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April 2009 Making Mattes W hen integrating CG and real elements for visual ef- fects, the captured images often need to be segmented into foreground and background regions in order to achieve realistic occlusions. is kind of separation of pic- ture areas is usually defined by a matte, or mask image, where brightness corresponds to the transparency or the opaqueness of the given layer. If computer-generated objects are in the foreground, we simply need to let our render engine produce the masks, which are often stored as the fourth alpha channel. However, it is a bit more com- plicated to composite something behind the real elements. e creation of a proper matte image for photographs, video, or film is something artists and scientists have been struggling with since the birth of photography. In fact, compositing—the combi- nation of visual elements from different sources—is not a "digital" concept. Long before digital image processing, or even early video technology, filmmakers utilized the double-exposure (or multiple- exposure) technique. By exposing the film twice, images could be overlaid on top of each other. If only one part of the frame was recorded for the first pass, then by using a holdout matte, the film could be rewound and the blank, unexposed area could be record- ed with an inverse mask. If the separate elements were lit properly and the camera perspective matched, the resulting image would appear perfectly integrated. at kind of compositing did not need to be created "in-cam- era," since filmmakers could use optical printers during the post- production stage. ese machines could project film frames onto another filmstrip while applying different filters, pan and scan, or masks. rough the use of the multiple-exposure technique, not only could live-action frames be blended, but also miniature and painted scenes. Matte Magic Painted backgrounds, or matte paintings, were—and still are— used extensively to save production costs by negating the need to build or visit large, distant, or expensive sets. By limiting live ac- tion to occur in a well-defined window inside the picture frame, static mattes could be used. However, when moving objects were to be masked out, traveling mattes were needed. ese give film- makers more freedom but are harder to accomplish, especially with traditional chemical and optical techniques. e most laborious way to create traveling mattes was, and still is, through rotoscoping, invented in the early 1900s by Max Fleischer (Fleischer Studios). e original rotoscope was a piece of equipment that projected prerecorded film frames onto a glass panel, where an animator could precisely redraw its shape. While rotoscoping was an effective way to create realistic animated char- acters, like Disney's Snow White (1937) and Cinderella (1950), it PostProduction By GerGeley Vass Using the rotoscope, patented in 1917 by Max Fleischer, animators could trace the contours of real actors frame by frame. A former Maya TD and instructor, Gergely Vass eventually moved to the Image Science Team of Autodesk Media and Entertainment. Currently he is developing advanced postproduction tools for Colorfront in Hungary, one of Europe's lead- ing DI and post facilities. Vass can be reached at

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