"All that glitters has a high refractive index."
- anonymous

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"Special effects" is a relative term in computer graphics. In the film and television industry, today's new cool effect eventually becomes just another tool in the toolbox. Convincing hair and cloth are currently a hot property, done well at the high end, but will become mainstream for still image rendering (not real-time, yet) as the technology moves into commercial products. Image morphing techniques that were unique and exciting just a decade ago are now a standard part of children's shows. Once upon a time Gouraud shading, or even simple fill shading, was special.

The field of real-time rendering in many ways recapitulates the evolution of computer graphics for still images. As processors get faster, algorithms that took minutes now take fractions of a second. Graphics accelerators help perform operations that are widely used, such as filling triangles. However, because the new factor of dedicated graphics hardware is added to the mix, new ways of performing old algorithms arise. Multipass rendering (section 5.4) is a prime example. In traditional computer graphics the lighting equation is resolved at each pixel in a single pass, either by Gouraud interpolation or per-pixel shading. Software real-time rendering engines use this approach. Because graphics accelerators provide some basic operations that are extremely fast, an elaborate lighting equation can broken into separate pieces which hardware can handle and combine in a few passes, in less time than a single software pass can complete.

Special effects work for real-time rendering depends upon both classic computer graphics techniques and using available hardware acceleration to best effect. Algorithms leverage the existing triangle fill, filtered texturing, and transparency support. As new capabilities become available in hardware, new special effects become possible, or more general, or at least faster. A good example of a capability that is becoming standard is the stencil buffer. The stencil buffer is a special buffer that is not displayed (similar to how the Z-buffer is not displayed). Instead, it is normally used to mask areas of the screen off and make them unwritable. Primitives are written to the stencil buffer, and where these primitives appear in the buffer becomes the only areas of the screen where succeeding objects will be displayed. As will be seen in this chapter, techniques such as true reflections and shadows can be done more rapidly when a stencil buffer is available in hardware. There are many other operations that can be aided by a stencil buffer, such as capping objects cut by an arbitrary clipping plane [10] and visualizing models formed directly by adding and subtracting solids [10,15].

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