Compositing

Special effects for film or video are often created by shooting a scene against a color screen. After the footage is digitized, the color screen is then made transparent with a key. The first scene, or clip, is placed over a second clip, which usually includes some sort of background scene. The result forms a composite, where the background is visible wherever the first clip is transparent, making the first clip appear to belong with the background.

Composites are often used in feature films where, for example, an actor appears to dangle from a helicopter or appears to be floating in outer space. In this case, the actor is shot in an appropriate position against a color screen, and after making the color transparent, the actor's scene is superimposed over the backdrop. Color screens are usually blue or green because these colors are relatively absent from skin or hair color.

Superimposing and Compositing > About transparency > Compositing