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Chroma key' is a technique for mixing two images or frames together, in which a color (or a small color range) from one image is removed (or made transparent), revealing another image behind it. This technique is also referred to as color keying, colour-separation overlay (CSO; primarily by the BBC), greenscreen, and bluescreen. It is commonly used for weather forecast broadcasts, wherein the presenter appears to be standing in front of a large map, but in the studio it is actually a large blue or green background. The meteorologist stands in front of a bluescreen, and then different weather maps are added on those parts in the image where the color is blue. If the meteorologist himself wears blue clothes, his clothes will become replaced with the background video. This also works for greenscreens, since blue and green are considered the colors least like skin tone. This technique is also used in the entertainment industry, the iconic theatre shots in Mystery Science Theater 3000, for example.
The principal subject is filmed or photographed against a background consisting of a single color or a relatively narrow range of colors, usually blue or green because these colors are considered to be the furthest away from skin tone. The portions of the video which match the preselected color are replaced by the alternate background video. This process is commonly known as "keying", "keying out" or simply a "key".
Green is currently used as a backdrop more than any other color because image sensors in cameras are most sensitive to green. Therefore the green camera channel contains the least "noise" and can produce the cleanest key/matte/mask. Additionally, less light is needed to illuminate green because of the higher sensitivity to green in the image sensors. Blue was used before digital keying became commonplace because it was necessary for the optical process, but it needed more illumination than green. Bright green has also become favored as a blue background may match a subject's eye color.
In analog color TV, color is represented by the phase of the chroma subcarrier relative to a reference oscillator. Chroma key is achieved by comparing the phase of the video to the phase corresponding to the preselected color. In-phase portions of the video are replaced by the alternate background video.
In digital color TV, color is represented by three numbers (red, green, blue). Chroma key is achieved by a simple numerical comparison between the video and the preselected color. If the color at a particular point on the screen matches (either exactly, or in a range), then the video at that point is replaced by the alternate background video.
The chroma key technique is used in many aspects of the filming process, including: Battle scenes between the Resistance and HK-Aerials, exposed endoskeleton areas on infiltrators, and the compositing of 3D models (HKs, terminators, etc.) with live action shots.