Keying

The process of compositing multiple images together. There are multiple different forms of keying. The most well-known is chroma-keying (“blue screen”, “green screen”). In broadcast, typically various forms of alpha-keying are used. In alpha-keying, a so-called Fill signal is superimposed onto the so-called Background signal. A third signal, the so-called Key signal determines whether the Fill, the Background, or a mix of the two is visible: the Key is a gray-scale image where all the White parts result in the Fill being shown in the result, Black parts result in the Background being shown in the result, and Gray values produce a mix of Background and Fill, allowing for smooth edges where the Fill dissolves into the Background.