Widely employed in film and video motion picture production, compositing is the combining of visual elements from separate sources into single images, often to create the illusion that all those elements are parts of the same scene. (For compositing in graphic design and still photography, see Photomontage.) Live-action shooting for compositing is variously called “blue screen,” “green screen,” “chroma key,” and other names. Today, most though not all compositing is achieved through digital image manipulation. Pre-digital compositing techniques, however, go back as far as the trick films of Georges Méliès in the late 19th century; and some are still in use.
from: wikipedia.org