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Cubemap environment lighting+anisotropic surfaces

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I use HDR cubemap-based environment lighting throughout my projects. I use ATI's CubeMapGen to filter cubemaps to create diffuse and glossy cubemaps from the usual ones used for specular reflection. However, I've been wondering how to do anisotropic materials. The problem is that if I filter the cubemap with anisotropy in a particular direction, when the object to which this environment map is applied is rotated, the direction of anisotropy would change on the object and stay with the world. I'm looking for ideas as to how to approach this. The only idea I've had is to have cubemaps filtered using a sampling of the possible directions, then, in the shader interpolate between the two closest directions. However, this will only work if the anisotropy is low and overall glossiness is low, otherwise a lot of cubemaps would be needed to provide a sufficient sampling...

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You should be able to sample anisotropically from the cube map by specifying derivatives when you do the lookup. That way you could prefilter isotropically and do the anisotropic part at runtime.

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