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Hemisphere Lighting With Radiosity Maps Additional Considerations This form of lighting can easily be simplified, using cheaper versions to implement shader LOD on distant objects. Most significantly, the per-pixel radiosity lookups and sky color calculations can be replaced by a single CPU texture lookup at the center of the object, with the resulting sky and ground colors set as vertex shader constants, the hemisphere tween evaluated per vertex, and no work at all required in the pixel shader. Doing single-texel CPU lookups into the radiosity map is extremely fast, and this data can be useful in many places. For instance, a particle system might do a ground color lookup when spawning a new dust particle, to see if it should be in shadow and also so it can be tinted to match the hue of its surroundings. The radiosity maps can easily become very large, but they are also highly compressible. Large areas of the map will typically contain either flat color or smooth gradients, so good results can be obtained by splitting it into a grid of tiles and adjusting the resolution of each tile according to how much detail it contains. At runtime, a quick render to texture can expand the area around the camera back out into a continuous full resolution map. Because the radiosity map is only a two dimensional image, there will obviously be problems with environments that include multiple vertical levels. Such cases can be handled by splitting the world into layers, with a different radiosity map for each, but this lighting model is not well suited to landscapes with a great deal of vertical complexity. View Animation I have provided a sequence of videos that show the technique in action. These five videos (each approximately 7.5MB) show the same sequence of events with various parts of the lighting model highlighted in each clip. (Requires DivX player to view.)
Color Plates
References Philip Taylor (Microsoft Corporation) discusses hemisphere lighting: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dndrive/html/directx11192001.asp The non-approximate version: Image Based Lighting, Cunjie Zhu, University of Delaware: http://www.eecis.udel.edu/~czhu/IBL.pdf This
lighting model is used in the game MotoGP 2, on PC and Xbox,
developed by Climax and published by THQ. MotoGP 1 used a
similar radiosity technique, but without the Fresnel term on the
environment cubemap.
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