(L) [2014/01/16] [tby shiqiu1105] [Re: Anybody tried implementing Disney's Principled BRDF?] Wayback!>> ypoissant wrote:You could select 50/50 between the coating or the base BRDF for reflection using a Russian Roulette. Or you could use a Russian Roulette on the coating microfacet Fresnel reflectance. Both are good. Personally, I select based on the coating Fresnel reflectance.
Just wanna make sure, I was asking about importance sample a single microfacet BRDF, not a layered brdf consists of two microfacets.
In a single microfacet brdf, there is a diffuse component, plus the microface specular component that includes F, D and G.
So when sampling such brdf, do I importance sample the specular D only, or both the diffuse component and the specular D?
(L) [2014/01/16] [tby ypoissant] [Re: Anybody tried implementing Disney's Principled BRDF?] Wayback!It is the same idea. The two components, diffuse and specular really refer to the base material and the coating material respectively. In traditional 3D applications, the diffuse and specular are combined together among a set of surface properties. But this just hides the fact that we are really refering to two different BRDF except that in this case, the base material is always perfectly diffuse. In other words, the system does not expose any parameters to adjust the roughness of the base BRDF. It is frozen into a diffuse state. This is an important limitation.
Anyway, You have to think of specular and diffuse as two BRDF. Whatever energy is reflected by the specular component is not available to light the diffuse component. You importance sample this as I exlained previously using a Russian Roulette to choose one or the other either based on fixed probability or based on the Fresnel reflectance of the specular component.