3D Space Perception(aka Depth Perception)

3D Space Perception

• The flat retinal image problem:

How do we reconstruct 3D-space from 2D image?

What information is available to supportthis process?

• Interaction between Perceived Size and Perceived Distance (both depend upon “scaling”)

Size ConstancyPerceived size is not slavishly linked to retinal size; otherwise your car would appear to be smaller when observed at increasing distances.

Instead, perceived size tends to remain invariant across observation distance…a phenomenon known as size constancy.

Perceived size depends upon the psychological scaling of retinal size relative to perceived distance. Hence, size, distance and 3D visual perception are all based upon a more complex process known as spatial scaling.

Corollary: Perceived Speed = retinal velocity x scaled distance

Failures of Size Constancy

The Moon Illusion

Failures of Size Constancy

The Buechet ChairClick here for more

The Ames Room(Iowa State University at Ames, IA)

Failures of Size Constancy

3D-Depth Information “Cues”

Oculomotor Information

• State of Accommodation• State of Vergence

AccommodationIn theory, the efferent signal driving the ciliary muscles (and/or afferent feedback from stretch sensors in the ciliary muscles) could be used by higher-order visual processes to help scale 3D space and/or visual distance.

There is little evidence to support this hypothetical role of accommodation.

Vergence Eye Movements

Support for the role of efferent/afferent 3D information from vergence eye movements comes from:

“Tower Speed Illusion”and

Botox Treatment of Rectus Muscles in

Strabismus Surgery

Static MonocularSources of 3D Information

• Occlusion• Familiar Size (Relative Size)• Texture Gradients• Linear Perspective• Aerial Perspective (Atmospheric extinction)• Shadow Casting

Occlusion

Near objects block visual access to far objects

Linear Perspective

Parallel lines on thevisual plane convergetoward the “vanishingpoint” with increasing observation distance

This law of projectivegeometry provides a strong cue about distanceand 3D space.

Linear Perspective

Linear Perspective in the Service of Art

Familiar Size/Relative Size

Objects of the same physical sizeproject different size retinal images depending upon the observation distance.

This knowledge and prior experience contribute to 3D space perception.

What is the height of thissculpture in feet?

Familiar Size Cue

Easter Island Sculpturewithout Familiar Size Cue

Easter Island Sculpturewith Familiar Size Cue

Novel objects can be psychologically scaled given visual referencesof known size. For example…

Texture Gradients

An extended surfacewith uniform spatialtexture will project a retinal image with a non-uniform texture gradient that increases in spatial frequency as observation distance increases.

Aerial Perspective

Particulate matter in the atmosphere scatters light; reducing contrast and intensity of the retinal image.

The light from distant objects must pass through more atmosphere than the light from near objects.

Shadow Casting

Just as occlusion of objects serves as a powerful cue for depth…occlusion of the illuminant (sun) forms shadows which provide a powerful source of information for extracting 3D representations from a 2D retinal image.

Identify the Monocular Depth Cues

A Rainy Day in ParisGustav Caillebotte (1848-1894)

Identify the Monocular Depth Cues

A Rainy Day in ParisGustav Caillebotte (1848-1894)

Linear Perspective

Occlusion

Texture Gradient

Aerial Perspective

Shadow Casting

Familiar Size

Dynamic MonocularSources of 3D Information

• Motion Parallax• Relative Angular Velocity• Radial Expansion/Looming• Moving Shadows

Motion ParallaxMotion parallax occurs when an observer fixates a point at intermediate distance and then rotates their head.

Objects in the distance appear to move WITH head motion; while objects closer than the fixation plane appear to move AGAINST the rotation of the head.

Motion Parallax and DynamicShadow Casting Demo

Optic Flow: Radial Expansion

Optic Flow and Driving Demo

Delta Angular Velocity/Angular Size

Lecture Note:Need for improved Slow Moving Vehicle sign/

Binocular Depth Perception

Advantages of Binocularity

• Redundancy (survival value)• Stereopsis (Predators)

Large Field-of-View (Prey)• Binocular summation improves sensitivity

by √2 (signal:noise ratio sampling theory)Binocular acuity better than monocular;Same for CSF and many other functions

Binocular Vision (cont.)

• Binocular rivalry(Role of the “dominant” eye)

• Autokinesis phenomenon

Stereopsis

• Ability to use binocular retinal disparity information to extract relative depth information from the retinal image pairs

• Retinal “mismatch” can be used to reconstruct much of the missing 3rd dimension from the flat retinal images

Retinal Disparity

Understanding begins with a consideration of the geometry of the horopter

Horopter(Corresponding Retinal Images)

The HOROPTER is an imaginary surface whose points are all at thesame distance as the fixation point.

Points on the horopter project tocorresponding locations on the temporal and nasal retinas,respectively.

These corresponding locationsexhibit zero retinal disparity

i.e., D = dtemporal – dnasal = 0

“Crossed” and Uncrossed” Retinal Disparity

The corresponding locations for the“closer” green stimulus exhibitspositive retinal disparity

D = dtemporal – dnasal > 0(or “crossed” disparity)

The corresponding locations for the“farther” red stimulus exhibitsnegative retinal disparity

D = dtemporal – dnasal < 0(or “uncrossed” disparity)

DepthRecovery

byBinocularCortical

Cells

Panum’s Fusion Area

Nativists v. Empiricists “Debate”

• Nativist positionThe CNS is capable of processing many environmental invariants at birth – giving rise to direct perception (e.g., James Gibson)

• Empiricist positionSensory information is too impoverished to explain perceptual experience without recourse to knowledge about the world; it is based upon “unconscious inference” (e.g., Bishop Berkeley)

Support for Nativism

• Eleanor Gibson’s Visual Cliff Experiment(and HRD replication studies)

• Bela Julesz’s Random Dot Stereogram paradigm

Random DotStereograms

Bela Julesz

Can retinal disparity yield perception of depth independent of knowledge about the nature of the world?

Nativist vs. Empiricist Debate

Red-Blue Anaglyph Technique(black background)

Anaglyph glasses transmit RED and MAGENTA dots to the left eye; and, the BLUE and MAGENTA dots to the right eye.

Demo stimulus from USD’s PSYC 301 Lab