Chapter 5 Human Stereopsis, Fusion, and Stereoscopic Virtual Environments

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

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

3. Stereoscopic Displays • Stereoscopic 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. • Time 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? • when info is presented in perspective • when monocular cues are ambiguous • when static displays are used • for complex scenes and ambiguous objects • for complex 3D manipulation tasks • when inexperienced users are performing 3D manipulation tasks • Stereopsis can help provide info about spatial layout

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

6. Convergence & Retinal Disparity • Convergence - the amount of rotation of the eyes when they are fixed on a target. • Disparity is the difference between two convergence angles of two points in space. • Any 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 • Crossed disparity - The object is in front of the fixation point. • Uncrossed disparity - The object is behind the fixation point. • See handouts

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

9. Horopter • Corresponding retinal locations in each eye result in perception of a single image at that point. • i.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). • Stereopsis 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 • Large disparities can result in diplopia (double images). • Larger amount of horizontal disparity can be fused (10 to 20 arc min) than vertical disparity (2.5 to 3.5 arc min). • Do not induce vertical disparities in stereoscopic displays!

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

12. Spatial Factors • Relative Spacing of stimuli • length of the compared features and distance between them affect stereoacuity • lines 10 to 15 arc min • dots pairs separated by 10 to 15 arc min • gaps of 10 to 30 arc between stimuli (larger distances or closer together results in lower stereoacuity • Be careful when designing spatial enhancers to improve distance or elevation judgements.

13. Spatial Factors • Disparity scaling and disparity gradient limit • The maximum disparity fused is proportional to the distance between objects. • Highly dense visual scenes may result in difficulties fusing images. • Orientation • Stereoacuity worse for oblique or horizontal visual patterns than for vertical patterns

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

15. Stereo Displays for VE • Stereo displays do not provide the same visual cues that are available in the real environment. Some problems include: • a) large amounts of disparity result in double images • b) ghosting of stereo time multiplexed displays • c) only horizontal disparity will result in depth perception. Vertical disparity results in suppression of one eyes image or in the perception of double images. • d) stereo pairs can't be fused by 10% of the population • e) 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 • Wickens, C.D., Todd, S., and Seidler, K. Three-dimensional displays: perception, implementation, applications. CSERIAC SOAR-89-01. (1989). • McAllister, D.F. (Ed.) (1993). Stereo Computer Graphics and Other True 3D Technologies. New Jersey:Princeton University Press. • Yeh, Y. & Silverstein L.D. (1990). Limits of fusion and depth judgement in stereoscopic color displays. Human Factors, 32, 45-60.

17. What are some general recommendations? • Avoid stereoscopic displays with low spatial resolution for tasks with fine detail. • Lower resolutions could be used for