Dark spots in GGX renders with higher tail values

[Joelaff]

Any else ever see dark spots in materials using GGX BDRF with reflection? I have a surface making some dark spots (not noise, like black patches) when I have the tail value above 2.0. Oddly this same texture looked fine in another close up render, but this time it is rather small in frame.

Is this a known issue? Or should I try to send a sample scene? VRay 3.10.03 release.

[vlado]

What values are you using? I know that values above 4 or 5 can produce this effect and we need to clamp them probably.

In any case the GGX tail calculations will be refined a bit in the next official build to work somewhat more accurately.

Best regards,
Vlado

[Joelaff]

Initially it was 4.0. I tried lowering it to as low as 2.2 and it was still there. Black triangles (maybe, hard to tell since they are so small).

[vlado]

Alright, can you post that part of the scene or send it to me at vlado@chaosgroup.com ?

Best regards,
Vlado

[Joelaff]

Hmmm... I see now it is from what are likely negative values in the reflection gloss channel. I had a map that was in subtract mode in a composite map.

Is this what you mean by you need to clamp it? But an Output node set to clamp doesn't help. Will get you a scene.

[Joelaff]

Well, nevermind. It seems someone had a map set to use VRayHDRI's Cubic mapping rather than a standard UV map. At this scale that was causing larger black spots.

Nothing to see here. Move along. Sorry to waste your time.

[kosso_olli]

Actually I had the same problem in the last days. Using GGX produces negative pixel values in the reflection channel. There is no composite maps or VrayHDRI used. The tail value is between 2 and 2.5. Will strip down a scene and send it to Vlado.

[kosso_olli]

Scene was sent to Vlado with a link to this thread.

[vlado]

One of the glossiness textures becomes pure black, which is what causes the issue. This will be fixed for the next nightly/stable build.

Best regards,
Vlado

[kosso_olli]

Good to know, thanks a lot!

[kosso_olli]

Vlado,
did the fix already make it into one of the last official releases?

[vlado]

It's in the 3.20 stable builds; we will be uploading those on the website sometime next week.

Best regards,
Vlado