Volume Grid + VDBs and object opacity issue

[seandunderdale]

Im trying to render objects in fog VDB volume grids, and Im finding when objects have full opacity, they still show up. Even if I turn off cast shadows etc, they still seem to be visible in the fog render. I also am finding that my particle snowflakes that have an opacity map on them to soften the edges, have opacity issues and you can see a black ring around them where they should show opacity. Do volume grids + VDBs have a known issue with using opacity in vray materials and light materials?

[hermit.crab]

If I understood correctly, you have some opacity-mapped particles that when intersecting a VolumeGrid lose their transparency? I tried something like that, but I can't seem to reproduce it. Could you send the scene? Also, when using VRayLightMtls make sure the "Multiply color by opacity" is active.

[seandunderdale]

Yep thats pretty much it...Ill send over a scene when I can.

[seandunderdale]

Here's a screen grab I could do quickly. Ill still send a scene maybe in jan, but this is just to show my issue.

This is the alpha channel of a particle snow sim, using the particle instancer, and volume grids and VDBs.

[seandunderdale]

Here is a zip file of the issue. Stripped the scene down, but this shows the two workflows I tried. Geo on camera left is a light source that was controlled on and off by opacity, but in a VDB it still shows up even when opacity is fully black and "multiply colour by opacity" is ticked. It slightly helps if you turn off "direct light" but the geo is still visible at full opacity. I ended up going with keyframing the visibility, which gives me less fading options.

[seandunderdale]

The same issue was basically happening with the snowflakes, where the opacity wasnt reading correctly, but becuase they were backlit in the animation, out of focus, or motion blurred, I got away with it.

[hermit.crab]

Thanks for the provided scene. Although I could not see the issue on your scene, I think I understood what was going on. You are probably confused about the fact that when the opacity is black, there is still light. This is on purpose. Quoting the docs: "Note that making the material less opaque does not affect the intensity of the self-illumination color. This is so that you can create perfectly transparent materials that nevertheless still emit light."

That being said, turning on the "Multiply Color by Opacity" parameter (while having a black opacity) would result in a black color as well (0 multiplied by color = 0), making the object invisible. However, if Direct Illumination is active that would result in a black light (which is not equivalent to VRayLights' invisible parameter) since the option makes the object act as a VRayMeshLight.

[seandunderdale]

Ok, here's some screen grabs from the scene I sent. I cant guarantee the scene is working as it is on my end.

I was not expecting to see anything rendering when the opacity was black, but I can see the geo in the VDB. Even if I turn colour to black, and multiplier to black, I can still see rendered geo in the VDB. If I turn the VDBs off, the geo disappears, as it should.

[seandunderdale]

And here are the renders when I hide the VDB. No geo visible, nothing in the alpha.

[hermit.crab]

Can you try increasing the Max Transparency Levels (Render Settings > Overrides > Materials) and see if that makes any difference?

[seandunderdale]

No changes. I tried a bunch of stuff, Turn off "clamp ray max" "secondary bias" "material max depth" "max transparency" "transparency cutoff". Im new to Volume Grids, but there doesnt seem to be a lot of rendering options to tweak, apart from quality / speed settings and colour / opacity.

[hermit.crab]

Which V-Ray build version do you have installed?

[seandunderdale]

V-Ray 6 for Maya, update 1.1

(v6.10.01 from Sep 27 2023)

[hermit.crab]

V-Ray 6 for Maya, update 1.1

(v6.10.01 from Sep 27 2023)

That's odd. I cannot reproduce the issue from the first screencap (black opacity, DI off) with the same version and without any modifications on the provided scene. The left object renders invisible as it should. Check the screenshots (overexposed for clarity).
If DI is on, the object does indeed show up, but we discussed why this is so earlier.

I load the .vdbs with "No preset". Which preset do you use? Have you modified anything in the VRayVolumeGrid?

[seandunderdale]

ok, very odd. Thanks for spending the time on it though. Makes sense about the mesh light / direct illumination option. I did find a work around to get the shot done, so if it is a quirk on my end, it didn't hold things up too long.

I have brought this up in another thread, but also wanted to know your perspective on the likelihood of Volume Grids getting VRay RT support anytime soon (They do render in RT,but any changes to values needs an RT restart to see). It'd make using these VDB assets so much nicer.