Everything about Stereopsis totally explained
Stereopsis (from meaning solidity, and
opsis meaning vision or
sight) is the process in
visual perception leading to the sensation of depth from the two slightly different projections of the world onto the
retinas of the two
eyes. The differences in the two retinal images are called
horizontal disparity,
retinal disparity, or
binocular disparity. The differences arise from the eyes' different positions in the head.
History of stereopsis
Stereopsis was first described by
Charles Wheatstone in
1838. ”… the mind perceives an object of three-dimensions by means of the two dissimilar pictures projected by it on the two retinæ…”. He recognized that because each eye views the visual world from slightly different horizontal positions, each eye's image differs from the other. Objects at different distances from the eyes project images in the two eyes that differ in their horizontal positions, giving the depth
cue of
horizontal disparity, also known as
retinal disparity and as
binocular disparity. Wheatstone showed that this was an effective depth cue by creating the
illusion of depth from flat pictures that differed only in horizontal disparity. To display his pictures separately to the two eyes, Wheatstone invented the
stereoscope.
Leonardo da Vinci had also realized that objects at different distances from the eyes project images in the two eyes that differ in their horizontal positions, but had concluded only that this made it impossible for a painter to portray a realistic depiction of the depth in a scene from a single canvas. Leonardo chose for his near object a column with a circular cross section and for his far object a flat wall. Had he chosen any other near object, he may have discovered horizontal disparity of its features. His column was one of the few objects that projects identical images of itself in the two eyes.
Stereopsis became popular during
Victorian times with the invention of the
prism stereoscope by
David Brewster. This, combined with
photography, meant that tens of thousands of stereograms were produced.
Until about the 1960s, research into stereosis was dedicated to exploring its limits and its relationship to
singleness of vision. Researchers included
Peter Ludvig Panum,
Ewald Hering,
Adelbert Ames Jr., and
Kenneth N. Ogle.
In the 1960s,
Bela Julesz invented
random-dot stereograms. Unlike previous stereograms, in which each half image showed recognizable objects, each half image of the first random-dot stereograms showed a square matrix of about 10,000 small dots, with each dot having a 50% probability of being black or white. No recognizable objects could be seen in either half image. The two half images of a random-dot stereogram were essentially identical, except that one had a square area of dots shifted horizontally by one or two dot diameters, giving horizontal disparity. The gap left by the shifting was filled in with new random dots, hiding the shifted square. Nevertheless, when the two half images were viewed one to each eye, the square area was almost immediately visible by being closer or farther than the background. Julesz whimsically called the square a
Cyclopean image after the mythical
Cyclops who had only one eye. This was because it was as though we've a cyclopean eye inside our brains that can see cyclopean stimuli hidden to each of our actual eyes. Random-dot stereograms highlighted a problem for stereopsis, the
correspondence problem. This is that any dot in one half image can realistically be paired with many same-coloured dots in the other half image. Our visual systems clearly solve the correspondence problem, in that we see the intended depth instead of a fog of false matches. Research began to understand how.
Also in the 1960s,
Horace Barlow,
Colin Blakemore, and
Jack Pettigrew found
neurons in the
cat visual cortex that had their
receptive fields in different horizontal positions in the two eyes. This established the neural basis for stereopsis. Their findings were disputed by
David Hubel and
Torsten Wiesel, although they eventually conceded when they found similar neurons in the
monkey visual cortex. In the 1980s,
Gian Poggio and others found neurons in
V2 of the monkey brain that responded to the depth of random-dot stereograms.
In the 1990s,
Christopher Tyler invented
autostereograms, random-dot stereograms that can be viewed without a stereoscope. This led to the popular
Magic Eye pictures.
Popular culture
A
stereoscope is a device by which each eye can be presented with different images, allowing stereopsis to be stimulated with two pictures, one for each eye. This has led to various
crazes for stereopsis, usually prompted by new sorts of stereoscopes. In
Victorian times it was the
prism stereoscope (allowing stereo
photographs to be viewed), in the 1950s it was
red-green glasses (allowing stereo
movies to be viewed), in the 1970s it was
polarizing glasses (allowing
coloured movies to be viewed), and in the 1990s it was
Magic Eye pictures (
autostereograms). Magic Eye pictures didn't require a stereoscope, but relied on viewers using a form of
free fusion so that each eye views different images.
Geometrical basis for stereopsis
Stereopsis appears to be processed in the visual cortex in
binocular cells having
receptive fields in different horizontal positions in the two eyes. Such a cell is active only when its preferred stimulus is in the correct position in the left eye and in the correct position in the right eye, making it a
disparity detector.
When a person stares at an object, the two eyes converge so that the object appears at the center of the
retina in both eyes. Other objects around the main object appear shifted in relation to the main object. In the following example, whereas the main object (dolphin) remains in the center of the two images in the two eyes, the cube is shifted to the right in the left eye's image and is shifted to the left when in the right eye's image.
Because each eye is in a different horizontal position, each has a slightly different perspective on a scene yielding different
retinal images. Normally two images are not observed, but rather a single view of the scene, a phenomenon known as
singleness of vision. Nevertheless, stereopsis is possible with double vision. This form of stereopsis was called
qualitative stereopsis by Kenneth Ogle.
If the images are very different (such as by going cross-eyed, or by presenting different images in a
stereoscope) then one image at a time may be seen, a phenomenon known as
binocular rivalry.
Computer stereo vision
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| Stereovision cameras are used on unmanned ground vehicles to measure the distance between the camera and objects in the field of view, for purposes of path planning and obstacle avoidance. (Courtesy of MobileRobots Inc) |
Computer stereo vision, is a part of the field of
computer vision. It is sometimes used in
mobile robotics to detect obstacles.
Two cameras take pictures of the same scene, but they're separated by a distance - exactly like our eyes. A computer compares the images while shifting the two images together over top of each other to find the parts that match. The shifted amount is called the
disparity. The disparity at which objects in the image best match is used by the computer to calculate their distance.
For a human, the eyes change their angle according to the distance to the observed object. To a computer this represents significant extra complexity in the geometrical calculations (
Epipolar geometry). In fact the simplest geometrical case is when the camera image planes are on the same plane. The images may alternatively be converted by reprojection through a
linear transformation to be on the same image plane. This is called
Image rectification.
Computer stereo vision with many cameras under fixed lighting is called
structure from motion. Techniques using a fixed camera and known lighting are called
photometric stereo techniques, or "
shape from shading".
Computer stereo display
Many attempts have been made to reproduce human stereo vision on rapidly changing computer displays, and toward this end numerous patents relating to
3D television and
cinema have been filed in the
USPTO. At least in the US, commercial activity involving those patents has been confined exclusively to the grantees and licensees of the patent holders, whose interests tend to last for twenty years from the time of filing.
Discounting
3D television and cinema (which generally require a plurality of digital projectors whose moving images must be synchronized by computer), several stereoscopic
LCDs are going to be offered by
Sharp, which has already started shipping a notebook with a built in stereoscopic LCD. Although older technology required the user to don goggles or visors for viewing computer-generated images, or CGI, newer technology tends to employ
fresnel lenses or plates over the liquid crystal displays, freeing the user from the need to put on special glasses or goggles.
Further Information
Get more info on 'Stereopsis'.
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