Patent Publication Number: US-9405365-B2

Title: Systems and methods for identifying gaze tracking scene reference locations

Description:
RELATED APPLICATION DATA 
     The present application is a continuation of application Ser. No. 13/113,003, filed May 20, 2011, issuing as U.S. Pat. No. 8,885,877, the entire disclosure of which is expressly incorporated by reference herein. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     The U.S. Government may have a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Department of Defense (US Army) Contract No. W81XWH-05-C-0045, U.S. Department of Defense Congressional Research Initiatives No. W81XWH-06-2-0037 and W81XWH-09-2-0141, and U.S. Department of Transportation Congressional Research Initiative Agreement Award No. DTNH 22-05-H-01424. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to apparatus, systems, and methods for unobtrusively identifying reference locations within the environment of a device wearer for gaze tracking and other applications. 
     BACKGROUND OF THE INVENTION 
     The apparatus, systems, and methods herein utilize machine vision techniques to track locations and objects being viewed by an observer. Gaze tracking algorithms can be considered as requiring two continuous data streams in order to produce accurate tracking results: 1) eye tracking methods to detect the edges of pupils or other identifiable reference points within the eye to compute pivot angles and viewing directions of the eye, and 2) head tracking methods to locate the position and orientation of the head within our three-dimensional world. 
     Generally, head tracking can involve identifying the location of a rigid object affixed to the head (as opposed to the head itself). In this case, headwear or eyewear that is affixed to the head has known geometries and displacements relative to the head or reference points on the head that can be computed. More specifically, for accurate gaze tracking, a head tracking apparatus should have a known displacement from the pivot point of one or both eyeballs of the observer. Furthermore, for most applications, gaze tracking locations are determined relative to reference locations or objects within the environment of a device wearer, such as the corners of a display monitor, a mobile computing device, a switch, a light source, a window, and the like. 
     Applications that involve machine vision are becoming increasingly common-place. In part, this has arisen as a result of technological advances in the electronics and software development industries, and decreases in the cost of cameras, information processing units, and other electronics components. Gaze tracking, in particular, is increasingly being used in a number of diagnostic, human performance, and control applications. A small number of examples include monitoring the degree of fatigue of an individual, assessing driver or pilot awareness, assessing the effects of drugs or alcohol, diagnosing post-traumatic stress disorder, tracking human performance with age, determining the effectiveness of training or exercise, assessing the effectiveness of advertising and web-page design by measuring ocular dwell times, magnifying or changing the brightness of specific objects or images (including words) under observation, controlling various aspects of games, acquiring foundational clinical data to assess neurological or cognitive disorders, diagnosing and monitoring degenerative eye conditions, and allowing individuals with limited or no mobility below the neck to communicate by controlling a computer cursor using one or more eyes and eyelids. Sectors and industries that utilize gaze tracking include military, medicine, security, human performance, sports medicine, rehabilitation engineering, police, research laboratories, and toys. 
     In almost all cases, an increase in the accuracy of gaze tracking leads to an increase in the performance and convenience of most applications. For example, with increased accuracy, ocular dwell times to quantify fixation times on smaller objects or components of objects can be more accurately measured. Gaze tracking can be more effectively employed with portable devices that utilize smaller screens including mobile phones and hand-held displays. When gaze tracking is used to control a cursor involving selection from a number of virtual objects or icons within a screen, an increased number of selectable objects can be displayed simultaneously because of the ability to use smaller virtual objects or icons. An increased number of objects within each level of a selection process has a dramatic effect on the efficiency (i.e., reduced number of selection levels and/or reduced time) that a virtual object and associated action can be chosen. Similarly, enlarging or increasing the brightness levels of objects and words under observation can significantly increase recognition and reading rates of individuals who are visually impaired. 
     Many gaze tracking systems use cameras and eye illuminators that are located at a considerable distance (e.g., greater than ten centimeters (10 cm)) from an eye. As the distance away from the eyes is increased, an eye tracking apparatus generally becomes less obtrusive; however, it becomes increasingly difficult to accurately measure the location of an eye because of the need for higher spatial resolution by cameras and because wide-ranging head movement can cause the complete loss of the ability to track an eye. Many gaze tracking systems also use bright (visible or invisible) “point” sources of light located some distance from the head to produce glints or bright spots on the surface of the eye. These glints can be used to generate reference vectors from the location of the glint on the surface of the eye to known locations in the environment (i.e., the light sources). Here again, wide-ranging movements of the head can cause loss of the ability to track glints and/or the ability to associate a glint with a particular light source. 
     With the advent of modern-day microelectronics and micro-optics, it is possible to unobtrusively mount the components for gaze tracking on eyewear (e.g., eyeglasses frames) or headwear (e.g., helmet, mask, goggles, virtual reality display) including those devices disclosed in U.S. Pat. No. 6,163,281, 6,542,081, or 7,488,294, 7,515,054, the entire disclosures of which are expressly incorporated by reference herein. Using high-precision micro-optics within the eyewear or headwear, it is possible to more clearly resolve structures and reflections within the eye and nearby regions, as well as the scene viewed by the device wearer. The use of low-power, miniature cameras and electronics permits a head-mounted system to optionally be non-tethered through the use of a battery power source. Furthermore, recent advances in wireless telecommunications allow gaze tracking results to be transmitted in real-time to other computing, data storage, or control devices. As a result of these technological advances in a number of fields, an eyewear- or headwear-based gaze tracking system can be unobtrusive, light-weight, portable and convenient to use. 
     SUMMARY OF THE INVENTION 
     Gaze tracking involves substantially continuously identifying the locations and/or objects being viewed by an observer. Accurate gaze tracking results from a combination of eye tracking and head tracking relative to identified reference locations within our 3-dimensional world. The apparatus, systems, and methods herein utilize an unobtrusive scene camera mounted on eyewear or headwear to identify naturally occurring or intentionally placed reference locations in the environment of the wearer. 
     More specifically, the apparatus, systems, and methods herein may facilitate unobtrusively identifying reference locations within the environment of the device wearer for gaze tracking and other applications. In one embodiment, systems and methods for determining scene reference locations may include a device configured to be worn on a person&#39;s head; a scene camera connected to the device and positioned for capturing images of the environment of the wearer; a scene processor operatively connected to the scene camera for determining scene reference locations within the scene camera images; an eye-tracking camera connected to the device and positioned for capturing eye-tracking locations of at least one of the wearer&#39;s eyes; and a processor that uses scene reference locations and eye-tracking locations to determine locations being viewed by the wearer. 
     Reference locations within a scene may be identified using one or more characteristics of objects including an object&#39;s shape size, or color. The spatial relation among various geometric shapes such as those found on one- and two-dimensional bar codes, QR (i.e., quick response) codes, matrix (i.e. two-dimensional) codes, and the like may also be used for location identification and orientation. Objects that define reference locations may be intentionally placed within the wearer&#39;s environment; such as colored pieces of paper or plastic, pigmented (e.g., paint or ink) spots, colored (or black and white) regions within a display screen, light sources, and/or reflective surfaces. Alternatively, reference locations may be extracted using object recognition techniques from an unaltered wearer&#39;s environment such as the corners of a display screen, the corners of a mobile phone or reader (e.g., iPad® or Kindle® device), the central location of a larger object, an icon or patch of color on a display monitor, a button, markings on an object, edges of colored patterns, and the like. Reference locations may be identified by visible or invisible light. They may be based on the locations of entire objects or subsets of objects, such as corners, voids, points, or edges. Light from reference locations may utilize ambient light, light projected from the eyewear or headwear, light generated by the reference locations themselves, and/or light from other sources. Combinations of both general approaches (i.e., recognizing both naturally occurring and intentionally placed objects) are also possible. 
     In light of the foregoing background, the apparatus, systems, and methods herein may provide an improved gaze tracking method and system for various applications. 
     In an exemplary embodiment, a method involves the use of a “scene camera” that is affixed to eyewear or headwear looking outwardly relative to the individual wearing the eyewear or headwear. The scene camera transmits images to a processor programmed to identify multiple reference locations within scene camera images. Optionally, the processor may be coupled to, may communicate with, or may otherwise access a database of “templates,” (i.e., images of known objects, configurations of reference locations, and the like) to identify reference locations. 
     In accordance with one embodiment, systems and methods are provided that identify reference locations using image recognition techniques to identify objects or components of objects with known geometries and colors. A common configuration using this method is to identify the four corners of a computer display monitor or mobile computing/phone device or other electronic object. This may be performed by recognizing the edge of the device frame relative to a background scene, the edge of the display screen (i.e., the backlit region in the case of a LCD-based device or object) relative to the frame of the display, or both. Corners and/or edges may be identified based on color, texture, sharp versus rounded geometry, size relative to other identifiable components, markings, and the like. 
     In accordance with another embodiment, systems and methods are provided that produce reference locations in which identifiable objects or surfaces have been added to a scene at known locations. For example, systems and methods may use pieces of paper or plastic conveniently affixed to objects (e.g., using adhesive, screws, clips, or other fasteners, and the like) that may be identified based on color and/or shape. Similarly, ink, paint or other pigmented substances may be applied to objects to generate reference locations with an identifiable color or shape. The color and/or shape of the applied reference surface may be based on measuring reflected, fluorescent, phosphorescent, or luminescent light that may be either visible or invisible. 
     In accordance with yet another embodiment, systems and methods are provided that produce bright reference points using reflective patches (e.g., constructed from paint, cloth, plastic, paper, and the like) that may be affixed to any surface (e.g., using adhesive, fasteners, and the like). These reflective surfaces may be based on prismatic or flat reflective mirrored surfaces. They may be illuminated using one or more light sources located on the eyewear or headwear, by ambient light, and/or other light sources. One example of a light source is a single or multiple light-emitting diodes (LEDs) located adjacent to or away from the scene camera on the eyewear or headwear. The light sources may use wavelengths of electromagnetic radiation that are visible or invisible, e.g., infrared or other light outside the visible spectrum to avoid interference with normal activities of the wearer and/or others. In this configuration, the timing of illumination may be controlled by the eyewear or headwear and no illumination sources powered external to the eyewear or headwear may be required. 
     In accordance with still another embodiment, systems and methods are provided that not only provide bright reference locations illuminated by the eyewear or headwear, but also produce reference glints by the light reflected from the reference points onto the eyeball. By controlling the timing of illumination relative to the timing of video image acquisition, it is possible to acquire images with and without illumination of reflective reference points and glints. Subtracting images with illumination turned on, from images with illumination turned off, may facilitate the ability to isolate the locations of reflective sources including the locations of the reference points within images acquired by scene cameras as well as the locations of corresponding glints within images gathered by eye tracking camera(s). 
     A controller may be coupled to the camera(s) and/or the light sources that is configured for sampling brightness in the respective reflected reference locations of the light sources using the camera(s) and modulating the light source(s) based on the sampled brightness to provide desired brightness levels within camera images. 
     A processing unit operationally coupled to the scene camera may acquire images of the environment of the device wearer, for example, to monitor and/or further analyze characteristics of the scene. The scene processing unit and eye-tracking processing unit may be one or more separate processors, or may be a single processor and/or may include illumination controllers to regulate the intensity of illumination of the environment to the device wearer. 
     In one embodiment, the illumination controller may be configured for amplitude modulation of at least one of the current and/or the voltage to the light source to provide desired brightness levels in the respective regions of scene camera images. In addition or alternatively, the controller may be configured for pulse-width modulation of the current and/or the voltage to the light sources to provide desired brightness levels. 
     In any of these examples, illumination, reference location tracking, eye tracking and gaze tracking may be operated substantially continuously or intermittently. For example, scene light sources may be deactivated when the scene camera is inoperative. This includes times between acquiring camera images. Processors, cameras and illumination may also be deactivated when not in use, e.g., to conserve power. Illumination sources and other electronics may also be reduced in power or turned off for increased safety of the device wearer. 
     In an exemplary embodiment, the system includes an eyewear or headwear frame, a scene camera directed to view the environment around a device wearer, at least one camera directed at an eye of the wearer, one or more illumination sources oriented towards at least one eye of the wearer, and one or more processors, e.g., a scene processing unit coupled to the scene camera to identify reference locations within scene camera images, and a processing unit for eye tracking. The system may also include one or more light sources on the frame oriented away from the wearer, e.g., to provide scene illumination when reflective reference locations are utilized. Machine vision techniques are used within the processing unit(s) to determine reference locations. Reference locations identified within the scene processing unit and the eye-tracking processing unit may then be used in gaze tracking calculations. 
     Other aspects and features of the present invention will become more apparent from consideration of the following description taken in conjunction with the accompanying drawings 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate exemplary embodiments of the invention, in which: 
         FIG. 1  is a perspective view of an example of a system mounted on eyeglasses frames for reference location tracking and eye tracking. 
         FIG. 2  is a partial cut-away, side view of the system of  FIG. 1 , showing the spatial relation between a scene camera and an eye-tracking camera; and connections among a processing unit, scene camera, eye-tracking camera, and other components. 
         FIG. 3  shows an exemplary method for detecting reference locations using object recognition within an unaltered scene that includes a mobile computing/phone device. 
         FIG. 4  shows another exemplary method for detecting reference locations that includes providing reference objects, e.g., four (4) identifiable, colored, round pieces of paper, on the four (4) corners of a display monitor. 
         FIG. 5  shows yet another exemplary method for detecting reference locations that includes providing “virtual” identifiable reference objects, e.g., four (4) colored, regions displayed in the four (4) corners of a display monitor. 
         FIG. 6  is an example of an illumination pathway that shows a reflective surface that may be detected by a scene camera as a reference location, and a glint on the surface of the eye that may be detected by an eye-tracking camera. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     Turning to the drawings,  FIG. 1  shows an exemplary embodiment of a system  10  including an eyeglass frame  11  with a scene camera  12 , two eye-tracking cameras  13   a ,  13   b , and a processing unit  14 . Scene camera  12  is oriented on the frame  11  to view the region away from the device wearer&#39;s head  15  in order to track one or more reference locations  16   a ,  16   b  within the environment of the device wearer. Eye-tracking cameras  13   a  and  13   b  are oriented on the frame  11  toward the head  15  in order to track the locations of the wearer&#39;s pupils, glints, and/or other reference points on one or both eyes of the wearer. 
     In this embodiment, a single processing unit  14  may be carried by the frame  11 , e.g., to acquire images from the scene camera  12  as well as the eye-tracking cameras  13   a ,  13   b , although it will be appreciated that separate processors (not shown) may be provided on the frame  11  or at a remote location (not shown) that communicates with the frame  11 . A power source (e.g., battery)  17  may be carried by the frame  11 , e.g., encased in the stem of the frame  11  opposite that containing the processing unit  14 . Scene illumination light sources  18   a ,  18   b  may optionally be located near the scene camera  12  or more distant from the scene camera  12 . 
     In an exemplary embodiment, the scene camera  12  may include a CCD or CMOS or other detector including an active area, e.g., having a rectangular or other array of pixels, for capturing images and generating video signals representing the images. The active area of the camera  12  may have any desired shape, e.g., a square or rectangular shape, and the like. In addition, the camera  12  may include one or more filters, lenses, and the like (e.g., filter  67  and/or lens  66  as illustrated in  FIG. 6 ), if desired, e.g., to focus images on the active area, filter undesired intensities and/or wavelengths of light, and the like. 
     In the embodiment illustrated in  FIG. 1 , the scene camera  12 , is unobtrusively located on the nose bridge  25  ( FIG. 2 ) of the frame  11 , thereby minimizing interference with the wearer&#39;s normal vision. Other locations for the scene camera(s) are also possible including near the outer edges of the frame  11 . Alternatively, in the case of headwear, one or more scene cameras may, for example, be located atop the head (not shown). Reflective and/or refractive optical components may be incorporated, e.g., to direct light from different regions of the environment towards scene camera(s). 
     In addition or alternatively, multiple scene cameras  19   a ,  19   b  may be provided that are spaced apart from one another and/or directed towards multiple reference locations  16   a ,  16   b , e.g., providing separate or overlapping fields-of-view. Multiple scene cameras  16   a ,  16   b  may provide higher resolutions, increased sensitivity under different lighting conditions and/or a wider field-of-view, e.g., in addition to or instead of scene camera  12 . Another potential advantage of using multiple scene cameras is the ability to use different optical filters (e.g., see filter  67  in  FIG. 6 ) with each camera, e.g., to isolate reference sources that differ in color or that are preferentially illuminated using different wavelengths of electromagnetic radiation. 
     If two (2) scene cameras are used, they may, for example, be conveniently located near each of the outer corners of the frame  11  (e.g., near locations indicated as  19   a  and  19   b  in  FIG. 1 ) or lateral sides of headgear (not shown). Reference locations and corresponding scene camera orientations may be within the normal visual field of the wearer or outside of this range including directed beside or behind the head. Field(s)-of-view may optionally be controlled in size and/or location by reflective surfaces and refractive lenses. 
       FIG. 2  shows a cut-away view and back side of the system  10  illustrated in  FIG. 1 . The fixed spatial displacement between scene camera  12  and eye-tracking camera  13   b  mounted within eyeglasses frames  11  in X, Y and Z directions may be seen from this perspective.  FIG. 2  also shows an example of a location where a single processing unit  14  for reference location tracking and eye-tracking may be embedded within the stem of the frame  11 . In this exemplary embodiment, the processing unit  14  is a field-programmable gate array (FPGA). 
     The processing unit  14  may include one or more controllers or processors, e.g., one or more hardware components and/or software modules for operating various components of the system  10 . For example, the processing unit  14  may include a separate (not shown) or integral controller for controlling light sources or cameras, for receiving and/or processing signals from cameras  12 ,  13   b , and the like. Optionally, one or more of the components of processing unit  14  may be carried on ear supports  24 , on the lens supports of the frame  11 , nose bridge  25 , and/or other locations within the eyewear or headwear, similar to embodiments described in the references incorporated by reference elsewhere herein. In the exemplary embodiment shown in  FIGS. 1 and 2 , a single processing unit  14  is used for image acquisition and processing for both reference location and eye tracking functions. 
     Cable(s)  26  may include individual cables or sets of wires coupled to cameras  12 ,  13   b , battery  17  ( FIG. 1 ), light sources  18   a ,  18   b  ( FIG. 1 ) and/or other components on the frame  11  and/or to processing unit  14 . For example, individual cables or sets of wires (not shown) may be embedded in the frame  11 , e.g., along the rim from the cameras  12 ,  13   b , and the like, until captured within the cable  26 , e.g., to reduce the overall profile of the frame  11  and/or to direct signals around any hinged regions or corners  27  within the eyewear or headwear, as desired. 
     The processing unit  14  may also include memory (not shown) for storing image signals from the camera(s)  12 ,  13   b , filters for editing and/or processing the image signals, elements for measurement calculations (also not shown), and the like. Optionally, the frame  11  and/or processing unit  14  may include one or more transmitters and/or receivers (not shown) for transmitting data, receiving instructions, and the like. In addition or alternatively, at least some processing may be performed by components that are remote from the frame  11  and/or on-board processing unit  14 , similar to embodiments disclosed in the references incorporated by reference elsewhere herein. For example, a data acquisition system may include one or more receivers, processors, and/or displays (not shown) at one or more remote locations from the processing unit  14  and/or frame  11 , e.g., in the same room, at a nearby monitoring station, or at a more distant locations. Such displays may include views generated by the scene camera(s)  12  and/or eye-tracking camera(s)  13   b , as well as gaze tracking measurements and related calculations. 
       FIG. 3  is an example of reference location tracking where machine vision techniques involving object identification are used to locate objects with known geometries and/or colors within an “unaltered scene” (i.e., a scene not altered intentionally for the purpose of establishing reference locations by the wearer/observer or anyone else involved in observations). In this example, the size, orientation, and/or location of a conventional mobile phone or hand-held computing device  30  may be tracked using a scene camera  31 . Images may be brought into focus on a scene camera  31  (which may be similar to the scene camera  12  shown in  FIGS. 1 and 2 ), e.g., using one or more lenses  33 , which may be carried by or otherwise coupled to the scene camera(s)  31  (not shown). 
     Within images acquired by the scene camera  31 , a processing unit (not shown) may scan the field-of-view  32  of images from the scene camera  31  for objects similar in shape and color to an object template for a mobile computing device. For example, the processing unit may include or otherwise access a database of known templates, e.g., a table associating known objects with data identifying their shapes and/or colors. The database may include vertical and horizontal reference points  36 ,  37  of known objects, detailed color and/or shape information on the reference objects, and the like, mapped to particular physical objects, thereby providing the processing unit sufficient information to identify the encountered object. If an object with appropriate attributes is found, a tetragon  34  (in this example of a rectangular cell phone) may be used to define the boundary of the device within images from the scene camera  31 . The dimensions of the sides of the tetragon  34  may be used to compute the orientation of the location of the scene camera  31  relative to reference points within the mobile computing device  30 . The overall size of the tetragon  34  within images from the scene camera  31  may be used in calculations of the distance between the scene camera  31  (i.e., affixed to the eyewear or headwear  11 ) and reference points within the mobile computing device  30 . 
     Examples of reference locations within reference objects include the four (4) corners of the tetragon  34  that correspond to the four (4) corners  35   a ,  35   b ,  35   c ,  35   d  of the mobile computing device  30 . The vertical  36  and horizontal  37  real-world dimensions of the reference object are known to the scene camera processing unit and, along with measurements made in scene camera images, may be used to translate distances measured within the images from scene camera  31  into real-world dimensions. 
       FIG. 4  is an example of reference location tracking where reference objects have been intentionally placed within a wearer&#39;s environment. Machine vision techniques involving object identification are used to locate these objects with known geometries and/or colors within scene camera images. In this case, four (4) discs  45   a ,  45   b ,  45   c ,  45   d  of known size(s) and color(s) have been affixed to the four (4) corners of display monitor  40 , e.g., by bonding with adhesive. Alternatively, the monitor  40  or other device may include reference objects permanently attached or otherwise incorporated into the device at desired locations. 
     Any number of reference objects may be added to the wearer&#39;s environment, e.g., two or three, or more than four (not shown), if desired. Reference objects may be of any size, shape or color. Reference objects may all be substantially the same size, shape and/or color; or one or more reference objects may differ in size, shape and/or color. In the latter example, differences in size, shape or color may be useful in unambiguously determining the exact orientation of reference locations and associated objects, e.g., to uniquely identify each corner of the mobile computing device  30 . 
     With further reference to  FIG. 4 , images may be brought into focus on scene camera  41  (which may be similar to the scene camera  12 ), e.g., using a lens  43 . Employing images acquired by the scene camera  41 , a processing unit (not shown) may scan the field-of-view  42  of the scene camera  41  for objects similar in shape and/or color to an object identification template for intentionally placed reference objects, e.g., accessing a database of templates, as described elsewhere herein. When objects with appropriate attributes are found, the distances between the centers or edges of reference objects  45   a ,  45   b ,  45   c ,  45   d  may be measured in vertical  46  and horizontal  47  directions. These distances may then be used to compute the orientation of the location of the scene camera  31  relative to reference points  45   a ,  45   b ,  45   c ,  45   d  within the scene. The overall size of the tetragon defined by the four (4) corners of reference objects  45   a ,  45   b ,  45   c ,  45   d  may also be used in calculations of the distance between the scene camera  41  and locations within the scene. Known real-world distances between vertical  46  and horizontal  47  reference points may be used to translate distances measured within the images from a scene camera  41  into real-world dimensions. 
     One application of head tracking and gaze tracking using these techniques is to control the position of a computer cursor  44  displayed on a monitor  40 . The accurate control of a cursor using gaze tracking may result in a wide range of applications including using a computer to surf the Internet, control a game, generate text-to-speech, turn on/off lighting or other environmental controls in household or industrial settings, and so on. Tracking head and eye movements while an observer is instructed to closely follow an object such as a cursor  44  may also be used during calibration procedures that, for example, may be used to account for spatial aberrations within a field-of-view  42 , such as those produced by most lenses  43 . 
       FIG. 5  shows another example of reference location tracking where “virtual” reference objects are intentionally displayed on a monitor or screen  50  that is within the field-of-view  52  of a scene camera  51 . “Virtual” reference objects may, for example, be patches of color, icons, QR codes, and/or other visual patterns that are distinct from the screen&#39;s background. For example, the drivers for the monitor  50  may be modified or replaced such that the virtual objects are superimposed on any images otherwise displayed on the monitor  50 . Thus, even when the monitor is used to display images and/or otherwise operate a variety of programs, the virtual objects may be present. The virtual objects may remain substantially static in the images presented on the monitor  50  or may moved during, e.g., as described elsewhere herein. 
     Machine vision techniques may be used to locate these “virtual” objects with known geometries, spatial relationships and/or colors within the scene. In the example illustrated in  FIG. 5 , four (4) “virtual” objects  55   a ,  55   b ,  55   c ,  55   d  are displayed in the four (4) corners of a display monitor  50 . Any number of “virtual” reference objects may be added to the field-of-view  52  of the scene camera  51 . The “virtual” reference objects may be of any size, shape or color. The “virtual” reference objects may all have substantially the same size, shape, spatial distribution of geometric forms and/or color; or one or more “virtual” reference objects may differ in size, shape and/or color. In the latter example, differences in size, shape, spatial distribution of geometric forms and/or color may be useful in unambiguously determining the rotational orientation of reference locations, similar to other embodiments herein. 
     When virtual objects with appropriate attributes are found, e.g. by a processing unit analyzing the images from the scene camera  51 , the distances between the centers of objects  55   a ,  55   b ,  55   c ,  55   d  may be measured in vertical  56  and horizontal  57  directions. These distances may be used to compute the orientation of the location of the scene camera  51  relative to reference points  55   a ,  55   b ,  55   c ,  55   d  within the environment of the device wearer. The overall size of a tetragon defined by reference objects  55   a ,  55   b ,  55   c ,  55   d  in the four (4) corners of the display screen may be used in calculations of the distance between the scene camera  51  and locations within the scene. Known real-world distances between vertical  56  and horizontal  57  reference points may be used to translate distances measured within images from the scene camera  51  into real-world dimensions. For example, the processing unit may include or access a database of templates that includes sufficient information to identify the object actually encountered, similar to other embodiments herein. Head tracking and gaze tracking measurements using these techniques may be used, e.g., to control the position of a cursor  54  displayed on the computer monitor  50  and/or otherwise interact with the encountered object and/or other nearby objects. 
     An advantage of using “virtual” reference objects as depicted in  FIG. 5  is the ability to generate identifiable reference objects without any (hardware) modifications of real-world objects. For example, if a computer (not shown) is to be used by a wearer of the system  10  of  FIG. 1 , software may be loaded onto the computer, e.g., modifying or replacing the monitor driver(s) and/or otherwise causing the virtual reference objects to be included in images displayed on the computer&#39;s monitor  50  during use of the system  10 . Conversely, the use of physical reference object placed, for example, on the edge of a computer monitor  40  as depicted in  FIG. 4  obviates the need for any superimposed display (and associated software modifications) within the displayable area of a monitor  40 . 
     With further reference to  FIGS. 4 and 5 , it is possible to combine any number of real reference objects with any number of “virtual” reference objects within a scene. Machine vision techniques using images from one or more scene cameras may be used to track any number of such objects. For example, the tracking of physical objects may be used initially when viewing reference objects in certain orientations and directions to identify a screen or device being operated by or communicating with a system, such as system  10  of  FIG. 1 . Identification of “virtual” objects may then be used when appropriate screen viewing angles are present, e.g., after the physical objects have been used to identify the monitor or screen. In order to produce highly precise gaze tracking within a localized area on a screen, for example, it may be desirable to dynamically change the positions or other tracking characteristics of “virtual” reference objects, e.g., once gaze tracking has determined where on the monitor or screen the wearer is looking. For example, more closely spaced, smaller “virtual” reference objects may be use as attention is focused to a particular subset or area of a monitor or screen. A processing unit may then discard image data outside of the field of the virtual objects on the monitor or screen, e.g., to enhance accuracy in gaze tracking, reduce the size of image data stored and/or processed, and the like. 
       FIG. 6  shows an example of an illumination and optical pathway that takes advantage of reflective reference patches and associated locations. In this example, an illumination source (e.g., one or more LEDs)  60  is included within or otherwise carried by the eyewear or headwear (not shown, such as the frame  11  of  FIGS. 1 and 2 ). Electromagnetic radiation from this illumination source  60  reflects off of one or more reflective patches or surfaces  61  that have been added to or embedded within one or more objects within the scene at known locations. In this exemplary embodiment, light is reflected from a disc  61  affixed to the corner of a display monitor or mobile computing device  62 . The location of this reflective surface and other reference surfaces in the scene may be determined from images gathered using a scene camera (not shown in  FIG. 6 , see, e.g., scene camera  12  in  FIGS. 1 and 2 ). 
     With additional reference to  FIG. 6 , light reflected from the reflective reference surfaces may produce glints  63  on the surface of the eye  64 . Glints may be detected as high-intensity bright spots within images gathered using eye-tracking camera(s)  65 . Within the eyewear or headwear, a short working distance lens  66  is generally required to focus images from eye  64  onto eye-tracking camera  65 , and a filter  67  may optionally be included in the light pathway to isolate optical wavelengths produced by the reflective (fluorescent, phosphorescent or luminescent) reference location surfaces. 
     A line segment between the center of glint  63  and the center of the corresponding reference location  61  produces a vector  68  that may be used as an input to gaze tracking calculations. This reference vector  68  along with the location of the center of the pupil  69  may then be used to compute a gaze tracking vector  70  relative to the reference vector  68 . Additional considerations in calculating a gaze tracking vector  70  include the slightly offset location of the center of the fovea (i.e., the image-sensing region of the retina) relative to the measured center of the pupil  69  and refraction within the light path through the cornea (not shown). The gaze tracking vector  70  points to the location  71  being viewed by the observer (i.e., the wearer of the eyewear or headwear). 
     Returning to  FIGS. 1 and 6 , an advantage of having illumination source(s) on the eyewear or headwear, e.g., frame  11 , is the ability to conveniently control the timing and/or intensity of illumination compared to the acquisition of images by scene camera  12  and eye-tracking  13   a ,  13   b  cameras. By subtracting scene images and/or eye-tracking camera images with illumination turned on from images with illumination turned off, reflections from reference locations  16   a ,  16   b  may be more readily isolated in scene camera images, and reflections from glints  63  may be more readily isolated in eye-tracking camera images. Furthermore, this scheme obviates the need for any light source or other powered component to be located away from, or tethered to, a power source  17  or controller within the eyewear or headwear. Thus, if reference objects are attached to or incorporated into a monitor or screen of a device, such reference objects do not need to be provided with a power source and/or controller to generate light, but may merely reflect light from the illumination source(s)  60 . 
     Any number of reflective surfaces  61  may be used as reference locations and/or sources for the production of glints  63 . Any number of sources of electromagnetic radiation may generate visible or invisible light. Using invisible light to produce reflections at reference locations and glints on the eye is particularly convenient, as this scheme generates little or no distraction (due to the presence of potentially bright, reflected light) on the part of a device wearer. CMOS cameras, in particular, are capable of detecting electromagnetic radiation in the near infrared spectrum that is not visible to the human eye. CMOS cameras are also particularly well suited in applications where low power and/or miniaturization are desired. 
     As described further elsewhere herein and with reference to  FIG. 6 , the brightness levels of glints  63  measured using an eye-tracking camera  65  and reflections from reference locations  61  measured using scene camera(s)  12  (not shown, see  FIG. 1 ) may be used in a feedback mode to control the intensity of the illumination source(s)  60 . One or more illumination sources  60  may be used to illuminate reference locations, for example, multiple illumination sources  60  (not shown) mounted at multiple locations throughout the eyewear or headwear. The use of multiple illumination sources  60  illuminating the environment of the device wearer from different angles may help to maintain high intensity reflections in camera images at different viewing angles. 
     In one embodiment, the amplitude of either the voltage or the current driving each illumination source  60  may be used to control light intensity. This is generally referred to as “amplitude modulation.” In another embodiment, the duration or “dwell time” of a controlling voltage or current may be modified to control light intensity. This is generally referred to as “pulse-width modulation.” Optionally, it is also possible to use both schemes simultaneously. 
     In an exemplary embodiment, each illumination source  60  may include a LED (light emitting diode) configured for emitting a relatively narrow or wide bandwidth of light, e.g., near infrared light at one or more wavelengths between about 640-700 nanometers, broadband visible light, white light, and the like. Optionally, one or more of the illumination sources  60  may include lenses, filters, diffusers, reflectors, or other features (not shown), e.g., for facilitating and/or controlling the uniformity of lighting of the environment of the device wearer. The illumination source(s)  60  may be operated substantially continuously, periodically, or otherwise intermittently, e.g., such that desired scene images are illuminated by the source(s)  60 , and then the images may be processed using the systems and methods described elsewhere herein. 
     The foregoing disclosure of the exemplary embodiments has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. 
     Further, in describing representative embodiments, the specification may have presented methods and/or processes as a particular sequence of steps. However, to the extent that the methods or processes do not rely on the particular order of steps set forth herein, the methods or processes should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. 
     While the invention is susceptible to various modifications, and alternative forms, specific examples thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the invention is not to be limited to the particular forms or methods disclosed, but to the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the scope of the appended claims.