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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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:
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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.
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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
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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.
Broad distinction among
types of cues
Oculomotor
Visual
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).
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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.
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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
zero- retinal disparity.
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.
Monocular depth information
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/Visio
n/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.
Special case: Occlusion and
Transparency
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
Colors move from one region to the next.
The role of occlusion is found in:
Amodal completion
Illusory figures
Transparency
Neon spreading
Size cues for depth
Perspective
Linear perspective (objects receding)
Aerial perspective (blur, haze)
Shading
Integration of depth information
Size, interposition, motion, and
perspective interact additively (Bruno and
Cutting, 1998).
Depth cues compete. When two cues give
conflicting evidence,