Chroma key

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Chroma key compositing (or chroma keying) is a technique for compositing 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[1]), 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 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.[2] This technique is also used in the entertainment industry, the iconic theatre shots in Mystery Science Theater 3000, for example.

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History

For motion-pictures, a complex and time consuming process once known as "travelling matte" was used prior to the introduction of digital compositing. The blue screen and traveling matte method were developed in the 1930s at RKO Radio Pictures and other studios, and were used to create special effects for The Thief of Bagdad (1940). At RKO, Linwood Dunn used travelling matte to create "wipes" – where there were transitions like a windshield wiper in films such as Flying Down to Rio (1933).

The credit for development of the bluescreen is given to Larry Butler, who won the Academy Award for special effects for The Thief of Bagdad. He had invented the blue screen and traveling matte technique in order to achieve the visual effects which were unprecedented in 1940. He was also the first special effects man to have created these effects in Technicolor, which was in its infancy at the time.

In 1950, Warner Brothers employee and ex-Kodak researcher Arthur Widmer began working on an ultra violet traveling matte process. He also began developing bluescreen techniques: one of the first films to use them was the 1958 adaptation of the Ernest Hemingway novella, The Old Man and the Sea, starring Spencer Tracy.[3]

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