Keying is defining transparency by a particular color value or luminance value in an image. When you key out a value, all pixels that have colors or luminance values similar to that value become transparent.
Keying makes it easy to replace a background, which is especially useful when you work with objects too complex to mask easily. When you place a keyed layer over another layer, the result forms a composite, in which the background is visible wherever the keyed layer is transparent.
You often see composites made with keying techniques in movies, for example, when an actor appears to dangle from a helicopter or float in outer space. To create this effect, the actor is filmed in an appropriate position against a solid-color background screen. The background color is then keyed out and the scene with the actor is composited over a new background.
The technique of keying out a background of a consistent color is often called bluescreening or greenscreening, although you don’t have to use a blue or green screen; you can use any solid color for a background. Red screens are often used for shooting non-human objects, such as miniature models of cars and space ships. Magenta screens have been used for keying work in some feature films renowned for their visual effects. Other common terms for this kind of keying are color keying and chroma keying.
Difference keying works differently from color keying. Difference keying defines transparency with respect to a particular baseline background image. Instead of keying out a single-color screen, you can key out an arbitrary background. To use difference keying, you must have at least one frame that contains only the background; other frames are compared to this frame, and the background pixels are made transparent, leaving the foreground objects. Noise, grain, and other subtle variations can make difference keying very difficult to use in practice.