area already exposed. The top layer contains the background scene, which is now exposed only in the areas protected during the previous pass. The result is a positive print of the combined background and foreground. A copy of this composite print yields a "dupe negative" that will replace the original foreground shot in the film's edited negative. Advantages of digital mattes Digital matting has replaced the traditional approach for two reasons. In the old system, the five separate strips of film (foreground and background originals, positive and negative mattes, and copy stock) could drift slightly out of registration, resulting in halos and other edge artifacts in the result. Done correctly, digital matting is perfect, down to the single-pixel level. Also, the final dupe negative was a "third generation" copy, and film loses quality each time it is copied. Digital images can be copied without quality loss. This means that multi-layer digital composites can easily be made. For example, models of a space station, a space ship, and a second space ship could be shot separately against blue screen, each "moving" differently. The individual shots could then be composited with one another, and finally with a star background. With pre-digital matting, the several extra passes through the optical printer would degrade the film quality and increase the probability of edge artifacts. Elements crossing behind or before one another would pose additional problems. See also Alpha compositing Broadcast designer Character generator Clean feed (TV) Compositing window manager Deep image compositing Derivative work Digital asset Digital compositing Digital on-screen graphic (Bug or DOG) Graphics coordinator Image stitching Matte painting Video matting Motion graphic design Multiple exposure Photographic mosaic Primatte chromakey technology References Further reading T. Porter and T. Duff, "Compositing Digital Images", Proceedings of SIGGRAPH '84, 18 (1984). Ron Brinkmann, The Art and Science of Digital Compositing () Steve Wright, Digital Compositing for Film and Video, Second Edition () American Cinematographer Manual, 2nd ed., Mascelli, Joseph V., A.S.C. and Miller, Arthur, A.S.C, eds. Los Angeles, 1966, p. 500 ff. External links http://www.videomaker.com/article/10914-green-screen-lighting Visual effects