On the Accuracy of Surface Scattering Theories
Abstract
Surface scattering models are typically validated by fitting measured reflectance data. However, predictive rendering requires deriving appearance directly from surface geometry. We test the predictive accuracy of Microfacet and Generalized Harvey-Shack (GHS) theories by comparing renderings derived strictly from measured surface profiles (AFM/profilometry) against photographs of metal samples. We demonstrate that no current model succeeds across all angles; specifically, Microfacet theory fails at grazing angles where optical roughness vanishes. Additionally, we analytically link the Trowbridge-Reitz (GGX) distribution to the K-correlation PSD. This suggests that the popular “GGX look” arises from wave-optical effects on Gaussian (i.e. Beckmann NDF) surfaces, rather than suggesting an underlying non-Gaussian geometry.
