Seeing a 3-d World : Depth Perception

8/14/2019 Seeing a 3-d World : Depth Perception

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The retinal images from which depthinformation is extracted are two-dimensional:they are inherently depthless

Depth perception occurs effortlessly andwithout thought, despite being derived from atwo-dimensional image on the retina


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Major sources of depth information

DEPTHINFORMATION

OCULOMOTOR

Perspective

Static Cues

Interposition

Motion Parallax

BINOCULAR MONOCULAR

VISUAL

Size

Convergence

Accommodation


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Oculomotor cues


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Oculomotor Cues to Depth

To be seen clearly, closeobjects require moreaccommodation and

convergence than doobjects farther away fromyou

By monitoring the degree

of muscular contraction, wecould compute either of 2values: your angle ofconvergence or the amount

of accommodation from thelenses of our e es


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Can people judge distance under conditionswhere accommodation and convergenceprovide the only cues to depth?

(1)Limited effective range for accommodation

objects in front of you(2)Limited effective range for convergence 20

feet and beyond


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Binocular cues


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Binocular visual fields

To make use of binocular depth cues, anorganism must have a binocular visual field -- aregion of overlapping visibility for the two eyes.

Different animals have different extents of

binocular visual fields. In general, predatorshave both eyes on the front of their heads, andconsequently have large binocular visual fields.In contrast, prey typically have one eye on eitherside of their heads, and consequently havesmall, if any, binocular visual fields.


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Binocular Visual Depth Information:Stereopsis

Stereopsis : perception of relative depth frombinocular vision

Allows us to judge relative depth with greataccuracy and it enables is to see objects that

are invisible to either eye alone


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Retinal disparity as the stimulus of stereopsisThe magnitude of the disparity depends on

the distance between objects

if one object is closer to the observer than

the other, large disparity if one object is only slightly closer to the

observer than the other, small disparity


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Retinal disparity

Although humans have large binocular visual fields,each eye is getting a slightly different view of the

world because the two eyes are in slightly different

positions.

Retinal disparity is the difference between the lateralposition of object in the left and right eyes


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Retinal disparity ariseswhenever objects arelocated in front of or behindthe object you are looking at

If an object lies farther fromyou than the object you arefixating on, the disparitybetween the two is said to

be uncrossed If an object is located closer

to you than the one you arelooking at, the disparity issaid to be crossed


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Horopter: imaginary plane where each object on the

plane casts images on corresponding parts of both eyes.

Includes the point of fixation. Objects same distance from observer as fixation yield zero

disparity

Objects on the horopter cast images on corresponding points ofboth eyes


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In this picture, the lifeguard is fixating on Ralph. Thecurved, dashed line represents the horopter: all points

on this line are the same distance from the lifeguardas the fixation point. So, Susan and Harry (and Ralph)fall on the horopter; Carol and Charlie do not.


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In the 1830s, Wheatstone developed a device with whichto view stereograms: the stereoscope. This devicepresented one two-dimsensional view of a scene to oneeye, and a slightly different two-dimensional view to the

other eye. When the brain received these slightly different views

from the two eyes, it integrated them into one three-dimensional scene. In other words, the disparity betweenthe two scenes enabled the observer to see depth.


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Random dot stereograms

In the 1960s (~130 years after the invention ofthe stereoscope), Julesz developed random dot

stereograms.

In a random dot stereogram, each eye sees only

a collection of spatially random dots. But whenthe information from the two eyes is combined,

the observer can see depth, and the depth can

define a form. In this case, then, depth

perception can precede form perception.


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In random dot stereograms, neither eye sees anyclear form on its own. The percept of form follows thepercept of depth. If no clear form is perceived in eithermonocular view, how does the visual system knowwhich features in one eye's view match up with which

feature's in the other eye's view?

This question is known as the correspondenceproblem for stereovision.

spatial frequency information plays a role in solving thecorrespondence problem we need overlapping spatial frequency information in the two

eyes to see stereo


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Do we all have stereopsis? NO!! There are great individual

difference in stereovision abilities. (5-10% of humans) is stereoblind Stereopsis requires properly working

binocular cells

Properly working binocular cells requirenormal visual experience Kittens raised with only one eye open at

a time do not develop normal binocularcells, even though they appear to benormal in every other respect

Humans who had abnormal early visualexperiences (e.g., strabismus), often do

not have stereovision. There is a criticalperiod for the development of binocularcells (and consequently stereovision). Ifthe problem causing abnormal earlyvisual experience is not caught earlyenough, it can lead to permanentdamage.


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Binocular vision can producepartial occlusion,a special form of retinal disparity

when one object partially occludes anothermore distant object, one eye will see portionsof the more distant object that are invisible to

the other eye


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Monocular cuesMonocular cues


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Occlusion: Closer objects partly block theview of more distant objects. Occlusion leads to a percept of depth (you see the

occluding object as closer than the occluded

object). Occlusion also enables us to complete and

recognize objects.

Occlusion is closely related to other perceptual

phenomena such as transparency and illusory

contours.


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By 7 months of age, human infants can judgerelative distance solely on the basis ofocclusion

Occlusion is such a strong depth cue that it

can override retinal disparity when the twocues conflict

The effectiveness of occlusion is amplifiedwhen the surface of the occluding object

contains high spatial frequency information


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Objects occluded by anearer figure are seenas complete, eventhough portions of theirsurfaces are obscuredby the nearer object

(amodal completion)


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Size: As the distance between you and an

object varies, the sixe of the image of thatobject on your retina changes

Smaller objects often appear to be moredistant than identical larger objects

Familiar size is an effective cue to distance inthe absence of other information. BUT thecue of retinal size critically depends onknowing the correct size of the object


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Perception of size and depth perception

An object can look the same size at differentdistances But: retinal image size changes with

distance Increase distance : decrease retinal image size

Decrease distance : increase retinal image size The fact that an object can look the same size

regardless of changing retinal image size is

referred to as size constancy


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These three disks look to be the same size,

but at different depths. In fact, they varygreatly in terms of their retinal image sizes.


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The fact that we have size constancy meansthat perceived size depends on more than justretinal image size.

We seem to take perceived distance into

account as well. This link between perceived size and

perceived distance is known as size-distancescaling.

We can formalize the notion of size-distancescaling in terms ofEmmert's law:


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Linear perspective: Lines that are parallel in the

real world appear to converge in a drawing. The greater the distance, the greater the

convergence. At infinity, lines meet at the vanishing point.


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Texture gradients: Texture is more dense in adistant object than in an identical closer object. If there is no variation in texture density, no depth will

be perceived.

An abrupt change in texture implies a depthdiscontinuity (a sharp bend).


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Shading: Three-dimensional objects cast

shadows, so 3D objects tend to haveluminance gradients. In perceiving depth from shading, we make certain

default assumptions about lighting

There is only one light source. Light comes from above. "Above" is defined retinally, not environmentally (i.e., the

light seems to come from the same direction as the top ofyour head, even when you turn your head on its side).


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Because we assume that there is only one light source, and thatthat light source is from above, if an object is brighter on the topthan on the bottom, we'll perceive that object as convex (like the"eggs" that form an X in this picture).

In contrast, if the object is brighter on the bottom than on the top,we'll perceive that object as concave (like the "holes" in this

picture).


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MOVING

Motion Parallax Objects at different distances from fixation move at

different rates and directions on your retina. Motion parallax can be thought of a disparity across

time in contrast to the disparity across eyes seen instereovision.

Net result is the same: by integrating information aboutslightly different views across time, you see depth.

Note that disparity from motion parallax is equivalent to

disparity from stereopsis when the head/eye is movedthe distance between the two eyes.


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Note how the retinal positions of the objects change as the eye moves

from left to right. These differences in position give disparity across time:

motion parallax.


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End.

Seeing a 3-d World : Depth Perception

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