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Chapter 8:

Chapter 8:. Seeing a Three-Dimensional World. The visual system must compute: . Depth (distance of an object from the perceiver) Egocentric direction (direction of an object relative to the perceiver). Allocentric frame of reference. Independent of the vantage point of a viewer. Examples:

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Chapter 8:

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  1. Chapter 8: Seeing a Three-Dimensional World

  2. The visual system must compute: • Depth (distance of an object from the perceiver) • Egocentric direction (direction of an object relative to the perceiver)

  3. Allocentric frame of reference • Independent of the vantage point of a viewer. • Examples: • Ten miles north of the Eiffel Tower. • Half way between Detroit and Chicago. • Not nearly as useful to perceiver’s as egocentric direction.

  4. Egocentric views are specified relative to fixation points in one’s field of view: • Cartesian co-ordinates. • Polar co-ordinates. • People are good at describing the locations of objects independent of their field of view.

  5. Egocentric viewing • People are good at locating points in free viewing. • Marksmen • Pointing at points of light in the dark • People are poor at locating points in the periphery of their visual field.

  6. Remarkable Vernier acuity • Can discriminate less than the width of human hair. • 1/6 the size of a single cone photoreceptor

  7. Fixation point ≠ point of attention • Posner (1980), Posner, Snyder, & Davidson (1980)

  8. Inverted Goggles • George Stratton (1897) • Linden, et al., (1999).

  9. Depth perception • Camera vs. the visual system • Both are initially 2D. • Retinal image is constantly moving. • Visual system has two eyes.

  10. Depth is not directly perceived. Depth is judged via a series of cues that work over different ranges. Depth is judged absolute distance and relative distance.

  11. Effectives distances of cues • Personal space: ~1.5 meters • Action space: ~30 meters • Vista space: beyond action space in visual space.

  12. Broad distinction among types of cues • Oculomotor • Visual

  13. Oculomotor depth cues • Angle of convergence of the eye muscles • Accommodation of the lens of the eyes

  14. Accommodation • Accommodation works only at relatively close distances (< a few meters). • Not very accurate

  15. Convergence • Works for short distance (< 6 meters). • Can be used in isolation from accommodation.

  16. Visual cues • Binocular • Monocular

  17. Binocular cues • Retinal disparity = the difference in distance between two objects as seen from the left eye and the right eye.

  18. Stereoscope • Charles Wheatstone (1838/1964). • Two drawings on an object. • One from a perspective ~65 mm from the perspective of the other. • Show one image to one eye and the other image to the other eye.

  19. Computing retinal disparity • Identify features to match. • Compute magnitude and direction of disparity.

  20. Computing retinal disparity • Identify features to match. • E.g. a face in one eye and a face in the other, a bottle in one eye and a bottle in the other. • Random dot stereograms. • Compute magnitude and direction of disparity.

  21. Computing retinal disparity • Identify and compare only low frequency information. • (Ignore or filter out high frequency information.)

  22. Digression: Binocular rivalry • When two patterns can be fused, they are. • When two patterns cannot be fused, they create a mosaic or sometimes one merely attends to one rather than the other.

  23. Some binocular cells are selective for zero- retinal disparity. • Some binocular cells are selective for some degree of retinal disparity. • Cats with monocular stimuli.

  24. Stereoblindness • Some people (5-10%) are unable to detect depth from disparity. These individuals may be those who cannot see “magic eye” images. • Most common cause may be strabismus, a misalignment of the two eyes.

  25. Monocular depth information

  26. Motion parallax • As you move through the world, objects at different distances move at different rates. This provides a powerful depth cue. • This occurs either when the viewer or the objects viewed move.

  27. Some depth from motion demos: • http://epsych.msstate.edu/descriptive/Vision/mparallax/DC4a.html

  28. Interposition • Occlusion of one object by another is perhaps the most elementary depth cue. • The potency of occlusion is revealed in Kanisza figures.

  29. Amodal completion • We perceive occluded objects as complete wholes, when it is logically possible that they are mere parts of objects.

  30. Sekuler & Palmer, (1992) • Perceptual representations of partially occluded objects start out as a mosaic-like snapshot of the individual pieces, then evolves over time into perceptually complete objects.

  31. Occlusion and transparency • Lightness values within the “covered” regions must be intermediate between the lightness values of the “uncovered” regions. • The occluding transparent object must be plausibly a single object.

  32. Special case: Occlusion and Transparency

  33. Occlusion and transparency • Lightness values within the “covered” regions must be intermediate between the lightness values of the “uncovered” regions. • The region must be plausibly a single object. • A region will be perceived as transparent only if binocular disparity specifies that the region is in front of the object.

  34. Neon spreading • Colors move from one region to the next.

  35. The role of occlusion is found in: • Amodal completion • Illusory figures • Transparency • Neon spreading

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