Transcript Chapter 8: Vision in three dimensions

Chapter 8: Vision in three dimensions
Basic issue: How do we construct a three-dimension visual experience from twodimensional visual input?
Important terms:
Allocentric direction (or location): understanding positions in space irrespective of
viewers’ perspective.
Egocentric direction (or location): understanding positions in space from a specific
viewer’s perspective.
Absolute distance: information about distances of objects from the observer
Relative distance: information about distances of various objects in visual field
from one another.
Inverting goggles: Are up and down innate or learned?
• In the late 1890’s George Stratton
put on inverting lenses to see if he
could adapt to a world upside down.
He did. Since then other studies
have investigated this phenomenon.
What do they indicate:
a. People can adapt remarkable well
with time to an inverted world.
b. The adaptation appears to be
largely motor-based, not visual.
c. Thus it appears that the up/down
axis is “imprinted” on the brain.
Dept and distance cues
Oculomotor: refers to those depth cues arising from the muscular adjustments of the eye to a changing
visual scene. These are the only cues which give unambiguous information about absolute
distances. There are two oculomotor cues.
Accommodation: refers to the degree of strain exerted by the muscle controlling the shape of the lens.
As objects get closer the muscle increases its strain (fattens the lens), as object moves further away
the muscle decreases its strain (flattens lens). Degree of strain then, is an indicator of distance.
Problem -- muscles assume most relaxed state for an object about 10ft away or so. So variations in
strain are only useful for a very limited range, and even in that range are not terribly accurate.
Convergence: refers to the degree of strain exerted by the muscles extra-ocular muscles controlling the
eye movements. Again, as object gets closer, muscle strain and angle of convergence increases,
thus this indicates distance from eyes.
Evaluation -- convergence is fairly reliable cue within about a 20ft range, after that muscle strain is not
present.
Dept and distance cues
Using two eyes to understand depth/distance: Stereopsis
Binocular dept/distance cue: Retinal disparity – difference
between the two eyes’ view of a single scene.
Retinal Disparity
Disparity increases as depth difference between objects increases (left)
Horopter: semi-circle of same perceived (egocentric) distance
Corresponding retinal points: along horopter, where images are fused
into single percept
Non-corresponding: off horopter where images remain unfused
(crossed – in front; uncrossed – behind)
But how does fusion happen?
• Random dot
stereograms
• Fusion, not
based on
common
features, but
something
even more
primitive:
spatial
frequency
Binocular rivalry
Different (un-fusable) images presented to
corresponding retinal points (foveas usually).
Results in perceptual switching over time
Stereoblindness: inability to use disparity as depth cue
Disparity is powerful dept cue; reason why eyes of are
frontally placed with overlapping visual fields. But
stereoblindness does not mean complete depth
blindness, other cues can be used. Common cause of
stereoblindness: strabismus – misalignment of eyes
such that they don’t work together effectively
Dept and distance cues
Monocular cues: useful
with only one eye
(pictorial cues)
1) Linear perspective:
parallel lines tend to
converge as distance
increases.
2) Texture gradient:
elemental structure of
ground tends to
become more
homogenous as
distance increases.
Monocular cues
3) Interposition: the
visual occlusion (or
blocking out) of one
object by another.
4) Retinal image size:
closer object tend to
cast larger retinal
images, familiarity with
true object size, or size
relative to other
familiar objects in visual
scene facilitate use of
this cue.
Monocular cues
5) Aerial perspective: haze tends to occlude objects as they get further away.
6) Motion parallax: as one moves close surfaces tend to move swiftly in the
opposite direction of motion, while far objects tend to move slowly in the same
direction of movement.
Illusions based on monocular cues
Moon illusion: moon at zenith smaller than moon at
horizon. Relation between retinal image size and perceived
distance (Emmert’ Law). Depth cues make moon look
farther away on horizon
Illusions based on monocular cues
• Ponzo illusion: linear perspective affects perceived distance
Illusions based on monocular cues
• Muller/Lyer illusion