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Depth Perception 1 Depth Perception Binocular … · Depth Perception 1 Depth Perception Part II...

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  • 1

    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

  • 2

    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

  • 3

    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 R

    R Ldisparity =

    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 & Panums Area

  • 4

    Depth Perception 4

    RL

    Locus of corresponding retinal points - Horopter

    Panumsfusional 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

  • 5

    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:

    Wheatstones 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

  • 6

    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

  • 7

    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

  • 8

    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

    Emmerts Law

    The perceived size of an afterimage is related to thedistance of the viewing surface from the eye

  • 9

    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

  • 10

    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

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Depth Perception 1 Depth Perception Part II Binocular Monocular Static Cues Perspective Size Interposition Shading Motion Parallax Visual Oculomotor Accomodation Convergence Depth Information Binocular Cues to Depth Fovea Fovea 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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