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Stereopsis (from stereo meaning solidity, and opsis meaning vision or sight) is the process in visual perception leading to perception of stereoscopic depth. In turn, stereoscopic depth is the sensation of depth that emerges from the fusion of the two slightly different projections of the world on the two retinas. The difference between the two eyes’ images, which is a result of the eyes' horizontal separation, is usually referred to as binocular disparity or retinal disparity. The fact that this binocular disparity is interpreted by the brain as depth was first discovered by the English genius Charles Wheatstone and described by him in a classic paper published in 1838: ”… the mind perceives an object of three-dimensions by means of the two dissimilar pictures projected by it on the two retinæ…”, (Wheatstone, 1838). To prove his ideas Wheatstone invented a simple device, which he dubbed a stereoscope. Using his newly invented stereoscope Wheatstone was able to convincingly show that a vivid sense of depth emerges from two completely flat pictures depicting two different projections of the same scene.
History of stereopsis
Stereopsis was first described by Charles Wheatstone in 1838. 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 sterescope 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 Panum, Hering, Adelbert Ames Jr., and Kenneth N. Ogle.
Also 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 stimulus after the mythical Cyclops who 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 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.
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.
Computer stereo vision
Stereo vision, an important field of computer vision, is sometimes a method used in mobile robotics to detect obstacles. 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 televison 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.
- Correspondence problem
- Epipolar geometry
- Stereoscopic vision
References & Bibliography
- 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.
- Google Scholar
- Birch, E. E., Gwiazda, J., & Held, R. (1983). The development of vergence does not account for the onset of stereopsis: Perception Vol 12(3) 1983, 331-336.
- Gillam, B., Blackburn, S., & Cook, M. (1995). Panum's limiting case: Double fusion, convergence error, or "da Vinci stereopsis." Perception Vol 24(3) 1995, 333-346.
- Hamori, E., Broad, R. D., & Reed, C. E. (1982). Study of unaided cross-eyed stereopsis: Perception Vol 11(3) 1982, 297-304.
- Harwerth, R. S., Smith, E. L., & Siderov, J. (1995). Behavioral studies of local stereopsis and disparity vergence in monkeys: Vision Research Vol 35(12) Jun 1995, 1755-1770.
- Harwerth, R. S., Smith Iii, E. L., Crawford, M. L. J., & von Noorden, G. K. (1997). Stereopsis and disparity vergence in monkeys with subnormal binocular vision: Vision Research Vol 37(4) Feb 1997, 483-493.
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