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Depth perception, a crucial aspect of human vision. Depth cues, such as retinal images, height, linear perspective, blur, aerial perspective, accommodation, motion parallax, shadows, interposition, convergence, and binocular disparity, help the visual system estimate depth and recover the missing dimension from two-dimensional retinal images. The document also discusses stereoscopes and disparity coding in the cortex.
Typology: Lecture notes
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Parallel lines sloping away from the observer converge as they recede into the distance.
Similarly sized shapes decrease in retinal size as they recede into the distance.
Eyes (and cameras) have limited depth of field
objects nearer or farther than the point of fixation or focus are blurred.
Blur variation across the image gives a cue to distance.
As fixation shifts from far to near, the shape of the lens must be changed to maintain a sharply focused image on the retina (accommodation).
Ciliary muscles are used to control lens shape. - Information on the state of the muscles offers a cue to fixation distance.
As the observer moves, points at different distances move at different velocities over the retina.
Points beyond fixation (red) move in the direction of observer motion; points nearer than fixation (green) move in the direction opposite to observer motion. F T im e 1 T im e 2 F ie ld o f v ie w
Near objects often partially obscure far objects.
The retinal image contains ‘T
junctions’ at points where contours of the nearer object intersect contours of the far object.
Convergence angle is the angle formed by the two eyes when fixat ing an object at a specific distance.
As fixation shifts from a far object to a near object, convergen ce angle increases. - Extra
ocular muscles control eye position, including convergence angle
Information on the state of the muscles offers a cue to fixation distance. F a r F ix a tio n Ne a r F ix a tio n F F
Images of points at the same distance fall on corresponding retinal positions in the two eyes.
The horopter is a line drawn through all such points. - Images of points nearer or farther than fixation fall on non
corresponding or disparate retinal locations.
The sign or direction of the disparity signifies near vs. far depth. - The magnitude of the disparity signifies magnitude of depth. Ho r o p te r L e ft Re tin a Rig h t Re tin a F o v e a F F a r th e r a w a y : Un c r o s s e d o r fa r d is p a r ity F ix a te d : C o r r e s p o n d in g p o in ts o r z e r o d is p a r ity Ne a r e r : C r o s s e d o r n e a r d is p a r ity L e ft Re tin a Rig h t Re tin a F o v e a F
A variety of devices simulate the slightly different views creat ed by a real stereo image. Na tu r a l S te r e o Im a g e P r is m S te r e o s c o p e Re d -G r e e n An a g ly p h L e ft E y e V ie w Rig h t E y e V ie w
Primate cortex contains binocular cells that respond selectively to stimuli falling on disparate retinal locations in the two eyes.
Position of right
eye receptive field (RED) relative to left
eye receptive field (GREEN) determines preferred disparity.
Data shown are based on Poggio & Talbot (1981)
Physiol
**Stimulus C ell R esponse L R
deg (n e a r ) 0 deg -0. deg (fa r ) D isparity C ell 1 C ell 2**
Psychophysical evidence for disparity coding cells
Random dot stereograms (RDS) contain only disparity cues.
A sub
set of dots in one eye’s view is displaced relative to the other eye’s view.
Subjects perceive the displaced dots as standing out in depth against the other dots. - The success of RDS is evidence for a pure disparity coding mechanism.
Natural images contain multiple depth cues. How are they combine d?
The magnitude of different cues is very highly correlated: cue magnitude is proportional to (1/distance). - To derive a single estimate of depth from multiple cues, the vis ual system seems to take the average of different cue values. - Cues are weighted according to their reliability in a given set of stimulus conditions.