Editing Chroma key (section)

==Background colour==
[[File:MuseumOfScienceBoston BlueScreenAtSpecialEffectsShow.jpg|thumb|right|267px|Demonstration of the creation of visual effects techniques using chroma key]]

[[Blue]] was originally used for the film industry as making the separations required a film that would only respond to the screen colour, and film that responded only to blue and higher frequencies (ultraviolet, etc.) was far easier to manufacture and make reliable than film that somehow excluded both frequencies higher and lower than the screen colour.

In television and digital film making, however, it is equally easy to extract any colour, and green quickly became the favoured colour. Bright green is less likely to be in the foreground objects, colour film emulsions usually had much finer grain in the green, and [[lossy compression]] used for analog video signals and digital images and movies retain more detail in the green channel. Green can also be used outdoors where the light colour temperature is significantly blue. Red is avoided as it is in human skin, and any other colour is a mix of primaries and thus produces a less clean extraction.

A so-called "[[Sodium vapor process|yellow screen]]" is accomplished with a white backdrop. Ordinary stage lighting is used in combination with a bright yellow sodium lamp. The sodium light falls almost entirely in a narrow frequency band, which can then be separated from the other light using a prism, and projected onto a separate but synchronized film carrier within the camera. This second film is high-contrast black and white, and is processed to produce the matte.<ref name=Foster>{{cite book | title=The Green Screen Handbook: Real-World Production Techniques | first=Jeff | last=Foster | publisher=John Wiley & Sons | year=2010 | isbn=978-0-470-52107-6}}</ref>{{rp|16}}

A newer technique is to use a [[retroreflective]] curtain in the background, along with a ring of bright [[LED]]s around the [[camera lens]].  This requires no light to shine on the background other than the LEDs, which use an extremely small amount of power and space unlike big [[stage light]]s, and require no [[rigging (theatre)|rigging]].  This advance was made possible by the invention in the 1990s of practical blue LEDs, which also allow for [[Emerald (color)|emerald green]] LEDs.

There is also a form of colour keying that uses light spectrum invisible to human eye. Called Thermo-Key, it uses [[infrared]] as the key colour, which would not be replaced by background image during [[Video post-processing|postprocessing]].<ref>{{cite web|url=http://www.hc.t.u-tokyo.ac.jp/project/thermo-key/|url-status=dead|archive-url=https://web.archive.org/web/20090221094257/http://www.hc.t.u-tokyo.ac.jp/project/thermo-key|archive-date=21 February 2009|title=What is Thermo-Key?|publisher=[[University of Tokyo]]|access-date=21 January 2009}}</ref><ref>{{cite journal|title=Thermo-key: human region segmentation from video|journal=IEEE Computer Graphics and Applications|year=2004|volume=24|issue=1|pages=26–30|pmid=15384664|doi=10.1109/MCG.2004.1255805 |last1=Yasuda|first1=K. |last2=Naemura|first2=T. |last3=Harashima|first3=H. |s2cid=8378941}}</ref>

For ''[[Star Trek: The Next Generation]]'', an [[ultraviolet light]] matting process was proposed by Don Lee of CIS Hollywood and developed by [[Gary Hutzel]] and the staff of [[Image G]].  This involved a [[fluorescent]] orange backdrop which made it easier to generate a holdout [[Matte (filmmaking)|matte]], thus allowing the effects team to produce effects in a quarter of the time needed for other methods.<ref>{{Cite book |last1= Sternbach |first1= Rick |author-link1= Rick Sternbach |last2= Okuda |first2= Michael |author-link2= Michael Okuda |title= Star Trek: The Next Generation Technical Manual |year= 1991 |publisher= [[Pocket Books]] |isbn= 978-0-671-70427-8 |page= [https://archive.org/details/startreknextgene00ster/page/13 13] |title-link= Star Trek: The Next Generation Technical Manual }}</ref>

In principle, any type of still background can be used as a chroma key instead of a solid colour. First the background is captured without actors or other foreground elements; then the scene is recorded. The image of the background is used to cancel the background in the actual footage; for example in a digital image, each pixel will have a different chroma key. This is sometimes referred to as a ''difference matte''.<ref>{{cite book|first=Steve|last=Wright|title=Digital Compositing for Film and Video: Production Workflows and Techniques|url=https://books.google.com/books?id=n-xHDwAAQBAJ&q=%22difference+matte%22&pg=SA2-PA23|date=22 November 2017|publisher=Taylor & Francis|isbn=978-1-315-28399-9|page=27}}</ref> However, this makes it easy for objects to be accidentally removed if they happen to be similar to the background, or for the background to remain due to camera noise or if it happens to change slightly from the reference footage.<ref>{{cite book|first=Steve|last=Wright|title=Digital Compositing for Film and Video: Production Workflows and Techniques|url=https://books.google.com/books?id=n-xHDwAAQBAJ&q=%22difference+matte%22&pg=SA2-PA25|date=22 November 2017|publisher=Taylor & Francis|isbn=978-1-315-28399-9|page=29}}</ref> A background with a repeating pattern alleviates many of these issues, and can be less sensitive to wardrobe colour than solid-colour backdrops.<ref>{{cite conference | first1= Atsushi | last1=Yamashita | first2=Hiroki | last2=Agata | first3=Toru | last3=Kaneko | conference= 2008 19th International Conference on Pattern Recognition | title= Every color chromakey | doi = 10.1109/ICPR.2008.4761643 | pages= 1–4 | year=2008| isbn= 978-1-4244-2174-9 | s2cid= 1424110 }}</ref>

{{anchor|Magic pink}}
There is some use of the specific full-intensity [[magenta]] colour {{tt|#FF00FF}} in digital colour images to encode (1-bit) transparency; this is sometimes referred to as "magic pink".<ref>{{cite web | url = http://www.skinyourscreen.com/site/Articles/so-you-wanna-make-a-theme | title = So you wanna make a theme? | work = skinyourscreen.com articles | access-date = 23 August 2008 | url-status = dead | archive-url = https://web.archive.org/web/20071029042547/http://www.skinyourscreen.com/site/Articles/so-you-wanna-make-a-theme | archive-date = 29 October 2007 | df = dmy-all }}</ref> This is not a photographic technique, and the extraction of the foreground from the background is trivial.