Artifacts in Alpha when using Fog Color

[kosso_olli]

Hi,
I have a scene here that shows very weird banding artifacts when using fog color in the material. The geometry is looking correct (no flipped or double-sided faces), the shader is simple (refractive shader with fog color, no textures). It happens in any scene with that object, also on a simple test plane with a little thickness. Is this a known limitation?
Max 2021.1, V-Ray Next 3.2

[nikoleta.garkova]

Hi kosso_olli,

Would it be possible to attach a sample scene containing this object so we can investigate? Thanks.

[kosso_olli]

Sure, here you go. Just look at the alpha to see the issue.

fog_color_alpha_issue.zip

[matanov]

Hey Oliver,

I don't think the geo is fine. It's made from 3 different elements that don't have volumes and for the refraction to function properly it needs a proper closed volume. I'm attaching a screenshot with the elements just shifted -

[kosso_olli]

Yes, I know. But that is how we get the data, it has always been like that. The glass for the front and side windows for example is made in the same way, and it does not show this error.
For refractions, you do not need a closed volume, only a front- and backside with correctly facing normals. The little edge thickness of 3-5mm (depending on the manufacturer) can be ignored, because there will be rubber seals all around the outer edge.
Also, if you put a shelled plane next to it with the same material, you will also get the error, although not as strong. The pattern looks different, but there is banding.

[nikoleta.garkova]

This is caused by the material's fog parameter - the default behavior is like that when objects are either too close to each other or have a very thin shell. It works likewise in other render engines.

In this particular case, you can move the objects a bit farther from each other or if you only need to use the alpha, set the Refract's Affect channels option to Color only.