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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