Radiosity normal map

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Lighting can be baked as a set of three light maps, storing the lighting vectors instead of just the brightness/color. This allows the surface normal maps to receive directional lighting so the bumps/recesses are lit more accurately by the baked lighting information.

Valve used this method extensively in Half-Life 2, which they called "Radiosity Normal Mapping" in their GDC 2004 paper Half-Life®2 / Valve Source™ Shading.

Three light maps are used for radiosity normal mapping. Here they're shown projected on geometry. Image by Valve Software.

A further optimization of this technique was used by Insomniac Games for their game Resistance: Fall of Man. The directions of the light rays were combined into a single RGB bitmap, using a kind of normal map, where each color channel was used to store one of the three directional components.

All three RGB channels are used to store the light vectors, so the light maps could not also store the colors of the lights. Apparently they used vertex color to store the light colors.

A directional light map used in Resistance: Fall of Man, from the article Creating the Lighting for Resistance: Fall of Man. Image by Eric Gooch.

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