Shallow angle reflection on ashik2 materials

[Nathaniel_Jones]

I’ve been working with the ashik2 material, and I’ve noticed that there is often a highlight where rays reach the material at shallow incidence, like this:

From the references I’ve found for Ashikhmin-Shirley shaders, it doesn’t seem as if the halo around the ball is expected.

For comparison, here’s the same ball rendered with a normal plastic material:

Might there be some issue with the way the ashik2 material is treating rays that hit the surface at a shallow angle?

Here are the material definitions from the above exampes:

void ashik2 mat1
4 1 0 0 .
0
8 0.179715112 0.188096672 0.189975708 0.015579238 0.017006337 0.016649794 145.7715107 145.7715107

void plastic mat2
0
0
5 0.182739195 0.192068015 0.200636119 0.015316636 0.108

[Greg_Ward]

The Ashikhmin-Shirley model includes a Fresnel coefficient that is always active. In contrast, the standard “plastic” and “metal” materials in Radiance only include this coefficient when the roughness is exactly 0.

This is somewhat of a design choice, as any significant surface roughness will tend to obscure grazing-angle Fresnel effects. This is why the default material leaves it off. We were trying to stay true to the published model for A-S.

Cheers,
-Greg