VolumeGrid boundary box wrongly affects refraction in VRay GPU

[felix_brner]

Hello everyone!

I found some weird behavior of the VolumeGrid in GPU mode when combining it with some refractive surface i.e. a water surface. The boundary box of the grid is somehow affecting the appearance of the refractive material and casting shadows on the underlaying geometry, while it doesn't in CPU mode. (see image below).

However this error doesn't occur in GPU mode when the refractive object is turn off. The VolumeGrid then generates shadows as intended (and without the boundary box) on the underlaying geometry.

I attached a file including the VDB below.
Could you please have a look if I maybe set up something wrong, or - if it's really a bug - could you address this issue as soon as possible?

Thanks,
Felix

Btw: during LightCache generation everything looks fine (no error), but as soon as the calculation starts, the box appears. Maybe this will help you find the reason.

[georgi.zhekov]

Hey,

Thanks for the report. Indeed it is a bug and seems to be due to the Fog bias in the V-Ray material for the Water surface.
Will log it in our system. Meanwhile as a workaround you can set the Fog bias to 0 and use the Fog multiplier instead to control the color.

Cheers!

[felix_brner]

Hey,

Thank you georgi.zhekov for the immediate reply and the offered workaround!

Is it normal, that the Fog Color accepts only very low saturated colors (HSV) or else it will generate super dense R, G or B colors that are closest to the intended color? Or am I setting up something wrongly? (see the images attached below)
Or does it have something to do with the fact that I'm using just a surface instead of a closed volume mesh?

Another thing is, that the Fog Multiplier is limited (by the C4D Interface itself probably) to 0.001, which is - especially regards to the constrained Fog Bias - not very low. Maybe something like a Pre-Multiplier could be generally a good thing to add in this place.

Thanks,
Felix

Edit: It seems to appears only in the corners of the surface so dense. So it's working for me now. Thank you!

[georgi.zhekov]

The fog color depends on the absolute size of the objects and is therefore scene-dependent. It all depends on the scene, sometimes more saturated colors will work better when you have larger and thicker objects.

If you do use a closed object will have an impact as well.

Here is how a cube looks like in your scene with your material and the default fog multiplier of 1.

Cheers!