We perceive
the world in three-dimensions even though the input to our visual system, the images projected onto our two retinas, has only
two spatial dimensions. How is this accomplished? It is well known that the visual system can infer the third dimension, depth,
from a variety of visual cues in the retinal images. One such cue is binocular disparity, the positional difference between
the two retinal projections of a given point in space. This positional difference results from the fact that the two eyes
are laterally separated and therefore see the world from two slightly different vantage points.
The
idea that retinal disparity contributes critically to depth perception derives from the invention of the stereoscope by Wheatstone
in the 19th century, with which he showed conclusively that the brain uses horizontal disparity to estimate the relative depths
of objects in the world with respect to the fixation point, a process known as stereoscopic depth perception or stereopsis.
Because simple geometry provides relative depth given retinal disparity, the problem of understanding stereovision reduces
to the question: How does the brain measure disparity from the two retinal images in the first place?
This
was demonstrated through his invention of random dot stereograms. A typical stereogram consists of two images of randomly
distributed dots that are identical except that a small distance with respect to the other image shifts a central square region
of one image horizontally. When each image is viewed individually, it appears as nothing more than a flat field of random
dots. However, when the two images are viewed dichoptically (i.e., the left and right images are presented to the left and
right eyes, respectively, at the same time), the shifted central square region “jumps” out vividly at a different
depth.
It separates the foreground with the background and thus limits our search
to the foreground object of interest. Stereovision is of paramount importance in perceiving depth. In stereovision the two
eyes (cameras) have the same image with a variable offset depending upon the depth of the object under consideration. This
offset is inherent in the setup of the two cameras, which are placed at an offset or a small distance apart. In order to capture
objects at different depths the cameras have to turn about its axis.
Our brain does not depend upon only such binocular cues to perceive depth. There are several monocular cues that give
us the sensation of depth using only one eye. So there are two types of cues that aid in depth perception:-
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