lean mapping
We introduce Linear Efficient Antialiased Normal (LEAN) Mapping, a method for real-time filtering of specular highlights in bump and normal maps. The method evaluates bumps as part of a shading computation in the tangent space of the polygonal surface rather than in the tangent space of the individual bumps. By operating in a common tangent space, we are able to store information on the distribution of bump normals in a linearly-filterable form compatible with standard MIP and anisotropic filtering hardware. The necessary textures can be computed in a preprocess or generated in real-time on the GPU for time-varying normal maps. The method effectively captures the bloom in highlight shape as bumps become too small to see, and will even transform bump ridges into anisotropic shading. Unlike even more expensive methods, several layers can be combined cheaply during surface rendering, with per-pixel blending. Though the method is based on a modified Ward shading model, we show how to map between its parameters and those of a standard Blinn-Phong model for compatibility with existing art assets and pipelines, and demonstrate that both models produce equivalent results at the largest MIP levels.
shading models mip mapping bump maps texture filtering