Thanks polinalkrimizei, image inspired by fused's btw.
Zom-B: A quote from http://www.the123d.com/tutorial/general ... s2-1.shtml
I've seen a lot of different implementations of Oren-Nayar models, so it's hard for me to tell you some facts. Most of them do not use masking and self-shadowing and most of them also do not take different wave-lenght computations into account. Most of the time this model looks like a lambertian diffusor combinated with some darkening in the viewing direction and some backscattering.
So darkening in the wiewing direction seems to belong to the "global" model. I explain that to myself by laws of energy conservation, it's just the lambertian dark rims blending into the surface (that's the explanation to myself, nothing I can affirm really).