Computer Graphics World

APRIL 09

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n n n n Viewpoint was also a convenient approach to creating precise traveling mattes by tracing the outline of actors or moving objects. Rotoscoping is not only a very time-consuming task, but it also requires great animation skills. A poor and inconsistent hand-drawn mask is very disturbing to see on screen, as contours may move and flicker. Color keying, or chroma keying, introduced in the 1930s, is a well-known process for creating traveling mattes of moving actors or objects shot in front of a uniform blue or green background. e holdout matte for the foreground, or the "male matte," was chemi- cally developed by rerecording the frames on an optical printer using filters to block out the tinted background. Similarly, filtering out all non-blue colors created the female matte. Using these matte im- ages, the foreground and background layers could be composited together seamlessly. Alas, the bluescreen technique had several draw- backs: Actors were not allowed to wear blue clothes or makeup, and even so, there was noticeable color spill on the edges of the matte, in particular at semi-transparent regions, such as the hair. An alternative process, developed by the Walt Disney Company, was the sodium vapor process, whereby the actor—wearing any color—was filmed in front of a white screen. A strong sodium va- por lamp—which has a very narrow spectrum not picked up by the sensitive layers of standard color film—was used to light the scene. With a custom-made camera recording every frame on two separate filmstrips simultaneously, a second black-and-white film (sensitive to the sodium light) was then exposed. On this second film, a neat female matte was created. At the time of its use (during 1940–1960 in movies like e Birds or Mary Pop- pins), the sodium vapor process yielded cleaner results than bluescreen. Keying Concepts Today As technology advanced in the past several decades, greenscreen and bluescreen technology—and keying, in general—has been perfected in many ways. With custom materials (such as retro-reflective fab- rics), advanced lighting kits, and modern cameras, it is easier than ever to create an evenly lit screen for keying. Hardware and software solutions are now able to pull a perfect key by maintaining the origi- nal edge quality and removing color fringing/spill, even if strong film grain or video (especially DV) artifacts degrade the images. Rotoscoping is still a commonly used technique to deal with shots where no automatic keying/matting is possible. Current roto applications use a wide range of tools—including advanced free- form curves with variable softness, realistic motion blur, automatic tracking of planar shapes, and paint functionality—to make this still-laborious task easier and the resulting matte look better. Today's popular matte-creation tools, based on keying or skilled animators' frame-by-frame tracing, are essentially built upon the principles of the chemical and optical processes developed half a century ago. Matte creation still is, however, an actively researched field with fascinating new results. Natural image-matting algorithms are de- signed to extract a matte of an object (even a hairy one) in front of an arbitrarily textured background. To guide the process, one has to provide an initial segmentation of the image: foreground, background, and unknown regions. is input image, called the trimap, does not require significant time to create; sometimes even a few brushstrokes are sufficient. It is the software's job to figure out the transparency value for each pixel in the unknown region. Almost all the current natural-image matting techniques are based on the assumption that in each small image window near the object boundary, the color of both the foreground and the background is smooth. Hard edges on the input image are due to opacity change at the object's boundary. If this statement does not hold, the user has to manually adjust the matte to correct the poor areas of the resulting matte. us, by computing not only trans- parency, but also the "clean" image layers—free from any color spill—we can composite the extracted foreground object onto an- other, perhaps computer-generated, background seamlessly. ese new innovations are certainly finding their way into soft- ware. However, we have to wait a bit longer for easy and quick, high-quality matte generation for film-resolution image sequenc- es based on a few brushstrokes. Until then, color keying and ad- vanced tracking solutions may help us out. Having a nice matte for all our image layers, however, does not guarantee a perfectly realistic end result. It is the compositor's job to deal with the issues of non-uniform grain structure and video noise, differences in color, contrast, and exposure, or motion artifacts when merging images from different sources. Anyone interested in integrating rendered CG elements into film or video should not only learn the nuts and bolts of compositing, but also how to "pull a perfect matte" effectively." n Matte generation with a few brushstrokes (Levin, Lischinski, Weiss: "A Closed Form Solution to Natural Image Matting").
 April 2009

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