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Chroma key is an editing technique in which two images are composited together to create a single, finished image. This editing technique is sometimes referred to as blue or green screen, referencing the backdrop used to film the subject of an image which will be subjected to the chroma key process. This editing technique is used extensively in film and television to create a range of special effects, and the basic concept is familiar to many film fans as a result.
In a classic example of a film segment produced with a chroma key technique, a director might want to create the illusion that two actors are sitting in a car together, driving down the road. While the film crew could potentially travel to a site, have the actors drive down a road, and film from inside and around the car, this is time-consuming and sometimes extremely difficult. Instead, the actors and the car are placed on a sound stage surrounded by a classically green backdrop, and then the scene is filmed.
In the editing room, the editor directs the computer to select the green background and “key it out” of the image, allowing the editor to overlay the segment over footage of a landscape sweeping past. In the finished image, it looks like the actors are indeed driving down the road as they talk. The same technique can be used to create weather maps for backgrounds on weather reports, to place actors in exotic locales without actually visiting them, and to perform a host of other special effects tricks.
Blue and green screens are classically used for chroma key because they are as far away from human skin tones as it is possible to get. Actors are also careful about wearing clothing which is similar in tone to the background, to avoid having parts of themselves keyed out, and the background is lit with an even wash of light to keep the tone as even as possible, making it easier to edit it out in the editing room. Without an evenly lit background, an image produced through the chroma key process can have jagged edges or halos caused by uneven lighting.
Sometimes, the chroma key technique is used to create special effects like invisibility, with the actor wearing garments and makeup which approximate the tone of the chroma key background so that he or she will disappear in the finished image. Chroma key can be used to make it look like actors are in dangerous, crowded, or fantastical situations with a greatly reduced cost to the producers, and with less stress for cast and crew.