binocular cues to depth - purduegfrancis/classes/psy310/l21b.pdf · 2006-03-03 · prof. greg...
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Prof. Greg Francis
1PSY 310: Sensory and Perceptual Processes
Purdue University
Binocular cues to depth
PSY 310
Greg Francis
Lecture 21
How to find the hidden word.
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Depth perception
You can see depth in static images with just oneeye (monocular)
Pictorial cues
However, motion and binocular cues generallyplay an important role in depth perception
Motion parallax
Stereopsis
Prof. Greg Francis
2PSY 310: Sensory and Perceptual Processes
Purdue University
Motion and depth
Consider a point far away
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Motion and depth
When it moves in the world, the point also moves on theretina
Prof. Greg Francis
3PSY 310: Sensory and Perceptual Processes
Purdue University
Motion and depth
Consider a point that is closer
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Motion and depth
Have it move the same amount in the world
Notice how much it moves on the retina
Prof. Greg Francis
4PSY 310: Sensory and Perceptual Processes
Purdue University
Motion and depth
Compare the movements for near and far dots
In the worldOn the retina
Far
Near
It’s avariation ofthe retinalsize withdistanceproperty
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Motion and depth
If you move, then the projection of light from nearby objects movesmore quickly on the retina than for far objects
Motionparallax.gif
Prof. Greg Francis
5PSY 310: Sensory and Perceptual Processes
Purdue University
Motion and depth Motion parallax is a property of physics
Motion and projection on to a surface
The retina
A camera
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Motion parallax
Many organisms use motion parallax to identify depth of objects
Rabbits, prairie dogs, insects, humans, robots…
Prof. Greg Francis
6PSY 310: Sensory and Perceptual Processes
Purdue University
Two views
We noted last time that with a single view there is no way toidentify the depth of a point
Most of the monocular cues work with objects, not points
Motion parallax can identify the depth of a point because itincludes several different views
For motion to exist, there must be more than one view
Either you move
Or the object moves
We can get depth of a point from different kinds of views
Two eyes
Binocular vision
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Binocular vision
Suppose you look at a scene with two shapes at differentdepths
You focus (converge your eyes) on object B
Prof. Greg Francis
7PSY 310: Sensory and Perceptual Processes
Purdue University
Binocular vision
Suppose you look at a scene with two shapes at differentdepths
You focus (converge your eyes) on object B
B is in the same place on the retina for the left eye and theright eye
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Binocular vision
Suppose you look at a scene with two shapes at differentdepths
You focus (converge your eyes) on object B
A is in a different place on the retina for the left eye and theright eye
Prof. Greg Francis
8PSY 310: Sensory and Perceptual Processes
Purdue University
Binocular vision
The difference in the position of A across the twoeyes is called disparity
Measured in terms of visual angle
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Two views
Similarly, if you look at thetree, the policeman is indifferent places for thefoveae of the left and righteye
Prof. Greg Francis
9PSY 310: Sensory and Perceptual Processes
Purdue University
Two views Even though the eyes are not far apart, the views can be quite
different
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Two views Even though the eyes are not far apart, the views can be quite
different
Make them alternateto give a goodimpression of depth.Motion parallax.TwoViews.gif
Prof. Greg Francis
10PSY 310: Sensory and Perceptual Processes
Purdue University
Horopter
Differencesbetween theview of the twoeyes can beused to identifydepth
What aboutpoints that fallon the samerelative positionof the tworetinas?
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Horopter
Suppose you stareat point D
The point D falls onthe central part ofthe fovea of eacheye
Prof. Greg Francis
11PSY 310: Sensory and Perceptual Processes
Purdue University
Horopter
Suppose you stareat point D
The point D falls onthe central part ofthe fovea of eacheye
Then point C willalso fall on the same
relative position
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Horopter
Suppose you stareat point D
The point D falls onthe central part ofthe fovea of eacheye
Then point B willalso fall on the same
relative position
The angle betweenD and B is the samefor both eyes
Prof. Greg Francis
12PSY 310: Sensory and Perceptual Processes
Purdue University
Horopter
The horopter is theset of points that fallon the same relativepositions of the twoeyes
No disparity forthese points
Consider a point W
The angles in thetwo eyes aredifferent
W
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Horopter
Easier to see ifwe get rid of thebackground
WD
Prof. Greg Francis
13PSY 310: Sensory and Perceptual Processes
Purdue University
Horopter
Easier to see if weget rid of thebackground
Easier to see if were-position thelines
W
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Horopter
Consider a pointbehind D
The projection of Wis on opposite sidesof the projection of D
Both on the nasal(nose) side of theretina
Uncrossed disparity
W
Prof. Greg Francis
14PSY 310: Sensory and Perceptual Processes
Purdue University
Horopter
Consider a point infront of D
The projection of Wis on opposite sidesof the projection of D
Both on the temporal(temples) side of theretina
Crossed disparity
W
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Lots of depth cues
How do we know the two views of the eye are used tocompute a depth percept?
What about all the monocular cues?
Does disparity do anything?
Need two views that have no monocular cues
Random dot stereogram
Prof. Greg Francis
15PSY 310: Sensory and Perceptual Processes
Purdue University
Random dot stereogram
Random dots contain no monocular cues to depth
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Random dot stereogram
Dots in the middle are in the same relative position
Prof. Greg Francis
16PSY 310: Sensory and Perceptual Processes
Purdue University
Random dot stereogram
Dots in the middle are in the same relative position
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Random dot stereogram
Dots in the surround are shifted
Prof. Greg Francis
17PSY 310: Sensory and Perceptual Processes
Purdue University
Random dot stereogram
Dots in the surround are shifted
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Random dot stereogram
The points in the different eyes introduce disparity
You have to look at the images with a special device(stereoscope)
Or learn how to cross or uncross your eyes to get the images toconverge together
And you see depth!
Prof. Greg Francis
18PSY 310: Sensory and Perceptual Processes
Purdue University
Single Image Stereograms
The disparate points can be hidden in a single image
We had an image here during lecture, but it was Causing problems for people to print out the notes.
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Single Image Stereograms Relax your eyes to look “far away” and get the white boxes to
merge together
We had an image here during lecture, but it was Causing problems for people to print out the notes.
Prof. Greg Francis
19PSY 310: Sensory and Perceptual Processes
Purdue University
Single Image Stereograms It takes practice
Some people are stereo-blind
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Stereopsis
It pretty much all works the way youwould expect from the geometry
And there are neurons in visual cortexthat respond to different amounts ofdisparity
Disparity is always relative to where youare looking
The point of fixation has no disparity How do you know the depth of this point?
Prof. Greg Francis
20PSY 310: Sensory and Perceptual Processes
Purdue University
Conclusions Depth perception
Motion parallax
Binocular cues
Two views
Disparity
Stereopsis
Lots of other issues
Motion and stereopsis together
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Next time
Size perception
How big is something?
Related to depth perception
Size constancy
Moon illusion