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Cosc 6326/Psych6750X

Cosc 6326/Psych6750X. Vision and Visual Displays. Why do we have two eyes?. Binocular vision has several advantages including increased field of view redundancy increased detection probability depth perception from vergence stereopsis. Geometry of Binocular Correspondence.

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Cosc 6326/Psych6750X

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  1. Cosc 6326/Psych6750X Vision and Visual Displays

  2. Why do we have two eyes? • Binocular vision has several advantages including • increased field of view • redundancy • increased detection probability • depth perception from • vergence • stereopsis

  3. Geometry of Binocular Correspondence • Images in the two eyes differ (or are disparate) due to separation of vantage points • Points of light project to the same retinal locations in the two eyes only on the horopter • Points off the horopter have horizontal and/or vertical disparity

  4. The Geometrical Point Horopter Fixation Point Uncrossed disparity Crossed disparity LE RE

  5. Even flat frontal surfaces have disparity • Differential perspective • Both horizontal and vertical disparities • Gradient of vertical size disparity • Varies with distance • Affects depth, shape judgments … Left Eye Image Right Eye Image

  6. Vergence

  7. Vergence eye movements • opposite (disjunctive) movements of the eyes • a principle function is to align both high acuity foveae on target of interest • vergence helps bring images of objects of interest into correspondence (so that the images fall on similar parts of both retinae)

  8. since eyes are laterally separated in the head the optical axes of the two eyes are parallel only for fixation at infinity • nearer targets require crossing one’s eyes (convergence)

  9. Several catergories • tonic vergence • voluntary vergence or proximal vergence • induced voluntarily or by an object that appears to be at a different distance • accommodative vergence • when accommodating there is a link with vergence that drives a compatible vergence change; also vergence accommodation

  10. disparity vergence • vergence in response to disparity in the images in the two eyes • allows for aligning the images of the target in the two eyes

  11. Two main questions for us • Does vergence give a sense of depth? • could theoretically triangulate depth of target from angle of convergence • static vergence is probably a fairly weak indicator for distance judgements but • indirectly important for size and shape perception, calibration of stereopsis • e.g. micropsia in the wallpaper effect • size and shape constancy

  12. Dynamic vergence may play a role in signalling change in depth • also controversial • Regan (1986) found no changing depth from changing vergence in large stimuli but weak effect in small stimuli; substantial changes in apparent size

  13. 2. Dissociation of accommodation and vergence • Normally vergence, accommodation and pupil size are controlled in a tightly coupled manner (the ‘near triad’) • In most binocular displays, the display is focused at a fixed distance and thus accommodation should be fixed

  14. This fixed accommodation leads to vergence/accommodation mismatch • depth cue conflicts between vergence and accommodation; stereopsis and blur • difficulty in obtaining clear vision for stimuli requiring vergence nearer or further than the optical distance of the display OR • difficulty ‘fusing’ stimuli • contributes to eyestrain, simulator sickness

  15. adaptation, re-adaptation, visual stress (seminar next week Wednesday)

  16. Stereoscopic vision

  17. Binocular Disparity • Horizontal disparity arises from lateral separation of the eyes (parallax) • Relative depth is coded by disparity scaled inversely by the square of viewing distance  (ad) D2 • Stereopsis (solid sight) refers to depth perception from disparity

  18. Fixation Point, F P PR F PL Screen d Fixation Point, F P PR PL F Screen D Left Eye Right Eye a Left Eye Right Eye Depth from Disparity • Horizontal disparity arises from lateral separation of the eyes

  19. Stereoscopic Voxels from Davis and Hodges 1995

  20. http://www.pandigitalmedia.com/bgussin

  21. Sensitivity to disparity • People are extremely sensitive to relative disparity. Stereopsis is a hyperacuity • discrimination thresholds of a few seconds of arc • Upper limit: patent versus qualitative stereopsis • much poorer sensitivity to absolute disparity • thus very sensitive to depth differences between stimuli; poor ability to estimate absolute distance

  22. Sensitivity to disparity • Disparity has an inverse square dependence on distance • Thus, stereopsis is most effective for estimating depth and surface shape at near distances

  23. Sensitivity to disparity • many people are stereo-anomalous • cannot see depth from disparity (about 5% of population) or • cannot see motion-in-depth from changing disparity or • have deficits in particular part of the visual field, or for crossed disparity, or for uncrossed disparity • cannot rely on all subjects seeing stereoscopic depth

  24. Two of the main issues in stereopsis are • matching of corresponding parts of the two images • estimation of depth from disparities

  25. Correspondence problem • If features are similar a matching ambiguity arises • Human stereopsis usually matches images very effectively • Often the most difficult problem in stereoscopic computer vision Keplerian projection

  26. Random-dot stereograms

  27. not all features in one eye’s image have a corresponding feature in the other image • these monocular zones play an important role in perception of objects and surface properties in binocular vision • depth from monocular occlusion • luster • binocular rivalry

  28. Fusion • Typically a wide range of disparities present in a scene • If disparities are modest people ‘fuse’ the images of the object in the left and right and perceive a unitary object • Outside this range objects are seen doubly (diplopic) or one eye’s image is suppressed (binocular rivalry)

  29. Fusion • Only a range of disparities near the horopter are seen singly (Panum’s fusional area) • Also a limited range of disparities for which depth can be obtained from disparity. But depth can be seen in diplopic images • Fusion depends on size, contrast, disparity gradient, temporal factors …

  30. Estimation of Depth from Disparity • Relation between horizontal disparity and depth varies with distance and location of surface wrt head (recall the curvature of the horopter) • Need to account for viewing system parameters of distance and eccentricity to judge • metric depth, relief, surface curvature, surface slant … • absolute retinal disparity does not provide viewing system parameters need: ‘sensed’ eye position, vertical disparities, other depth cues, …

  31. Stereoscopic issues in displays

  32. Types of display based on ‘ocularity’ • Monocular : Image presented to one eye only • no stereopsis, rivalry with dark images • Biocular : same image presented to both eyes • Stereopsis specifies a flat surface • stereoscopic artefacts can arise with misalignment • Binocular : eye specific (dichoptic) images presented to each eye • Stereoscopic imagery possible • Autostereoscopic - binocular display that requires single display and no special eyewear

  33. Methods of presenting stereoscopic displays • hmds • shutters • polaroid- linear, circular • anaglyphs • barrier, lenticular • holograms • ….

  34. From www.stereographics.com

  35. From www.stereographics.com

  36. Example - 3ality autostereoscopic display • Many new autostereoscopic displays based on lenticular sheets or parallax barrier methods • trade spatial resolution for stereopsis • Shutter and some other systems trade temporal resolution for stereopsis (flicker) • 3ality display codes stereoscopic information in luminance, preserving spatial and temporal resolution (at expense of dynamic range).

  37. Viewer wears glasses with a pair of orthogonal polarizing filters • At each pixel, left eye should see a certain amount of light of one polarization, and the right eye a certain amount of the other • Left and right eyes images mixed by summing luminance of the two images (vector sum) - ‘magnitude’ of the vector

  38. Images from Kleinberger et al, Stereoscopic Displays and Applications X, SPIE vol #5006

  39. An ‘extra’ liquid crystal panel twists the light for each pixel to the appropriate intermediate polarization - ‘angle’ of the vector • Eyewear decomposes the combined light into appropriate amount of left and right eye polarization

  40. Autostereoscopic version

  41. When are stereo displays useful? • 3D visualisation • ambiguous/poor monocular information • static displays • direct 3D manipulation • cluttered, complicated scenes

  42. some issues • field of view • depth resolution • ghosting and crosstalk • refresh rate for time multiplexed techniques • distortion, alignment and calibration • focus • head tracking, view dependence • ‘orthostereoscopy’ vs visual comfort

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