Editing Chroma key (section)

===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>