Spatial Perception Flashcards
What is spatial perception?
how we perceive the spatial layout of the environment
–> distance, depth, size, etc
What are the different classes of depth cues?
kinematic, stereoscopic, oculomotor, pictorial information
Depth
Relative position from observer (nearer/farther); qualitative
Distance
Absolute position given using some kind of metric or scale; quantitative
note: perception may be “body-scaled” (e.g. arm’s length, steps)
Non-metric
depth, qualitative
Metric
distance, quantitative
Sources of Information for Depth and Distance (motion cues)
Lecture:
Monocular cues
Motion cues
Binocular vision and stereopsis
---- Textbook: - Kinematic Information - Stereoscopic Information - Oculomotor Information - Pictorial Information
Kinematic Information
depth cues from motion
Motion perspective (aka motion parallax)
when the observer moves, displacement of an object’s image on the eye depends on its distance.
Optic flow
a form of motion parallax when the whole visual field is considered in a motion perspective
* pattern of retinal displacement relative to the observer
(demo: watching close tree + far cow on moving train)
What is the algorithm for segmenting a retinal image into different objects based on movement
Simple algorithm: get new value at each point by subtracting value there from average of its neighbors
- computation produces a map of significant object and surface edges in visual field; edges are marked by non-zero value (due to discontinuity)
Two types of kinematic information
based on movement:
1) Motion perspective (parallax)
2) Optical expansion/contraction
3) Accretion/deletion of texture
Optical expansion/contraction
when an object approaches, its image expands; if it is on a “hit” path, the expansion is symmetric
Accretion/deletion of texture
when a surface moves relative to another, the nearer surface progressively occludes background texture on the farther surface
Stereoscopic Information
depth information gained using the two eyes together
*depends on binocular disparity
Binocular disparity
refers to differences in the two eyes’ views of an object
- the amount of binocular disparity depends on the distance of an object from the observer
- depends on how far, and what point you’re looking from SO it can be used as a cue for distance
Horopter
sets of points in the world having identical binocular disparities; theoretically arranged in a circle
Crossed disparity
“NEARER from us”
– indicates that a point is nearer to the observer than the point being fixated
Uncrossed disparity
“FARTHER FROM US”
– indicates that a point is farther from the observer than the point being fixated
What are the points of 0 disparity?
- focused on fovea of both eyes
* falling on horopter
The farther away the ___er the disparity
bigger
Convergence
The ability of the two eyes to turn inward, often used in order to place the two images of a feature in the world on corresponding locations in the two retinal images (typically on the fovea of each eye)
– Convergence reduces the disparity of that feature to zero (or nearly zero)
Divergence
The ability of the two eyes to turn outward, often used in order to place the two images of a feature in the world on corresponding locations in the two retinal images (typically on the fovea of each eye).
– Divergence reduces the disparity of that feature to zero (or nearly zero)
Limitation of accommodation and divergence
Only works on near objects, not far
– eyes are diverged about as much as possible, so neither cue provides much useful information
Accommodation
changes in the shape of the lens in each eye to achieve focused images at varying distances
– accommodation may provide distance information via unconscious sensing of the muscular movements (in the ciliary muscles) that produce the lens changes
Are retinal images 2D or 3D?
2D
* all retina knows is that photons are hitting photopigments and activating ganglion cells –> do not know anything about how far the photons were
note: how retinal 2D image becomes a 3D image is an ill-posed problem
Oculomotor cues
having to do with eye muscles; physical movement of the eyes
*provide metric information; QUANTITATIVE
Pictoral Cues
depth cues that can operate in flat pictures; they are all also monocular cues, in that they can operate when you view with only one eye.
– most pictorial cues relate to rules of optics and geometry that govern the projection of the world onto the retina
Which cues were originally discovered by artists?
pictoral cues
Use of pictorial cues for depth perception involves using…
the rules of projection (law of optics) in reverse
Laws of Optics: Scene
vs.
Retina Inverse Optics: Retina
Pictoral cue of texture gradient
density of background texture impacts how close or large we perceive objects to be
Pictoral cue of familiar size
for objects we have encountered in the past, our perceptual system has an idea of what size they tend to be
– uses that information for interpreting how FAR they are given their actual size and the image
Convergence – metric, non-metric, or relative metric?
metric
Accommodation – metric, non-metric, or relative metric?
metric
Occlusion – metric, non-metric, or relative metric?
non-metric
Familiar size – metric, non-metric, or relative metric?
metric
Motion parallax – metric, non-metric, or relative metric?
relative metric
Relative size – metric, non-metric, or relative metric?
relative metric
Relative height – metric, non-metric, or relative metric?
relative metric
Binocular disparity – metric, non-metric, or relative metric?
relative metric
Aerial perspective
pictoral cue where far away objects tend to be low contrast/
near objects are high contrast
