chapter 8: seeing a three-dimensional world. the visual system must compute: depth (distance of an...

Chapter 8:Chapter 8:

Seeing a Three-Dimensional Seeing a Three-Dimensional WorldWorld

The visual system must compute:The visual system must compute:

Depth (distance of an object from the Depth (distance of an object from the perceiver)perceiver)

Egocentric direction (direction of an object Egocentric direction (direction of an object relative to the perceiver)relative to the perceiver)

Allocentric frame of referenceAllocentric frame of reference

Independent of the vantage point of a Independent of the vantage point of a viewer.viewer.

Examples:Examples: Ten miles north of the Eiffel Tower.Ten miles north of the Eiffel Tower. Half way between Detroit and Chicago.Half way between Detroit and Chicago.

Not nearly as useful to perceiver’s as Not nearly as useful to perceiver’s as egocentric direction.egocentric direction.

Egocentric views are specified relative to Egocentric views are specified relative to fixation points in one’s field of view:fixation points in one’s field of view: Cartesian co-ordinates.Cartesian co-ordinates. Polar co-ordinates.Polar co-ordinates.

People are good at describing the People are good at describing the locations of objects independent of their locations of objects independent of their field of view.field of view.

Egocentric viewingEgocentric viewing

People are good at locating points in free People are good at locating points in free viewing.viewing. MarksmenMarksmen Pointing at points of light in the darkPointing at points of light in the dark

People are poor at locating points in the People are poor at locating points in the periphery of their visual field.periphery of their visual field.

Remarkable Vernier acuityRemarkable Vernier acuity

Can discriminate less than the width of Can discriminate less than the width of human hair.human hair.

1/6 the size of a single cone photoreceptor1/6 the size of a single cone photoreceptor

Fixation point ≠ point of Fixation point ≠ point of attentionattention

Posner (1980), Posner, Snyder, & Posner (1980), Posner, Snyder, & Davidson (1980)Davidson (1980)

Inverted GogglesInverted Goggles

George Stratton (1897)George Stratton (1897)

Linden, et al., (1999).Linden, et al., (1999).

Depth perceptionDepth perception

Camera vs. the visual systemCamera vs. the visual system Both are initially 2D.Both are initially 2D. Retinal image is constantly moving.Retinal image is constantly moving. Visual system has two eyes.Visual system has two eyes.

Depth is not directly perceived.Depth is not directly perceived.

Depth is judged via a series of Depth is judged via a series of cuescues that that work over different ranges.work over different ranges.

Depth is judged absolute distance and Depth is judged absolute distance and relative distance.relative distance.

Effectives distances of cuesEffectives distances of cues

Personal space: ~1.5 metersPersonal space: ~1.5 meters

Action space: ~30 metersAction space: ~30 meters

Vista space: beyond action space in visual Vista space: beyond action space in visual space.space.

Broad distinction among Broad distinction among types of cuestypes of cues

OculomotorOculomotor

VisualVisual

Oculomotor depth cuesOculomotor depth cues

Angle of convergence of the eye musclesAngle of convergence of the eye muscles

Accommodation of the lens of the eyesAccommodation of the lens of the eyes

AccommodationAccommodation

Accommodation works only at relatively Accommodation works only at relatively close distances (< a few meters).close distances (< a few meters).

Not very accurateNot very accurate

ConvergenceConvergence

Works for short distance (< 6 meters).Works for short distance (< 6 meters).

Can be used in isolation from Can be used in isolation from accommodation.accommodation.

Visual cuesVisual cues

BinocularBinocular

MonocularMonocular

Binocular cuesBinocular cues

Retinal disparity = the difference in Retinal disparity = the difference in distance between two objects as seen distance between two objects as seen from the left eye and the right eye.from the left eye and the right eye.

StereoscopeStereoscope

Charles Wheatstone (1838/1964).Charles Wheatstone (1838/1964). Two drawings on an object.Two drawings on an object. One from a perspective ~65 mm from the One from a perspective ~65 mm from the

perspective of the other.perspective of the other. Show one image to one eye and the other Show one image to one eye and the other

image to the other eye.image to the other eye.

Computing retinal disparityComputing retinal disparity

Identify features to match.Identify features to match.

Compute magnitude and direction of Compute magnitude and direction of disparity.disparity.

Computing retinal disparityComputing retinal disparity

Identify features to match.Identify features to match. E.g. a face in one eye and a face in the other, E.g. a face in one eye and a face in the other,

a bottle in one eye and a bottle in the other.a bottle in one eye and a bottle in the other. Random dot stereograms.Random dot stereograms.

Compute magnitude and direction of Compute magnitude and direction of disparity.disparity.

Computing retinal disparityComputing retinal disparity

Identify and compare only low frequency Identify and compare only low frequency information.information.

(Ignore or filter out high frequency (Ignore or filter out high frequency information.)information.)

Digression: Binocular rivalryDigression: Binocular rivalry

When two patterns can be fused, they are.When two patterns can be fused, they are.

When two patterns cannot be fused, they When two patterns cannot be fused, they create a mosaic or sometimes one merely create a mosaic or sometimes one merely attends to one rather than the other.attends to one rather than the other.

Some binocular cells are selective for Some binocular cells are selective for zero- retinal disparity.zero- retinal disparity.

Some binocular cells are selective for Some binocular cells are selective for some degree of retinal disparity.some degree of retinal disparity.

Cats with monocular stimuli.Cats with monocular stimuli.

StereoblindnessStereoblindness

Some people (5-10%) are unable to detect Some people (5-10%) are unable to detect depth from disparity. These individuals depth from disparity. These individuals may be those who cannot see “magic eye” may be those who cannot see “magic eye” images.images.

Most common cause may be strabismus, Most common cause may be strabismus, a misalignment of the two eyes.a misalignment of the two eyes.

Monocular depth informationMonocular depth information

Motion parallaxMotion parallax

As you move through the world, objects at As you move through the world, objects at different distances move at different rates. different distances move at different rates. This provides a powerful depth cue.This provides a powerful depth cue.

This occurs either when the viewer or the This occurs either when the viewer or the objects viewed move.objects viewed move.

Some depth from motion Some depth from motion demos:demos:

http://epsych.msstate.edu/descriptive/http://epsych.msstate.edu/descriptive/Vision/mparallax/DC4a.htmlVision/mparallax/DC4a.html

InterpositionInterposition

Occlusion of one object by another is Occlusion of one object by another is perhaps the most elementary depth cue.perhaps the most elementary depth cue.

The potency of occlusion is revealed in The potency of occlusion is revealed in Kanisza figures.Kanisza figures.

Amodal completionAmodal completion

We perceive occluded objects as complete We perceive occluded objects as complete wholes, when it is logically possible that wholes, when it is logically possible that they are mere parts of objects.they are mere parts of objects.

Sekuler & Palmer, (1992)Sekuler & Palmer, (1992)

Perceptual representations of partially Perceptual representations of partially occluded objects start out as a mosaic-like occluded objects start out as a mosaic-like snapshot of the individual pieces, then snapshot of the individual pieces, then evolves over time into perceptually evolves over time into perceptually complete objects.complete objects.

Occlusion and transparencyOcclusion and transparency

Lightness values within the “covered” Lightness values within the “covered” regions must be intermediate between the regions must be intermediate between the lightness values of the “uncovered” lightness values of the “uncovered” regions.regions.

The occluding transparent object must be The occluding transparent object must be plausibly a single object.plausibly a single object.

Special case: Occlusion and Special case: Occlusion and TransparencyTransparency

Occlusion and transparencyOcclusion and transparency

Lightness values within the “covered” regions Lightness values within the “covered” regions must be intermediate between the lightness must be intermediate between the lightness values of the “uncovered” regions.values of the “uncovered” regions.

The region must be plausibly a single object.The region must be plausibly a single object.

A region will be perceived as transparent only if A region will be perceived as transparent only if binocular disparity specifies that the region is in binocular disparity specifies that the region is in front of the object.front of the object.

Neon spreadingNeon spreading

Colors move from one region to the next.Colors move from one region to the next.

The role of occlusion is found in:The role of occlusion is found in:

Amodal completionAmodal completion

Illusory figuresIllusory figures

TransparencyTransparency

Neon spreadingNeon spreading

Size cues for depthSize cues for depth

PerspectivePerspective

Linear perspective (objects receding)Linear perspective (objects receding)

Aerial perspective (blur, haze)Aerial perspective (blur, haze)

ShadingShading

Integration of depth informationIntegration of depth information

Size, interposition, motion, and Size, interposition, motion, and perspective interact additively (Bruno and perspective interact additively (Bruno and Cutting, 1998).Cutting, 1998).

Depth cues compete. When two cues give Depth cues compete. When two cues give conflicting evidence, conflicting evidence,