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| - The chroma keying technique is commonly used in video production and post-production. This technique is also referred to as color keying, color-separation overlay (CSO; primarily by the BBC), or by various terms for specific color-related variants such as green screen, and blue screen - chroma keying can be done with backgrounds of any color that are uniform and distinct, but green and blue backgrounds are more commonly used because they differ most distinctly in hue from most human skin colors and no part of the subject being filmed or photographed may duplicate a color used in the background.
- 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".
- More commonly known as green screen or blue screen (though that one also has a different meaning), the process by which a subject filmed on a camera can be seamlessly inserted into a scene generated by other means. It relies on filming the subject in front of a solid-color background -- any color will do, so long as it's not used in the foreground -- and adjusting the editing system to replace that color with the background signal. The main methods of controlling the background for live-action shots are, in increasing order of technological sophistication: Examples of Chroma Key include:
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| abstract
| - 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 keying technique is commonly used in video production and post-production. This technique is also referred to as color keying, color-separation overlay (CSO; primarily by the BBC), or by various terms for specific color-related variants such as green screen, and blue screen - chroma keying can be done with backgrounds of any color that are uniform and distinct, but green and blue backgrounds are more commonly used because they differ most distinctly in hue from most human skin colors and no part of the subject being filmed or photographed may duplicate a color used in the background.
- More commonly known as green screen or blue screen (though that one also has a different meaning), the process by which a subject filmed on a camera can be seamlessly inserted into a scene generated by other means. It relies on filming the subject in front of a solid-color background -- any color will do, so long as it's not used in the foreground -- and adjusting the editing system to replace that color with the background signal. The main methods of controlling the background for live-action shots are, in increasing order of technological sophistication:
* Finding one that already exists, and film on location.
* Build a set.
* Film the background separately, and project it onto a screen behind the actors while filming, typically via rear projection.
* Double-exposing the film, which results in a slightly transparent foreground but is cheap.
* The old analog Matte Shot, done with precise blocking of the camera frame.
* Chroma Key. The background inserted via Chroma Key can be any visual image. CGI is the most common today, but it can be other live action footage, models, stop motion or cel animation just as easily. The color used is now entirely arbitrary. Blue was a popular choice in the early days of color motion pictures, because it is complementary to the reds found in human skin. Green became popular because digital editing systems can isolate green with less light in the background, and because lime green is less common than bright blue in costuming Magenta is sometimes used, as is black, but the latter is problematic, as it's almost impossible to shoot a person without having some black visible on their person, in eyes or shadows. If any part of an actor or prop is colored the same as the background, that part will disappear. Thus, sometimes the background color is chosen because of the colors to be used in the foreground action. The original run of Doctor Who, for instance, used green or yellow backgrounds even when blue was the most common color at The BBC, because a large number of its effects shots involved the TARDIS, a timeship that takes the form of a blue police phone box The problem with using yellow was that foreground objects and actors always had a prominent yellow fringe around them. Normally, wardrobe and prop designers simply avoid using greens in the capture range, but this is not always possible; you'll occasionally see bloopers where weather forecasters have part of the meteorology map show up on their ties, for example. The invisibility effect can be used intentionally to allow a performer, or part of his body, to interact with props while remaining unseen. A garment that can be used for this purpose is a one-piece jumpsuit in the background color, with a full-face mask, and a mesh eye piece, called a "gimp suit" or, in the case of a blue background, a "blueberry" in the trade. Performers in recent Jim Henson Productions shows have used these suits to perform with puppets without having to raise them above their heads. The suit looks like a Ninja outfit, and that is not a coincidence, as it serves the same purpose as the black outfits traditionally worn by Japanese stagehands. See notes at Ninja. Almost all productions use Chroma Key at some point, but there are some standout examples. Also notable for causing occasional unintentional hilarity - when background and foreground are poorly matched, or the SFX budget is low, the effect is anything but seamless. It can be fairly tricky to create a viable Chroma-Key effect, especially with amateur equipment -- often, it requires fiddling with hue and saturation, and even then, there is often a faint, tell-tale 'border' around the subject where the green-screen footage and the 'real' actor don't match up. Examples of Chroma Key include:
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![[RDF Data]](/fct/images/sw-rdf-blue.png)