Keying
In
cryptography,
keying is the installation of
key material into a device.
In telecommunications, keying is a form of modulation where the modulating signal takes one of two values at all times. For example: "on" or "off", "mark" or "space". The name derives from the Morse code key used for telegraph signalling.
In graphics, keying is an informal term for compositing two full frame images together, by discriminating the visual information into values of color and light.
- A chroma key is the removal of a color from one image to reveal another "behind" it.
- A luma key similarly replaces color from an images which falls into a particular range of brightness. This technique is less controllable, but can be used on graphic elements.
- A matte key uses three images: the two images that will be composited, and a black-and-white third image that dictates the blending of the two, with white revealing one image, black the other, and grey revealing a blend of the two together.
Generally, the "bottom" image is called the
beauty, the image that appears on top is the
fill and the discriminating element (chroma, luma or matte) is called the
key or
matte.