Chromakeying

The process of overlaying one video signal over another, the areas of overlay being defined by a specific range of color, or chrominance, on the foreground signal. For this to work reliably, the chrominance must have sufficient resolution, or bandwidth. PAL or NTSC coding systems restrict chroma bandwidth and so are of very limited use for making a chromakey which, for many years, was restricted to using live, RGB camera feeds. An objective of the ITU-R 601 digital sampling standard was to allow high quality chromakeying in post production. The 4:2:2 sampling system allowed
far greater bandwidth for chroma than PAL or NTSC and helped chromakeying, and the
whole business of layering, to thrive in post production. High signal quality is still
important and anything but very mild compression tends to result in keying errors
appearing--especially at DCT block boundaries. Chromakeying 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.

See also: Digital chromakeying.