The 405

Early Compositing


All compositing involves the replacement of selected parts of an image with other material, usually, but not always, from another image. In the digital method of compositing, software commands designate a narrowly defined color as the part of an image to be replaced. Then every pixel within the designated color range is replaced by the software with a pixel from another image, aligned to appear as part of the original. For example, a TV weather person is recorded in front of a plain blue or green screen, while compositing software replaces only the designated blue or green color with weather maps.

Virtual sets are also used in motion pictures, some of which are photographed entirely in blue or green screen environments; for example, Sky Captain and the World of Tomorrow. More commonly, composited backgrounds are combined with sets – both full-size and models – and vehicles, furniture, and other physical objects that enhance the “reality” of the composited visuals. “Sets” of almost unlimited size can be created digitally because compositing software can take the blue or green color at the edges of a backing screen and extend it to fill the rest of the frame outside it. That way, subjects recorded in modest areas can be placed in large virtual vistas. Most common of all, perhaps, are set extensions: digital additions to actual performing environments.

In the film, Gladiator, for example, the arena and first tier seats of the Roman Coliseum were actually built, while the upper galleries (complete with moving spectators) were computer graphics, composited onto the image above the physical set. For motion pictures originally recorded on film, high-quality video conversions called “digital intermediates” are created to enable compositing and the other operations of computerized post production. Digital compositing is a form of matting, one of four basic compositing methods. The others are physical compositing, multiple exposure, and background projection.