What is Chroma Keying ?
“Chroma” is the word used to describe color that a camera can record in technical terms. (As opposed to “Luma”, which is the word used to describe the light that a camera can record.) Keying is an old production term that refers to removing an object from a picture using a form of matte.
The processes of Chroma.
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".
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 a triple of 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
