Three Dimensions Flashcards
monocular vision
one-eyed vision
binocular vision
two-eyed vision
visual field
visual field of a human is limited ~190° horizontally, 110° of which is covered by both eyes
binocular summation
the combination of signals from both eyes that makes performance on many tasks better than with either eye alone. Binocular vision makes the probability to detect small, fast-moving objects much higher, because there are 2 “detectors” looking for the object
Oculomotor cues
cues based on our ability to sense the position of our eyes and the tension in our eyes muscles. Oculomotor cues are created by convergence and accommodation
Convergence
the inward movement of the eyes that occurs when we look at nearby objects
Accomodation
the change in the shape of the lens that occurs when we focus on objects at various distances. We can feel the inward movement of the eyes during convergence and the tightening of eye muscles during accommodation. The feeling itself is the cue
Monocular depth cue
a depth cue that is availabe even when the world is viewed with one eye only. Accomodation is a monocular depth cue.
Pictorial depth cue
a depth cue that can be depicted in a 2D picture
Occlusion
a depth cue in which one object obstruct the view of (part of) another object. Occlusion is reliable in almost all cases, with the exception of accidental viewpoints
nonmetrical depth cue
it provides information about depth order, but not about depth magnitude (i.e., it does not provide information about distance between the occluders and occludees)
relative height
objects on the ground that are higher in the field of view appear farther away. Objects in the skay that are lower in the field of view appear farther away
familiar size
judging distance based on prior knowledge of object sizes
relative size
when 2 objects are known to be of equal physical size, the one that is farther away will appear smaller
Perspective convergence (linear perspective)
parallel lines converge in the distance
atmospheric/aerial/haze perspective
distant objects appear less sharp than nearer objects and sometimes have a slight blue tint. This occurs because the farther away an object is, the more air and particles we have to look through. The blue tint is due to the fact that the atmosphere preferentially scatters short-wavelength light, which appears blue
texture gradient
when a number of similar objects are equally spaced throughout a scene, they create a texture gradient. This results in a perception of depth: we see the more closely spaced elements farther away.
shadows
decreases in light intensity caused by the blockage of light- can provide information about the location of objects. Shadows also enhance the three-dimensionality of objects
motion-produced cues
depth cues that emerge when the observer is moving and enhance depth perception
motion parallax
as we move, nearby objects appear to glide rapidly past us, but more distant objects appear to move more slowly. The reason is that moving the eye from one point to another while looking at a closer object causes its retinal image to move more than that of a more distance object
deletion and accretion
as an observer moves sideways, some things become covered (deletion) and other become uncovered (accretion)
binocular disparity
the 2 retinas receive a slightly different visual image of the same scene due to the different positions of the eyes. The visual system reconstructs a 3-D world based on the inverted (slightly different) 2-D images of the world on the retina
corresponding retinal points
points on the retina that would overlap if the eyes were superimposed on each other
horopter
a surface in the world on which all points fall on corresponding retinal points
noncorresponding points
the images of objects that are not on the horopter fall on noncorresponding points
absolute disparity
the degree to which e.g., Bill’s image deviates from falling on corresponding points. Measured with an angle of disparity
angle of disparity
the angle between the corresponding point on the right eye for the left-eye image of bill and the actualy location of the image on the right eye
crossed disparity
an object that is closer to the observer than his/her fixation point (in front of the horopter) is seen to the right of fixation point by the left eye and to the left of fixation point by right eye
uncrossed disparity
the left eye sees an object to the left of the observer’s fixation point and the right eye sees the same object to the right of the fixation point. Occurs when an object is behind the horopter
stereopsis
depth perception that results from information provided by binocular disparity. Stereopsis is not necessary for 3D vision, but adds richness to the perception of the 3D world
random-dot stereogram
a stimulus that contains no depth cues other than disparity. It consists of 2 images of random-dot patterns, which are identical except for a square region from one of the patterns moved to the side
the correspondence problem
the visual system needs to solve the problem of matching every bit of the image in the left eye to bit in the right eye. It is not yet known how this happens
uniqueness constraint
a feature in the world is represented exactly once in each retinal image -> each monocular image feature should be paired with exactly one feature in the other monocular image
continuity constraint
except at the edges of objects, neighboring points in the world lie at similar distances from the viewer -> disparity chould change smoothly at most places in the image
binocular depth cells (or disparity-selective cells)
neurons that respond best when stimuli presented to the left and right eyes create a specific amount of absolute disparity. Disparity-selective neurons appear in most of the visual cortex and parts of the parietal cortex
disparity tuning curve
indicates the neural response that occues when stimuli create different amounts of disparity
role of V2
involved in computing depth order based on contour completion and border ownership processes
V4
encode depth intervals based on relative disparities
IT cortex and other higher cortical areas
involved in the representation of complex 3D shapes
visual angle
the angle of an object relative to the observer’s eye. The visual angle depends both on the size of the stimulus and on its distance from the observer -> a small near object can have the same visual angle as a far larger object. The visual angle says how lage an object will be on the retina
size constancy
our perception of an object’s size is relatively constant even when we view the object from different distances (and thus the visual angle gets smaller)
Size-distance scaling
a proposed mechanism for how size constancy works. It is expressed by the formula S = K(R x D)
S = an object’s perceived size
K = a constant
R = the size of the retinal image of the object
D = the perceived distance from the object
Muller-Lyer illusion
the right vertical line in the figure appears to be longer than the left vertical line, even though their length is equal
Misapplied size constancy scaling
a mechanism that explains the Muller-Lyer illusion.
Conflicting cues theory
our perception of line length depends on 2 cues:
1. The actual length of the line
2. The overall length of the figure
-> the overall length of the right figure is bigger than the overall of the left figure, which is why the illusion could be perceived
the ponzo (railroad track) illusion
both animals in the figure have the same visual angle, but the one on top appears longer. Misapplied size constancy scaling explanation -> the top animal appeaers bigger because of depth information provided by converging railroad tracks
moon illusion
when the moon is on the horizon it appears much larger then when it is high in the sky. However, the visual angles of the horizon moon and the elevated moon are the same
Apparent distance theory
the moon on the horizon appears more distant because it is viewed across the filled space of the terrain, which contains depth information. Since S = R x D and R is constant here, but D increases for the horizon moon, S for the horizon moon is larger
Angular size contrast theory
the elevated moon appears smaller because the large expanse of sky surrounding it makes it appear smaller by comparison
Study with cats
Cats were reared so that their vision was alternated between left and right eyes every other day during the first 6 months of their lives.
-> After the 6-month period, the cats had fewer binocular neurons
-> cats also performed poorly on a depth perception task
-> Disparity-selective neurons appear in most of the visual cortex and parts of the parietal cortex
Holway and Boring’s experiment
A person has to adjust the size of a comparison circle to be the same as the size of a test circle, whose distance is being changed, but retinal size stays constant.
-> When all depth cues are available, people tend to judge size correctly. They make the comparison circle small when the test circle is close and they make it large when the test circle is far away.
-> When depth cues are eliminated 1 by 1, the judgments of size become less accurate
-> When all depth information is eliminated, the observer’s perception of size is determined by the visual angles of the circle’s images on the retinas.
