Chapter 8:

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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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### 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:
• Ten miles north of the Eiffel Tower.
• Half way between Detroit and Chicago.
• Not nearly as useful to perceiver’s as egocentric direction.
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.
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.
Remarkable Vernier acuity
• Can discriminate less than the width of human hair.
• 1/6 the size of a single cone photoreceptor
Fixation point ≠ point of attention
• Posner (1980), Posner, Snyder, & Davidson (1980)
Inverted Goggles
• George Stratton (1897)
• Linden, et al., (1999).
Depth perception
• Camera vs. the visual system
• Both are initially 2D.
• Retinal image is constantly moving.
• Visual system has two eyes.
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.

Effectives distances of cues
• Personal space: ~1.5 meters
• Action space: ~30 meters
• Vista space: beyond action space in visual space.
Oculomotor depth cues
• Angle of convergence of the eye muscles
• Accommodation of the lens of the eyes
Accommodation
• Accommodation works only at relatively close distances (< a few meters).
• Not very accurate
Convergence
• Works for short distance (< 6 meters).
• Can be used in isolation from accommodation.
Visual cues
• Binocular
• Monocular
Binocular cues
• Retinal disparity = the difference in distance between two objects as seen from the left eye and the right eye.
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.
Computing retinal disparity
• Identify features to match.
• Compute magnitude and direction of disparity.
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.
Computing retinal disparity
• Identify and compare only low frequency information.
• (Ignore or filter out high frequency information.)
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.
• Some binocular cells are selective for some degree of retinal disparity.
• Cats with monocular stimuli.
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.
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.
Some depth from motion demos:
• http://epsych.msstate.edu/descriptive/Vision/mparallax/DC4a.html
Interposition
• Occlusion of one object by another is perhaps the most elementary depth cue.
• The potency of occlusion is revealed in Kanisza figures.
Amodal completion
• We perceive occluded objects as complete wholes, when it is logically possible that they are mere parts of objects.
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.
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.
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.