Light mapping and normal mapping are the most successful shading techniques used in commercial games and applications today because they require only few resources and result in a significant increase in the quality of the rendered image. The problem combining the two methods is that light maps store irradiance information for only one normal direction, the geometric surface normal, and therefore cannot be evaluated using the normals stored in a normal map. To overcome this problem, the irradiance (i.e., the incoming radiance integrated over the hemisphere) has to be pre-calculated for all possible normal map directions at every sample point.
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