1. Stereo Viewing Mel Slater Virtual Environments http://www.montereytechnologies.com/hmd.htm

2. Introduction n Depth Cues n Ideals in Achieving Depth n Computing Stereo Pairs n Head-mounted displays

3. Physiological Depth Cues n Accommodation u Focal length of the eyes adjust in attempt to focus at points in the scene. u Based on changing thickness of lens caused by relaxing and tensing the ciliary muscles. n Convergence u Eyes rotate inwards (near objects) become parallel (far objects)

4. Accommodation and Convergence n Usually work in conjunction with each other. n This correspondence is not physiologically determined. n Learned by experience n Is broken when looking at eg screen based stereo views.

5. Terminology n Binocular disparity u The difference between the two images produced by left eye and right eye. n Motion Parallax u How points move relative to one another with respect to head moves. u Greater apparent movement usually implies smaller distance.

6. Psychological Depth Cues n Linear perspective n Shading n Shadows n Aerial Perspective n Occlusion n Retinal image size (constancy scaling) n Texture gradient

7. Stereo n Stereo pairs: Two projections, left and right eye on flat display. n Horizontal parallax u R-L F R-L > 0 called positive horizontal parallax F R-L < 0 called negative horizontal parallax u Similar term for vertical parallax u IPD = inter-pupilary distance. Stereo window Left Eye Right Eye L R IPD

8. Effect of Parallax n Positive parallax points will be virtual points behind the stereo window. n Negative parallax points will be virtual points in front of the stereo window n Note that the projected image points of a single point are called ‘homologous points’. Stereo window Left Eye Right Eye L R IPD

9. Viewing Stereo Pairs n Uncrossed/parallel setup when right eye sees right image and left eye the left image u Requires focus beyond the images n Crossed setup when right eye sees left image and left eye sees right image u Requires crossing eyes. n Viewing the opposite way around will reverse the sense of depth.

10. http://www.eleves.ens.fr:8080/home/massimin/Images/teapot.gif

11. http://www.3dartist.com/3dao/stereo.htm

12. Ideals n Congruence - left and right images should be the same except as caused by the horizontal parallax: u colour, geometry, brightness n Avoid vertical parallax - it should be zero - otherwise discomfort. n The image plane itself must be mapped to itself.

13. Ideals n Wide parallax (separation in the views) produces good depth, but discomfort. n Provide maximum depth but lowest parallax. u Place principal objects so that approx half parallax values are positive, half negative. n Further distance of viewer from display the greater the parallax that can be tolerated.

14. Ideals n Cross talk is when left images reach right eye, and right images reach left eye u For time dependent methods F afterglow of phosphors F departures from correct shutter speed u For anagraphs (red/green filters) colours not properly filtered out. n Not same problem in other synchronous methods (HMDs).

15. Ideals n Minimise impact of accommodation and convergence breakdown u Use lowest possible parallax to get required depth effect u The closer homologous points the less the disparity between accommodation and convergence. n Make the parallax less than or equal to IPD.

16. Use Other cues! n Perspective is an extremely powerful cue - stereo pairs by themselves do not necessarily give the right depth. u Example of parallel projections with stereo - the depth looks wrong.

17. Ideals n Avoid screen edge effects u If objects are ‘coming out’ but are up against edges, then the stereo effect will be broken - conflicting sensory information.

18. Head-mounted displays n Simultaneously projects left-eye and right-eye disparate images. http://www.gel.ulaval.ca/~mbernat/rapporta/rapangl3.html#HMD helmet RGB Graphics Hardware RGB Graphics Hardware

19. Head-mounted displays n Images formed on LCDs or CRTs n Screens are small, low resolution, too close for direct viewing n Optical system used to magnify and allow focus on the displays n Distortion effects u pixels magnified u optics cause image warping and distortions

20. Robinett’s Discussion n Problems u incorrect convergence F optical axes not parallel F optical axes do not pass through centre of screens If so would correctly see far point at infinity. u Accommodation and convergence not linked F not much can be done about this

21. Robinett’s Discussion n FOV incorrect u physical FOV u geometric FOV F they don’t match n Geometric COP doesn’t match optical COP u need off-centre COPs F easily done in the general camera model

22. Robinett’s Discussion n Inter-pupillary distance ignored u could allow mechanical u optical u software correction n Optical distortion u non-linear optical transformations u straight lines become curves

23. Overcoming optical distortion n Screen image is distorted by optics u line becomes a curve F display to buffer, apply inverse distortion, and then map this to screen F Mapping from screen pixel to virtual pixel, including optical distortion (xv,yv) = D(xs,ys) u Find the inverse mapping (xs,ys) = D -1 (xv,yv)