display away from the user&#39;s eyes (e.g., along the z-axis), but may also alter the y-axis positions of the user&#39;s eyes. In some embodiments, there are two forehead pads including a thicker forehead pad and a standard forehead pad (which may merely be the absence of the thicker forehead pad). In some embodiments, there may be a variety of forehead pads of varying thicknesses. These are merely illustrative examples and may vary depending on the availability of additional fit pieces and fit adjustment mechanisms. Additional discussion of interchangeable fit pieces can be found above in connection with  FIGS.  15 A and  15 B . 
     As noted above,  FIG.  22 C  provides a set of particular recommendations based on particular deviations of the display from a desired position. Various ones of these recommendations have been described above, and are also listed below. 
     At block  2291 , the wearable system recommends to the user to install a thicker forehead pad and a nose pad two sizes taller. 
     At block  2292 , the wearable system recommends to the user to install a nose pad two sizes taller. 
     At block  2293 , the wearable system recommends to the user to install a thicker forehead pad and a nose pad one size taller. 
     At block  2294 , the wearable system recommends to the user to install a nose pad one size taller. 
     At block  2295 , the wearable system recommends to the user to install a thicker forehead pad. 
     At block  2296 , the wearable system recommends to the user to install a nose pad that is one size shorter. 
     At block  2297 , the wearable system recommends to the user to install a thicker forehead pad and a nose pad that is one size shorter. 
     At block  2298 , the wearable system recommends to the user to install a nose pad that is two sizes shorter. 
     After any of blocks  2291 - 2298 , the method  2200  may return to block  2204  (as indicated by off page references  0  on  FIGS.  22 A and  22 C ). In particular and after providing the user with a fit adjustment recommendation (as part of one of blocks  2291 - 2298 ) and allowing the user to install the new fit pieces, the method  2200  may determine new eye positions and eye tracking confidence levels. The method  2200  may continue to recommend additional fit adjustment recommendations until some end condition is satisfied. As examples, the method  2200  may continue until a proper fit is achieved, a user exits the fit process, or the system has repeated the process a sufficient number of times that additional improvements in fit are unlikely. As an example, the method  2200  may continue for a maximum of 3 iterations, a maximum of 4 iterations, or a maximum of 5 iterations. If desired, the method  2200  may return to block  2202  rather than block  2204  during one or more of the iterations (e.g., in order to remind the user of how to properly put the wearable system onto their head). 
     Computer Vision to Detect Objects in Ambient Environment 
     As discussed above, the display system may be configured to detect objects in or properties of the environment surrounding the user. The detection may be accomplished using a variety of techniques, including various environmental sensors (e.g., cameras, audio sensors, temperature sensors, etc.), as discussed herein. 
     In some embodiments, objects present in the environment may be detected using computer vision techniques. For example, as disclosed herein, the display system&#39;s forward-facing camera may be configured to image the ambient environment and the display system may be configured to perform image analysis on the images to determine the presence of objects in the ambient environment. The display system may analyze the images acquired by the outward-facing imaging system to perform scene reconstruction, event detection, video tracking, object recognition, object pose estimation, learning, indexing, motion estimation, or image restoration, etc. As other examples, the display system may be configured to perform face and/or eye recognition to determine the presence and location of faces and/or human eyes in the user&#39;s field of view. One or more computer vision algorithms may be used to perform these tasks. Non-limiting examples of computer vision algorithms include: Scale-invariant feature transform (SIFT), speeded up robust features (SURF), oriented FAST and rotated BRIEF (ORB), binary robust invariant scalable keypoints (BRISK), fast retina keypoint (FREAK), Viola-Jones algorithm, Eigenfaces approach, Lucas-Kanade algorithm, Horn-Schunk algorithm, Mean-shift algorithm, visual simultaneous location and mapping (vSLAM) techniques, a sequential Bayesian estimator (e.g., Kalman filter, extended Kalman filter, etc.), bundle adjustment, Adaptive thresholding (and other thresholding techniques), Iterative Closest Point (ICP), Semi Global Matching (SGM), Semi Global Block Matching (SGBM), Feature Point Histograms, various machine learning algorithms (such as e.g., support vector machine, k-nearest neighbors algorithm, Naive Bayes, neural network (including convolutional or deep neural networks), or other supervised/unsupervised models, etc.), and so forth. 
     One or more of these computer vision techniques may also be used together with data acquired from other environmental sensors (such as, e.g., microphone) to detect and determine various properties of the objects detected by the sensors. 
     As discussed herein, the objects in the ambient environment may be detected based on one or more criteria. When the display system detects the presence or absence of the criteria in the ambient environment using a computer vision algorithm or using data received from one or more sensor assemblies (which may or may not be part of the display system), the display system may then signal the presence of the object. 
     Machine Learning 
     A variety of machine learning algorithms may be used to learn to identify the presence of objects in the ambient environment. Once trained, the machine learning algorithms may be stored by the display system. Some examples of machine learning algorithms may include supervised or non-supervised machine learning algorithms, including regression algorithms (such as, for example, Ordinary Least Squares Regression), instance-based algorithms (such as, for example, Learning Vector Quantization), decision tree algorithms (such as, for example, classification and regression trees), Bayesian algorithms (such as, for example, Naive Bayes), clustering algorithms (such as, for example, k-means clustering), association rule learning algorithms (such as, for example, a-priori algorithms), artificial neural network algorithms (such as, for example, Perceptron), deep learning algorithms (such as, for example, Deep Boltzmann Machine, or deep neural network), dimensionality reduction algorithms (such as, for example, Principal Component Analysis), ensemble algorithms (such as, for example, Stacked Generalization), and/or other machine learning algorithms. In some embodiments, individual models may be customized for individual data sets. For example, the wearable device may generate or store a base model. The base model may be used as a starting point to generate additional models specific to a data type (e.g., a particular user), a data set (e.g., a set of additional images obtained), conditional situations, or other variations. In some embodiments, the display system may be configured to utilize a plurality of techniques to generate models for analysis of the aggregated data. Other techniques may include using pre-defined thresholds or data values. 
     The criteria for detecting an object may include one or more threshold conditions. If the analysis of the data acquired by the environmental sensor indicates that a threshold condition is passed, the display system may provide a signal indicating the detection the presence of the object in the ambient environment. The threshold condition may involve a quantitative and/or qualitative measure. For example, the threshold condition may include a score or a percentage associated with the likelihood of the reflection and/or object being present in the environment. The display system may compare the score calculated from the environmental sensor&#39;s data with the threshold score. If the score is higher than the threshold level, the display system may detect the presence of the reflection and/or object. In some other embodiments, the display system may signal the presence of the object in the environment if the score is lower than the threshold. In some embodiments, the threshold condition may be determined based on the user&#39;s emotional state and/or the user&#39;s interactions with the ambient environment. 
     In some embodiments, the threshold conditions, the machine learning algorithms, or the computer vision algorithms may be specialized for a specific context. For example, in a diagnostic context, the computer vision algorithm may be specialized to detect certain responses to the stimulus. As another example, the display system may execute facial recognition algorithms and/or event tracing algorithms to sense the user&#39;s reaction to a stimulus, as discussed herein. 
     It will 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. 
     Other Considerations 
     Each of the processes, methods, and algorithms described herein and/or depicted in the attached 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 implementations, particular operations and methods may be performed by circuitry that is specific to a given function. 
     Further, certain implementations 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, animations or 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. 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 implementations described herein is for illustrative purposes and should not be understood as requiring such separation in all implementations. 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. Many implementation variations are possible. 
     The processes, methods, and systems may be implemented in a network (or distributed) computing environment. Network environments include enterprise-wide computer networks, intranets, local area networks (LAN), wide area networks (WAN), personal area networks (PAN), cloud computing networks, crowd-sourced computing networks, the Internet, and the World Wide Web. The network may be a wired or a wireless network or any other type of communication network. 
     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. Various modifications to the implementations described in this disclosure may be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other implementations without departing from the spirit or scope of this disclosure. Thus, the claims are not intended to be limited to the implementations shown herein, but are to be accorded the widest scope consistent with this disclosure, the principles and the novel features disclosed herein. 
     Certain features that are described in this specification in the context of separate implementations also may be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation also may be implemented in multiple implementations 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. 
     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. 
     As used herein, a phrase referring to “at least one of” a list of items refers to any combination of those items, including single members. As an example, “at least one of: A, B, or C” is intended to cover: A, B, C, A and B, A and C, B and C, and A, B, and C. Conjunctive language such as the phrase “at least one of X, Y and Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to convey that an item, term, etc. may be at least one of X, Y or Z. Thus, such conjunctive language is not generally intended to imply that certain embodiments require at least one of X, at least one of Y and at least one of Z to each be present. 
     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 implementations. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, 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 implementations 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.