Improvements to the GTR BRDF

V-Ray 3.10 adds the possibility to use the generalized Trowbridge-Reitz (GTR) reflective BRDF in the V-Ray material (when its highlight tail falloff parameter becomes 2.0, the BRDF becomes equivalent to the GGX BRDF). However our initial implementation had a drawback where materials with low highlight tail falloff and high glossiness values would appear to “glow”, as shown in the left upper corner of the image below. This was because we used the same shadowing-masking function for all tail falloff values, even though it was really only valid when the falloff is 2.0. These materials not only tended to glow, but they were also noisier to render. Since these combinations turned out to be interesting for materials like cloth, we thought it would be good to fix this problem.

For the next service pack, we’ve improved the rendering of such material by computing a more accurate shadow-masking function for all possible values of the highlight tail falloff. This makes the BRDF more physically plausible and also reduces noise for such materials.