The process of overlaying one video signal over another, the areas of overlay being defined by a specific range of colour, or chrominance, on the background signal. For this to work reliably, the chrominance must have sufficient resolution, or bandwidth. PALorNTSC coding systems restrict chroma bandwidth and so are of very limited use for making a chroma key which, for many years, was restricted to using live, RGB camera feeds.

An objective of the ITU-R BT.601 and 709 digital sampling standards was to allow high quality chroma keying in post production. The 4:2:2 sampling system allowed far greater bandwidth for chroma than PAL or NTSC and helped chroma keying, and the whole business of layering, to thrive in post production. High signal quality is still important to derive good keys so some people favour using RGB (4:4:4) for keying – despite the additional storage requirements. Certainly anything but very mild compression tends to result in keying errors appearing – especially at DCT block boundaries.

Chroma keying techniques have continued to advance and use many refinements, to the point where totally convincing composites can be easily created. You can no longer ‘see the join’ and it may no longer be possible to distinguish between what is real and what is keyed.