Stereoscopic vision

Stereopsis is a process in visual perception leading to perception of the depth or distance of objects. It comes from two Greek roots, stereo meaning solidity, and opsis meaning vision or sight. That means it could refer to any sort of visual depth perception, but since about the 1960s it has come to refer to depth perception from binocular vision, requiring two eyes. Prior to then, it was often referred to as "binocular stereopsis".

Depth from stereopsis arises from the slightly different positions each eye occupies on the head, a form of parallax. 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 lead 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 did not require a stereoscope, but relied on viewers using a form of free fusion so that each eye views different images.

The two images required for stereopsis are called two half images. Two half images together comprise a stereogram.

History of stereopsis
Stereopsis was discovered by Charles Wheatstone in 1833. 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 proved 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 Weisel, 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 Magic Eye pictures becoming a new craze for stereopsis.

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.

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
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.