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The result of averaging or interpolating unit normals is less than unit length unless all normals within the footprint happen to be identical. Most algorithms simply renormalize, but this paper explores how the shortening can be used as a measure of normal variation to eliminate the common problem of strobing/sparkling noise due to aliasing of specular highlights.
A very inexpensive implementation that simply substitutes a power function with a 2D texture lookup is presented. Other applications of the technique, such as free gloss mapping, are also discussed.
With specular aliasing (left) and without specular aliasing (right)