Post on 29-Aug-2018
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Depth Perception 1Depth Perception
Part II
Binocular Monocular
Static Cues
Perspective Size Interposition Shading
Motion Parallax
VisualOculomotor
Accomodation Convergence
Depth Information
Binocular Cues to Depth
FoveaFovea
Fixating an Object
Binocular Vision:Vision with Two Eyes
Binocular cues to depth:• Binocular cues are based on the fact that we
have two forward facing eyes that are laterally separated
• This provides slightly displaced images in each eye
• This information can be converted into a signal about relative depth
• Based on the geometry of the images reaching the eye
Important concepts in binocular depth vision:
• Corresponding and non-corresponding points
• Fixation plane
• Horopter
• Retinal disparity
• Diplopia
• Stereopsis
• Stereoacuity
Our brains convert overlapping flat images projected onto the retina of each eye into a 3-D model of the surrounding world.
This creation of a 3-D world from the combining of information from the two eyes is called Stereopsis -from the Greek words stereos - for “solid” and opsis for “vision” - solid vision or solid sight.
Stereopsis - is the ability to perceive depth or relative object distance based on retinal disparity.
Stereopsis: Definitions
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Depth Perception 2
binocular stereopsis
Not the most important cue for depth
Why study it?
• Because its the only aspect of depth of which we have some physiological understanding
• “Eavesdropping” on binocular cells using electrophysiology
When we “look” at an object with two eyes we convergeour eyes so the the image of the object falls on the fovea of each eye - the retinal locus with the highest resolving power.
This convergence of the eyes so that the image of the object of interest falls on the foveas is called bifovealfixation and is generally considered to be the first stage in binocular function.
The foveas can be considered to be corresponding points on the two retinas.
Thus any object you fixate will fall on corresponding points on the two retinas - i.e. the foveas.
The Early Stage of Stereopsis Corresponding and non-corresponding points
• When fixating, image of target falls on fovea of each eye
• The images of an object at the same distance as the fixation plane will fall on the same relative position in the two eyes
• Images that fall on different relative locations are said to fall on non-corresponding points
Corresponding points and the horopter:
• The horopter is an imaginary plane through the fixation point that joins all corresponding points
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Depth Perception 3
The HoropterPoints Falling on the Horopter Fall on Corresponding
Points on the Retinae
Non-corresponding Points and Retinal Disparity
• If a target is closer or more distant than the fixation plane, its image falls on non-corresponding points in the two eyes
• If images fall on non-corresponding points, then there is retinal disparity and the potential for stereopsis
Retinal Disparity and Depth:• There is a systematic relationship between
the amount of retinal disparity on the retina and the distance of a target relative to the fixation plane
a b a bleft eye view right eye view
L Rθ θ
R Ldisparity = θ θ
ab
fixating here
perceived depth increases with increasing disparity (minus, closer than horopter,plus, farther than horopter)
definition of
The process by which we merge these retinally disparate images into a single percept is called fusion.
Not all images that fall on disparate points lead to double vision-- which is also known as diplopia.
There is a narrow region on either side of the horopterthat includes all points in visual space that are fused into single images. This region is called Panum's area - the region where fusion occurs.
Fusion & Panum’s Area
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Depth Perception 4
RL
Locus of corresponding retinal points - Horopter
Panum’sfusional space
Points that don't fall on the horopter fall on disparate(non corresponding) points in the two eyes.
That is, objects located nearer or farther than the fixated target form images in different positions on each retina giving rise to disparity.
The difference in the location of two retinal images of the same object is called binocular disparity.
R L
Crossed disparity
RL
RL
Uncrossed disparity
RL
RL
The smallest disparity that can be resolved =Stereoacuity
= 10 - 20 secondsof arc
Stereoacuity Random Dot Stereogram
Invented byBela Julesz1956 emigre engineerfrom Hungary
First innovative use of a computer for researchin perception
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Depth Perception 5
1 0 1 0 1 01 1 0 1 0 1
01
1 0 1 1 11 1 0 1 10 0 1 1 0 1
1
1 0 1 0 1 01 0 1 0 11
0 1 11 0 1
1 11 10 0 1 1 0 1
0
left eye right eye
LE RE
1 0 1 0 1 0
1 0 1 0 1 0R
L
row 1
1 1 0 1 0 11 0 1 0 11R
L
row 2
random black and white pixelswhich are essentially the same in each eye
some, however, are shifted laterallywith respect to the others
How a Random Dot Stereogram Works:
Wheatstone’s invention of the stereoscope (c. 1836)
mirrors
Each eye receives a separate image
Left eye image Right eye image
mirrors top view
mirrors
front view
notice that each eye receives a separateimage of just two lines having a different separation.
How does it work?
Brewster stereoscope
aluminized screen
Polaroid glasses method
P-
P+
P-
P+
Some other methods to show stereo pictures…
divergent convergentfree fusion, w/o optical aids
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Depth Perception 6
A
C BPlane of Fixation
A'
C'B'
Cell Responses Stimulation Location "Near" cell Cell tuned to fixation plane "Far" cell
Stimulation Location
(A,A') (B,B') (C,C')
Disparity-Tuned Cell Responses
Subject fixating B
• Individual neurons were “tuned” for different amounts and directions of disparity
• Several different classes of neuron, some finely tuned for small amounts of disparity, others simply responding to “near” or “far”
Autostereograms:
• In autostereograms we use our vergence eye movements as the stereoscope
• By converging or diverging we shift the image in one eye relative to the other
• With the correct amount of vergence we are now superimposing two parts of a repeating image which has been designed to contain disparity when viewed this way
A Simplified Example of HowAn Autostereogram Works…
Simplified “Magic Eye” Autostereogram
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Depth Perception 7
Binocular parallax
a b
Notice the difference in angular separation
Size constancy
Why does someone walking away not appear to shrink?
The perception of size is closely related to the perception of distance.
The brain is remarkably good at compensating for changes in retinal image size with distance in order to keep the perceived size constant
Size constancy
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Depth Perception 8
Size constancy:• Given the size of the image on the
retina (visual angle) and its distance, it is possible to compute the physical size of an object
• Size constancy is the mechanism that makes this computation
• Holway & Boring demonstrated the crucial importance of depth perception in an experiment
The Holway-Boring experiment:
• Observer views Test Disks located at different distances
• Task is to adjust size of Comparison Disk to match physical size of Test Disk
• Test disks all set to subtend 1o of visual angle
• Test under several condition in which the availability of depth cues is varied
• Observers matched closer to visual angle as cues removed
Relationship between size perception and perceived distance:
• Generate afterimage on retina
• View afterimage against surfaces at different distances
• Note changed size of afterimage
Emmert’s Law
The perceived size of an afterimage is related to thedistance of the viewing surface from the eye
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Depth Perception 9
Emmer t's law :
Sp = k x Sr x Dp
(Sp = pe rceived size; Sr = retinal size;Dp = pe rceived d ista nce; k = consta nt)
Ponzo Illusion
• This picture looks odd because the size and distance cues are in conflict
Size perception and visual illusions:
• A number of visual illusions may result from the misapplication of constancy scaling
• Gregory has argued that the misjudgement of size is because the illusory figure contains information that activates the constancy scaling mechanism
• Consequently, an object is seen as larger or smaller than it should be
Muller-Lyer Illusion:• If the arrowheads are seen as internal and
external contours, the closer, external corner should appear bigger
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Depth Perception 10
The moon illusion:• Moon (or sun) seems larger
at the horizon than at the zenith
• Recognised in classical times, many theories
• Current most-accepted explanation in terms of apparent distance, although issue is still controversial
• Assumption: if two objects have the same retinal image size, the one that appears closer will look smaller
• That means horizon moon must look more distant• Some evidence that horizon looks further away
than zenith sky
• If zenith sky appears closer, then moon will seem to be smaller