Keying

Some video processors allow you to select a particular colour from one source input and superimpose another input source to replace than colour. So, for example, if you record a presentation with a blue background ("bluescreen" in the movies) you can then select this colour with the processor and have another source replace it.

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".

There is also a form of color keying that uses light spectrum invisible to human eye. Called Thermo-Key, it uses infrared as the key color, which would not be replaced by background image during postprocessing.