Post on 06-Jun-2020
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PERCEIVING DEPTH AND SIZE
Visual Perception
DEPTH
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Cue Approach
Identifies information on the retinaCorrelates it with the depth of the sceneDifferent cuesPrevious knowledge
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Depth Cues
OculomotorMonocularBinocular
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Oculomotor Cues
ConvergenceInward movement for nearby objectsOutward movements for farther objects
AccommodationTightening of muscles the change the shape of the lens
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Monocular Cues (Pictorial)
OcclusionRelative heightCast shadowsRelative sizeFamiliar size
Atmospheric perspective Linear perspectiveTexture gradientMovement-based cues
Motion ParallaxDeletion and accretion
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Occlusion
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Relative Height
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Conflicting Cues
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Cast Shadows
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Relative Size
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Familiar Size
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Atmospheric Perspective
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Linear PerspectiveTexture Gradient
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Presence of Ground
It helps in perceptionRemoval of ground causes difficulty in perception
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Motion Parallax
Further points have lesser parallaxCloser points move more
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Deletion and Accretion
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Binocular Cues
Binocular disparity and stereopsisCorresponding points
Correspondence problemsDisparity information in the brain
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Binocular Disparity and Stereopsis
Binocular disparityDifference in the images of the two eyes
StereopsisDisparity is transformed into perception of depth
Discovery of stereoscope in 1800s
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Corresponding Retinal Points
Corresponding Points
Points on horopter are images at corresponding points of the two eyes
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Corresponding Retinal Points
Disparity is defined by the angle between the
image of a point and the corresponding point of its image on the other eye subtended from focus
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Crossed and Uncrossed Disparity
For points in front of the horopter
The image is on the outer side of the retina from the image of a point on horopter
For points behind the horopter
The image is on the inner side of the retina from the image of a point on horopter
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Disparity in Brain
Striate cortex has binocular depth cells or disparity detectorsDisparity selective cellsAlso selective about crossed or uncrossed disparityHowever, important is to solve the correspondence problem
Matching images of the same point on two different eyesOnly after this we can find the disparity
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How do they work together?
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Depth perception in other species
Frontal Eyes needed for binocular disparityRabbits etc cannot use disparity for depth Pigeon has 35 degree overlap
SIZE
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Depends on depth estimation
• All depth cues present• Closing one eye
• No binocular cues• Through a peephole
• No monocular cues•Adding drapes
• No shading cues
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Size Constancy
Size-distance ScalingS = K(RxD)K = 1/F
Focus of the eye
R = size of the retinal
image
D = depth of the object
S = perceived size of the object
F = focal length of eye
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Relative size
Visual Illusions
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Muller-Lyer Illusion
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Explanation 1: Misapplied size constancy scaling
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Explanation 2 : Conflicting Cues Theory
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Ponzo Illusion
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Ames Room