Stereoscopy
Stereoscopy, stereoscopic imaging or 3-D (three-dimensional) imaging is a technique to create the illusion of depth in a photograph, movie, or other two-dimensional image, by presenting both eyes two slightly different images. Stereoscopy is used in photogrammetry and also for fun.
The basic technique consists of creating a 3-D illusion starting from a pair of 2-D images. To create depth perception in the brain it is necessary to provide to the eyes of the viewer two different images, representing two perspectives of the same object, with a minor deviation similar to the perspectives that both eyes naturally receive in binocular vision.
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Techniques
Side-by-side
Two separate images are printed side-by-side. The user is required to force his eyes either to cross, or to diverge, so that the two images appear to be three. Then as each eye sees a different image, the effect of depth is achieved in the central image of the three. This is the oldest method of stereoscopy, having been discovered in the mid-19th century by Charles Wheatstone. In the late 19th and early 20th century stereo cards were popularly sold. The cards had a pair of photographs, usually taken with a special camera that took the pair of images from slightly separated views simultaneously. Cards were printed with these views (sometimes with explanatory text as well); when the cards were looked at through the double-lensed viewer, called a stereopticon, a three-dimensional image could be seen.
In the 1940s, a modified and miniaturized variation of this technology was introduced as "The View-Master®". Pairs of stereo views were printed on translucent film attached to a round card. A series of views could thus be seen on each card when the card was inserted in the View-Master while facing towards light.
More recently, random-dot stereograms have been created using computers to hide the different images in a field of apparently random noise, so that until viewed using this technique, the subject of the image remains a mystery.
The side-by-side method is extremely simple to create, but it can be difficult or uncomfortable to view with the naked eye. Devices such as the stereopticon and the View-Master make viewing easier and less fatiguing.
LCD shutter glasses
Glass containing liquid crystal and a polarizing filter has the property that it becomes dark when voltage is applied, but otherwise is translucent. A pair of eyeglasses can be made using this material and connected to a computer video card. The video card alternately darkens one eye, and then the other, in synchronization with the refresh rate of the monitor, while the monitor alternately displays different perspectives for each eye. At sufficiently high refresh rates, the viewer's visual system does not notice the flickering, each eye receives a different image, and the effect is achieved.
The problems associated are the cost for the additional equipment, and that the flickering can be noticeable if the refresh rate is not sufficiently high, as each eye is effectively receiving only half of the monitor's actual refresh rate.
Polarized glasses
To present a stereoscopic motion picture, two images are projected superimposed onto the same screen through orthogonal polarizing filters. The viewer wears low-cost eyeglasses which also contain a pair of orthogonal polarizing filters. As each filter only passes light which is similarly polarized and blocks the orthogonally polarized light, each eye only sees one of the images, and the effect is achieved.
The difficulty arises because light reflected from a motion picture screen tends to lose a bit of its polarization.
Recently (2003) Keigo Iizuka has discovered an inexpensive implementation of this principle on laptop computer displays using cellophane sheets. See the reference below for more information.
Two-color anaglyph
Two images from the perspective of the left and right eyes are projected or printed, one in red and the other in a contrasting color such as blue or green. An inexpensive pair of eyeglasses with two colored filters is worn by the viewer. A red lens over one eye makes the red image invisible to that eye and the blue appear in purple, while a blue lens over the other eye makes the blue invisible and the red look purple, so each eye sees only the perspective it is supposed to see.
Although this system works best for black-and-white images, difficulty arises in presenting color images, since the colored filters will affect the appearance of the colors in the image.