04 - Subsurface Scattering

An advanced CG effect to represent more natural reflections.

SUBSURFACE SCATTERING

Another lighting technique that's starting to cross over from film to real-time 3D is subsurface scattering. Subsurface scattering refers to the way in which translucent material responds when a beam of light is shone at it; some of the light is reflected by the surface, while some partly permeates the surface layer and is reflected at all sorts of strange angles inside the material. This is particularly important when rendering skin in films, as the nature of skin means that it needs subsurface scattering to prevent it from looking shiny and unrealistic. A great example of subsurface scattering in action is in Gollum's skin in 'The Lord of the Rings' films.

Subsurface scattering is a new area for games, but it isn't beyond the realms of possibility. When Nvidia launched the GeForce 8800 Ultra, it showed off the GPU's potential with its Human Head demo. Even Cinesite's Simon Maddocks describes the demo as 'tremendous'. You can download it from nzone to see it in action. In order to replicate a human head, the demo uses a standard 'sum of Gaussians' formulation of subsurface scattering and HDR lighting; the amount of off-screen texture rendering means that the GPU effectively has to process an incredible 1.2 billion pixels a second.

The demo is very demanding, and a little clunky even when running on a GeForce 8800 Ultra, but the level of detail is simply incredible for real-time rendering. Of course, it will be a few years before we see this level of lighting and detail when there are several game characters on-screen at the same time, but the Human Head demo shows that real-time subsurface scattering isn't impossible.

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