Eye tracking using aspheric cornea model
A display system can include a head-mounted display configured to project light to an eye of a user to display virtual image content at different amounts of divergence and collimation. The display system can include an inward-facing imaging system possibly comprising a plurality of cameras that image the user's eye and glints thereon, and processing electronics that are in communication with the inward-facing imaging system and that are configured to obtain an estimate of a center of cornea of the user's eye using data derived from the glint images. The display system may use spherical and aspheric cornea models to estimate a location of the corneal center of the user's eye.
1. A display system configured to project light to an eye of a user to display virtual image content in a vision field of said user, said eye having a cornea and a pupil, said display system comprising:
a frame configured to be supported on a head of the user;
a head-mounted display disposed on the frame, said display configured to project light into said user's eye to display virtual image content to the user's vision field;
one or more eye tracking cameras configured to image the user's eye;
a plurality of light emitters; and
processing electronics in communication with the display and the one or more eye tracking cameras, the processing electronics configured to:
receive images of the user's eye captured by the one or more eye tracking cameras, wherein glint reflections of the different light emitters are observable in said images of the eye captured by the one or more eye tracking cameras; and
estimate a location of a corneal center of the user's eye based at least in part on the locations of the glint reflections in said images captured by said one or more eye tracking cameras,
wherein said processing electronics employ an aspheric model of said cornea in numerical calculations to estimate said location of said corneal center of the user's eye,
wherein said processing electronics additionally employ a spherical model of said cornea in further numerical calculations to estimate said location of said corneal center, and
wherein said estimate of said corneal center determined from a spherical model of said cornea is applied to said aspheric model in the numerical calculations to determine a location of said corneal center.
2. The display system of claim 1 , wherein said spherical model of said cornea is used in the further numerical calculations to determine the location of said corneal center based on the location of the glint reflections in said images produced by the one or more eye tracking cameras and on the location of the one or more tracking cameras and the locations of the emitters that produced said respective glint reflections.
3. The display system of claim 1 , wherein said processing electronics are further configured to determine an estimate of said corneal center using an iterative process.
4. The display system of claim 1 , wherein said aspheric model is used to predict the locations of the glint reflections based on locations of light emitters.
5. The display system of claim 4 , wherein said locations of said glint reflections predicted based on said aspheric model are used to determine a center of curvature of a spherical surface.
6. The display system of claim 1 , wherein said processing electronics are configured to use a spherical model to determine one or more parameters of an aspheric surface.
7. The display system of claim 1 , wherein said processing electronics are configured to use an aspherical model to determine one or more parameters of a spherical surface.
8. The display system of claim 1 , wherein said aspheric model comprises an aspheric surface that is rotationally symmetric.
9. The display system of claim 1 , wherein said aspheric model comprises a surface that is non-rotationally symmetric.
10. The display system of claim 1 , wherein said one or more eye tracking cameras configured to image the user's eye comprise first and second eye tracking cameras.
11. The display system of claim 10 , wherein said processing electronics are further configured to:
based on the location of the glint reflections in one or more images produced by said first eye tracking camera and based on the location of the first eye tracking camera and the location of the emitters that produced said glint reflections, determine a first direction toward the center of corneal curvature of the user's eye; and
based on the location of the glint reflections in one or more images produced by said second eye tracking camera and based on the location of the second eye tracking camera and the location of the emitters that produced said glint reflections, determine a second direction toward the corneal center of the user's eye.
12. The display system of claim 11 , wherein said processing electronics are further configured to estimate a location of said corneal center of the user's eye based on said first and second directions toward the corneal center of the user's eye.
13. The display system of claim 11 , wherein said processing electronics are further configured to:
obtain an estimate of a corneal center of the user's eye based on the convergence of the first and second directions.
14. The display system of claim 1 , wherein said processing electronics are further configured to obtain an estimate of the corneal center of the user's eye based on multiple determinations of the corneal center of the user's eye for different eye poses.
15. The display system of claim 1 , wherein said processing electronics are further configured to obtain an estimate of the orientation of an aspheric surface for application of the aspheric model using the corneal center.
16. The display system of claim 10 , wherein said processing electronics are further configured to obtain an estimate of the orientation of an aspheric surface for application of the aspheric model based on an estimate of a corneal center and an estimate of the convergence of multiple vectors that pass through centers of the pupil determined from images obtained by the first and second eye tracking cameras.
17. The display system of claim 1 , wherein said one or more eye tracking cameras comprise first, second and third eye tracking cameras configured to image the user's eye, wherein said processing electronics are in communication with said first, second and third eye tracking cameras, and wherein the glint reflections of the light emitters are observable in the images of the eye captured by the first, second and third eye tracking camera.
18. The display system of claim 17 , wherein said processing electronics are further configured to estimate said corneal center of the user's eye based on a parameter of the user's eye determined by said first and third eye tracking cameras and a parameter of the user's eye determined by said first and second eye tracking cameras.