3/23/2005 © dr. zachary wartell 1 depth and size perception
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3/23/2005 © Dr. Zachary Wartell 1
Depth and Size Perception
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Human Depth Perception
• Cue Theory – brain learns to identify info. in retinal image that is correlated with depth– oculomotor cues– pictorial cues– movement-produced cues– binocular disparity cues
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Oculomotor Cues: Accommodation
●susp. lig. kept tight by eye fluid which stretches and thins lens (bend light less)
●cil. mus. (donut shape) contracts allows lens to spring into its natural rounder shape (bend light more)
suspensory ligments
ciliary muscle
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Oculomotor Cues: Vergence
physical box
eyes
fixation point
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Oculomotor Cues: Vergence
physical box
eyes
divergence
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Oculomotor Cues: Vergence
physical box
eyes
convergence
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Pictorial/Monocular Depth Cues
• overlap/occlusion• size in field of view• height in field of view• atmospheric perspective• familiar size• linear perspective• texture gradiant
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Overlap/occlusion
• only relative or “ordinal” (psy.) information (partial order - math)
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Size in field of view
• larger size causes an object to appear closer• Ames balloon experiment
– inflating illuminated balloons in darkened room
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Height in field of view
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Atmospheric/aerial Perspective
• farther objects less distinct, color may be less saturated
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Familiar Size
• knowledge of actual size influences distance judgement
• Epstein (1965)– photo of 3 dime, quarter and nickel at same
size as quarter– viewed by one eye at same distance– darkened room, photo lit by spotlight– judged smaller coin to be closer– binocular viewing foils illusion
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Linear Perspective
• Leon Battisa Alberti (1435) – principles of perspective drawing
• Leonardo da Vinci – drawing on plane of glass
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Texture Gradient
• elements that are equally spaced appear closer and closer together in distance
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ab
Movement-Produced Cue: Motion Parallax
• looking out of car – near objects “move” faster and blur, far objects “move” slower
A B
a'b'
Δa
Δb
ab
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• related to motion parallax and overlap
Movement-Produced Cue: Deletion & Accretion
Deletion
Accretion
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Stereopsis
• Wheatstone (1838) – stereoscope showed that differences alone in left/right eye image yields depth
• “stereopsis” – impression of depth due to two different image on retina
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Corresponding Points
• locations on retina connecting to same part of visual cortex
a'b'
ab
f' f
[Goldstein,44]
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Corresponding Retinal Points
• locations on retina connecting to same part of visual cortex – roughly equivalent to aligning retina atop one another
a'b'
ab
f' f
left right
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Horopter
A
B
bl
br
cl
crC
horopteral
bl
cl
cr
ar
br
• A is fixation point• horopter points have
correspondingretinal images
• horopter dependenton fixation pt.
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Noncorresponding (disparate) retinal points
AB
bl
br
cl
cr
C
horopteral
bl
cl
crar
br
retinal disparity:
θb= θbl-θbr < 0 θc= θcl-θcr > 0
θ<0 θ>0
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Crossed (<0) versus Uncrossed (>0) disparity
ABC
horopter
uncrossedcrossed
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Corresponding (Image) Points
• correspondence problem – how does brain determine what left eye image point should be matched with what right eye image point?
(Al Ar)
(Al Cr)
(Cl Cr)
(Ar Cl)
al
cl
crar
?
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Random-Dot Stereogram - Julesz (1971)
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Al, Bl
F
FlFr
BA
Br Ar
Rear View
Top ViewHoropter
Zone of SingleVision
Panum’s fusion area
Al, Bl Fl Br Ar
Fr
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Local vs Global Stereopsis
• local stereopsis – simple stimuli (1 line), small FOV, image correspondence use local info
• global stereopsis – complex stimuli, large FOV, image correspondence needs global info.
• Important: experimental psychophysical results in local & global case can differ
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Types of stereopsis
• no stereopsis – diplopia / diplopic images, perceived depth at fixation plane or undefined
• patent/quantative stereopsis – magnitude, direction, maybe diplopic, small disparities
• latent/qualitative stereopsis - only direction, always diplopic
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Human Size Perception
• Visual Angle
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Law of Size Constancy
• humans correctly perceive an object’s physical size no matter what its distance from us and no matter what the size of image on retina is
• Holway and Boring (1941)
hallways
subject
targetcircle
test circles
1o
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Holway and Boring (1941)
10 50 100
10
20
30
distance to test circle (ft)
size
of t
arge
t circ
le (
in)
vis. angle
idealall cues
1 eye
+peep hole
+drapes
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Emmert’s Law
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References
• E. Bruce Goldstein. Sensation and Perception, 4th Edition. Brooks/Cole Publishing Company, Pacific Grove.