Stereopsis (from "" meaning solidity, and "opsis" meaning vision or
sight) is the process in visual perceptionleading 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. Stereopsis is most commonly referred to as depth perception.
History of stereopsis
Stereopsis was first described by
Charles Wheatstonein 1838. ”… the mind perceives an object of three-dimensions by means of the two dissimilar pictures projected by it on the two retinæ…”. [http://www.stereoscopy.com/library/wheatstone-paper1838.html Contributions to the Physiology of Vision.—Part the First. On some remarkable, and hitherto unobserved, Phenomena of Binocular Vision. By CHARLES WHEATSTONE, F.R.S., Professor of Experimental Philosophy in King's College, London.] ] 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 cueof horizontal disparity, also known as retinal disparityand as binocular disparity. Wheatstone showed that this was an effective depth cue by creating the illusionof 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 Vincihad 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.Beck, J. (1979). Leonardo's rules of painting. Oxford: Phaidon Press.] 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.Wade, N. J. (1987). On the late invention of the stereoscope. Perception, 16, 785-818.] His column was one of the few objects that projects identical images of itself in the two eyes.
Stereopsis became popular during
Victorian timeswith the invention of the prism stereoscopeby 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 Juleszinvented random-dot stereograms. Julesz, B. (1960). Binocular depth perception of computer-generated images. The Bell System Technical Journal, 39(5), 1125-1163.] 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 imageafter the mythical Cyclopswho had only one eye. This was because it was as though we have 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 Pettigrewfound neurons in the cat visual cortexthat had their receptive fields in different horizontal positions in the two eyes.Barlow, H. B., Blakemore, C., & Pettigrew, J. D. (1967). The neural mechanism of binocular depth discrimination. Journal of Physiology, 193, 327-342.] This established the neural basis for stereopsis. Their findings were disputed by David Hubeland Torsten Wiesel, although they eventually conceded when they found similar neurons in the monkeyvisual cortex. Hubel, D. H., & Wiesel, T. N. (1970). Cells sensitive to binocular depth in area 18 of the macaque monkey cortex. Nature, 232, 41-42.] In the 1980s, Gian Poggioand others found neurons in V2 of the monkey brain that responded to the depth of random-dot stereograms.Poggio, G. F., Motter, B. C., Squatrito, S., & Trotter, Y. (1985). Responses of neurons in visual cortex (V1 and V2) of the alert macaque to dynamic random-dot stereograms. Vision Research, 25, 397-406.]
In the 1990s,
Christopher Tylerinvented autostereograms, random-dot stereograms that can be viewed without a stereoscope.Tyler, C. W., & Clarke, M. B. (1990). The autostereogram. SPIE Stereoscopic Displays and Applications, 1258, 182-196.] This led to the popular Magic Eyepictures.
stereoscopeis 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 timesit 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 Eyepictures ( autostereograms). Magic Eye pictures did not require a stereoscope, but relied on viewers using a form of free fusionso 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 disparitydetector.
When a person stares at an object, the two eyes converge so that the object appears at the center of the
retinain 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.Ogle, K. N. (1950). Researchers in binocular vision. New York: Hafner Publishing Company]
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
Computer stereo vision, is a part of the field of
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 vision. It is sometimes used in mobile roboticsto detect obstacles.
Two cameras take pictures of the same scene, but they are 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
televisionand 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 are mechanically coupled, in the case of IMAX 3D cinema), 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.
* Julesz, B. (1971). Foundations of cyclopean perception. Chicago: University of Chicago Press
* Scott B. Steinman, Barbara A. Steinman and Ralph Philip Garzia. (2000). "Foundations of Binocular Vision: A Clinical perspective". McGraw-Hill Medical. ISBN 0-8385-2670-5.
* [http://vision.middlebury.edu/stereo/ Middlebury Stereo Vision Page]
* [http://www.vision3d.com/stereo.html What is Stereo Vision?]
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