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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

Chapter 8:

Seeing a Three-Dimensional World

the visual system must compute
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
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.
slide4
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
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
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
Fixation point ≠ point of attention
  • Posner (1980), Posner, Snyder, & Davidson (1980)
inverted goggles
Inverted Goggles
  • George Stratton (1897)
  • Linden, et al., (1999).
depth perception
Depth perception
  • Camera vs. the visual system
    • Both are initially 2D.
    • Retinal image is constantly moving.
    • Visual system has two eyes.
slide10
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
Effectives distances of cues
  • Personal space: ~1.5 meters
  • Action space: ~30 meters
  • Vista space: beyond action space in visual space.
oculomotor depth cues
Oculomotor depth cues
  • Angle of convergence of the eye muscles
  • Accommodation of the lens of the eyes
accommodation
Accommodation
  • Accommodation works only at relatively close distances (< a few meters).
  • Not very accurate
convergence
Convergence
  • Works for short distance (< 6 meters).
  • Can be used in isolation from accommodation.
visual cues
Visual cues
  • Binocular
  • Monocular
binocular cues
Binocular cues
  • Retinal disparity = the difference in distance between two objects as seen from the left eye and the right eye.
stereoscope
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
Computing retinal disparity
  • Identify features to match.
  • Compute magnitude and direction of disparity.
computing retinal disparity1
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 disparity2
Computing retinal disparity
  • Identify and compare only low frequency information.
  • (Ignore or filter out high frequency information.)
digression binocular rivalry
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.
slide30
Some binocular cells are selective for zero- retinal disparity.
  • Some binocular cells are selective for some degree of retinal disparity.
  • Cats with monocular stimuli.
stereoblindness
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
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
Some depth from motion demos:
  • http://epsych.msstate.edu/descriptive/Vision/mparallax/DC4a.html
interposition
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
Amodal completion
  • We perceive occluded objects as complete wholes, when it is logically possible that they are mere parts of objects.
sekuler palmer 1992
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
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 transparency1
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.
neon spreading
Neon spreading
  • Colors move from one region to the next.
the role of occlusion is found in
The role of occlusion is found in:
  • Amodal completion
  • Illusory figures
  • Transparency
  • Neon spreading
perspective
Perspective
  • Linear perspective (objects receding)
  • Aerial perspective (blur, haze)
integration of depth information
Integration of depth information
  • Size, interposition, motion, and perspective interact additively (Bruno and Cutting, 1998).
  • Depth cues compete. When two cues give conflicting evidence,