Originally Posted by thev
There is currently no single analytic BRDF model that can accurately represent all real-world materials; the Cook-Torrance model comes fairly close, but it is not very convenient to work with from a sampling point of view. It would be most accurate to use measured BRDF data, but even then, a certain number of approximations and assumptions are typically involved.
Absolutely. All the work I've seen testing the validity of different BRDFs has been fitting to measured data (A&S came out top in the most recent paper I seem to remember) using multiple lobes. This isn't something a user's going to want to tweak.
What I mean is the BRDF models in maxwell and fry better represent conceptually what's really going on at the surface: where in nature is there a specular lobe plus a lambertian component? I find their model much more attractive for the purpose of artistically tuning shaders, although having said that, I've yet to see a realistic skin material from either of those renderers
EDIT: Just seen quite a nice one in the fry thread in general discussion. I stand corrected!