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In film, the term 3-D (or 3D) is used to describe any visual presentation system that attempts to maintain or recreate moving images of the third dimension, the illusion of depth as seen by the viewer. more...

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The technique usually involves filming two images simultaneously, with two cameras positioned side by side, generally facing each other and filming at a 90 degree angle via mirrors, in perfect synchronization and with identical technical characteristics. When viewed in such a way that each eye sees its photographed counterpart, the viewer's visual cortex will interpret the pair of images as a single three-dimensional image. Modern computer technology also allows for the production of 3D films without dual cameras.

Techniques

There are several methods of projecting natural, stereoscopic images. They can be categorized into mainly three categories.

LCD shutter glasses

Glasses containing liquid crystal that will let light through in synchronization with the images on the computer display, using the concept of alternate-frame sequencing. XpanD 3D are using this method with Digital cinema in over 400 digital cinemas

Linearly polarized glasses

To present a stereoscopic motion picture, two images are projected superimposed onto the same screen through orthogonal polarizing filters. A metallic screen surface is required to preserve the polarization. The projectors can receive their outputs from a computer with a dual-head graphics card. 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. Linearly polarized glasses require the viewer to keep his head level, as tilting of the viewing filters will cause the images of the left and right channels to bleed over to the opposite channel – on the other hand, viewers learn very quickly not to tilt their heads. In addition, since no head tracking is involved, several people can view the stereocopic images at the same time.

Circularly polarized glasses

To present a stereoscopic motion picture, two images are projected superimposed onto the same screen through circular polarizing filters of opposite handedness. The viewer wears low-cost eyeglasses which contain a pair of analyzing filters (circular polarizers mounted in reverse) of opposite handedness. Light that is left-circularly polarized is extinguished by the right-handed analyzer; while right-circularly polarized light is extinguished by the left-handed analyzer. The result is similar to that of stereoscopic viewing using linearly polarized glasses; except the viewer can tilt his head and still maintain left/right separation. RealD and masterimage uses electronically driven circular polarizers that alternate between left-and right- handedness, and does so in sync with the left or right image being displayed by the digital cinema projector.

Read more at Wikipedia.org