1. Chapter 5 Human Stereopsis, Fusion, and Stereoscopic Virtual Environments

2. What is Stereopsis? zBinocular disparity - lateral difference between the two retinal images. zStereopsis - depth sense based solely on stimulation of disparate locations on retina.

3. Stereoscopic Displays zStereoscopic displays are really 2D. Present two images, one to left eye and one to the right eye, to simulate disparity, and create 3D. Can not change viewing angle. yTime multiplexed systems present the images in an alternating fashion beyond the critical flicker frequency of the eye. First to left eye than to right eye.

4. When are 3D Stereoscopic Displays Useful? ywhen info is presented in perspective ywhen monocular cues are ambiguous ywhen static displays are used yfor complex scenes and ambiguous objects yfor complex 3D manipulation tasks ywhen inexperienced users are performing 3D manipulation tasks yStereopsis can help provide info about spatial layout

5. Stereopsis and Fusion zFusion - the ability to fuse the two images into one image. zStereoacuity - smallest depth that can be detected. y5” of arc under optimal conditions

6. Convergence & Retinal Disparity zConvergence - the amount of rotation of the eyes when they are fixed on a target. zDisparity is the difference between two convergence angles of two points in space. zAny part of the visual filed that does not project disparate retinal images is perceived as occupying a single point in space.

7. Convergence & Retinal Disparity zCrossed disparity - The object is in front of the fixation point. zUncrossed disparity - The object is behind the fixation point. ySee handouts

8. Horopter zLocus of points in space that fall on corresponding points on the retinae. y(see handouts) zThe observer interprets all intersected stimulus points as being at a common distance in 3D space.

9. Horopter zCorresponding retinal locations in each eye result in perception of a single image at that point. yi.e. fusion. However, binocular fusion is not an exclusive property of precise stimulus locations crossed by the horopter. Image points in small volume surrounding the horopter (Panum’s area) is also fused). zStereopsis is the ability of the visual system to extract from disparate retinal images the depth location of objects relative to the locus defined by the horopter.

10. Diplopia zLarge disparities can result in diplopia (double images). zLarger amount of horizontal disparity can be fused (10 to 20 arc min) than vertical disparity (2.5 to 3.5 arc min). zDo not induce vertical disparities in stereoscopic displays!

11. Spatial Factors z Spatial frequency yStereoacuity and fusion threshold depend on spatial frequency. Best for visual patterns that contain sf > 2.5 cycles per degree of visual angle. yLow sf results in poor stereoacuity ya complex stimulus contain broad range of sf can result in simultaneous precept of fusion and diplopia.

12. Spatial Factors zRelative Spacing of stimuli ylength of the compared features and distance between them affect stereoacuity xlines 10 to 15 arc min xdots pairs separated by 10 to 15 arc min xgaps of 10 to 30 arc between stimuli (larger distances or closer together results in lower stereoacuity yBe careful when designing spatial enhancers to improve distance or elevation judgements.

13. Spatial Factors zDisparity scaling and disparity gradient limit yThe maximum disparity fused is proportional to the distance between objects. yHighly dense visual scenes may result in difficulties fusing images. zOrientation yStereoacuity worse for oblique or horizontal visual patterns than for vertical patterns

14. Spatial Factors zVisual field location yStereoacuity is best for stimuli that fall on or near the fixation point. yProvide finer spatial resolution near the center of gaze then in peripheral areas of vision when using stereo.

15. Stereo Displays for VE zStereo displays do not provide the same visual cues that are available in the real environment. Some problems include: ya) large amounts of disparity result in double images yb) ghosting of stereo time multiplexed displays yc) only horizontal disparity will result in depth perception. Vertical disparity results in suppression of one eyes image or in the perception of double images. yd) stereo pairs can't be fused by 10% of the population ye) most effective for near space or objects, objects that are very distance produce the same disparities. But you can induce depth by artificially creating large disparities.

16. Some References zWickens, C.D., Todd, S., and Seidler, K. Three- dimensional displays: perception, implementation, applications. CSERIAC SOAR-89-01. (1989). zMcAllister, D.F. (Ed.) (1993). Stereo Computer Graphics and Other True 3D Technologies. New Jersey:Princeton University Press. zYeh, Y. & Silverstein L.D. (1990). Limits of fusion and depth judgement in stereoscopic color displays. Human Factors, 32, 45-60.

17. zWhat are some general recommendations? yAvoid stereoscopic displays with low spatial resolution for tasks with fine detail. yLower resolutions could be used for