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

Purdue University

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