A New Coefficient for a Two-Scale Microfacet Reflectance Model

Reflectance properties express how objects in a virtual scene interact with light; they control the appearance of the object: whether it looks shiny or not, whether it has a metallic or plastic appearance. Having a good reflectance model is essential for the production of photorealistic pictures. A considerate reflectance model needs to consider both specular peak and wavelength dependency. So a model combining reflection and diffraction is a reasonable idea. Holzschuch and Pacanowski proposed a two-scale microfacet model combining reflection and diffraction. However, the coefficient that connects the two parts of this model is not very physically-based since reflection part gets close to zero for a wide angle range while the surface roughness is excessive. In this paper, we design a new coefficient which controls the two parts of this model in a more reasonable range. As a result, the improved model produces a good approximation to measured reflectance. Moreover, we compute the integral of the diffraction part with higher efficiency by using Monte Carlo integration. Finally we use piecewise sampling to fit the parameters, and we can get model parameters faster.