Editing Chroma key (section)

==Process==
[[File:SpiderwickChroniclesSet.jpg|thumb|right|267px|Film set for ''[[The Spiderwick Chronicles (film)|The Spiderwick Chronicles]]'', where a visual effects scene using bluescreen chroma key is in preparation]]

The principal subject is filmed or photographed against a background consisting of a single colour or a relatively narrow range of colours, usually blue or green because these colours are considered to be the furthest away from skin tone.<ref name="BorisFX">{{cite web|url=http://www.borisfx.com/tutorials/chroma_key.php|title=The Chroma Effect|work=Chroma Key Tutorial|publisher=BorisFX|quote=If the foreground is a person then blue or green backing colour is recommended as these colours are not present in human flesh pigments. In fact, human skin colour is 70% red for all people regardless of race.|access-date=11 January 2010|archive-url=https://web.archive.org/web/20050315004609/http://en.chinabroadcast.cn/2246/2005-2-14/90%40206385.htm|archive-date=15 March 2005|url-status=dead|df=dmy-all}}</ref> The portions of the video which match the pre-selected colour are replaced by the alternate background video. This process is commonly known as "[[keying (graphics)|keying]]", "keying out" or simply a "key".

===Processing a green backdrop===

Green is used as a backdrop for TV and electronic cinematography more than any other colour because television weather presenters tended to wear blue suits. When chroma keying first came into use in television production, the blue screen that was then the norm in the movie industry was used out of habit, until other practical considerations caused the television industry to move from blue to green screens. Broadcast-quality colour television cameras use separate red, green, and blue image sensors, and early analog TV chroma keyers required RGB component video to work reliably. From a technological perspective it was equally possible to use the blue or green channel, but because blue clothing was an ongoing challenge, the green screen came into common use. Newscasters sometimes forget the chroma key dress code, and when the key is applied to clothing of the same colour as the background, the person would seem to disappear into the key. Because green clothing is less common than blue, it soon became apparent that it was easier to use a green matte screen than it was to constantly police the clothing choices of on-air talent.   Also, because the human eye is more sensitive to green wavelengths, which lie in the middle of the visible light spectrum, the green analog video channel typically carried more signal strength, giving a better signal to noise ratio compared to the other component video channels, so green screen keys could produce the cleanest key. In the digital television and cinema age, much of the tweaking that was required to make a good quality key has been automated. However, the one constant that remains is some level of colour coordination to keep foreground subjects from being keyed out.<ref>{{cite web|url=https://www.videomaker.com/article/f5/13055-the-keys-to-chromakey-how-to-use-a-green-screen |title=The Keys To Chromakey: How To Use A Green Screen|publisher=Videomaker |access-date=23 October 2017}}</ref>

===Processing a blue backdrop===
Before electronic chroma keying, compositing was done on (chemical) film. The camera colour negative was printed onto high-contrast black and white negative, using either a filter or the high contrast film's colour sensitivity to expose only blue (and higher) frequencies. Blue light only shines through the colour negative where there is ''not'' blue in the scene, so this left the film clear where the blue screen was, and opaque elsewhere, except it also produced clear for any white objects (since they also contained blue). Removing these spots could be done by a suitable double-exposure with the colour positive (thus turning any area containing red or green opaque), and many other techniques. The result was film that was clear where the blue screen was, and opaque everywhere else. This is called a ''female matte'', similar to an ''[[alpha matte]]'' in digital keying. Copying this film onto another high-contrast negative produced the opposite ''male matte''. The background negative was then packed with the female matte and exposed onto a final strip of film, then the camera negative was packed with the male matte and was double-printed onto this same film. These two images combined creates the final effect.

===Major factors===
The most important factor for a key is the colour separation of the foreground (the subject) and background (the screen)&nbsp;– a blue screen will be used if the subject is predominantly green (for example plants), despite the camera being more sensitive to green light.

In [[analog television]], colour is represented by the phase of the chroma [[Subcarrier#Television|subcarrier]] relative to a reference oscillator. Chroma key is achieved by comparing the phase of the video to the phase corresponding to the pre-selected colour. In-phase portions of the video are replaced by the alternate background video.{{citation needed|date=April 2024}}

In [[Digital television|digital colour TV]], colour is represented by three numbers (red, green, blue intensity levels). Chroma key is achieved by a simple numerical comparison between the video and the pre-selected colour. If the colour 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.

====Lighting====
In order to create an illusion that characters and objects filmed are present in the intended background scene, the lighting in the two scenes must be a reasonable match. For outdoor scenes, overcast days create a diffuse, evenly coloured light which can be easier to match in the studio, whereas direct sunlight needs to be matched in both direction and overall colour based on time of day.

A studio shot taken in front of a green screen will naturally have ambient light the same colour as the screen, due to its light scattering. This effect is known as ''spill''.<ref name=Foster />{{rp|p20}} This can look unnatural or cause portions of the characters to disappear, so must be compensated for, or avoided by using a larger screen placed far from the actors.<ref>{{Cite book |last1= Aronson |first1= David |author-link1= David Aronson |title= DV Filmmaking: From Start to Finish, Volume 1 |year= 2006 |publisher= O'Reilly Media, Inc. |isbn= 978-0596008482 |pages= [https://archive.org/details/dvfilmmakingfrom0000aron/page/52 52–53] |url= https://archive.org/details/dvfilmmakingfrom0000aron/page/52 }}</ref>

====Camera====
The depth of field used to record the scene in front of the coloured screen should match that of the background. This can mean recording the actors with a larger depth of field than normal.<ref name="bermingham">{{Cite book | first= Alan | last=Bermingham | title= Location Lighting for Television | year= 2013 | publisher=Focal Press | isbn=978-0-240-51937-1 | page= 42 | url=https://books.google.com/books?id=7bAcAAAAQBAJ&pg=PA42}}</ref>