(Resolved) Rendered volume density affected by voxel-resolution

[Dan Andersen]

The rendered density of a volume is affect by the grid-resolution of the volume (vdb) being rendered.

This means we need to dial in a density multiplier per volume. And whenever a volume changes resolution, we need to compensate in the shader (PhxShaderSim) to maintain the rendered density.

In Karma this doesn't seem to be the case and the volume appears equally dense whatever the resolution - it's controlled solely by the shader.

Any way to get the behavior from Karma?

Also, AFAIK this is not exclusive to Solaris/Hydra VRay (I think it was the same in Maya), it just happens to be where I'm currently battling this issue.

[ronald_a]

Yes, this is also in normal vray (houdini). This really needs to be fixed!

[Gosho.Genchev]

Hey folks,

There is a hidden option that we have now exposed under the Smoke tab of the Volume shader - it's called "Scale Opacity by Scene Units". It will tell V-Ray to base the opacity of the smoke on the real world units of the scene rather than the voxel count - both high-res and low-res caches of the same simulation should render with about the same alpha when enabled. We will also update the documentation shortly.

Best regards!

[Dan Andersen]

Hey Gosho, that's fantastic news - thanks!

I have questions
- 1) Is the feature already there in existing VRay builds, but just not exposed in the UI?
- 2) If yes, can we enable it by manually adding the relevant attribute to PhxShaderSim plugins in vrscenes?
- 3) Is this exclusive to VRay 6, or does this hidden option exist in VRay 5 too?

[Gosho.Genchev]

Hey,

1. Yes, the parameter is simply hidden in older builds - you can unhide it from the Edit Parameter Interface dialog.
2. No need to do that - simply search for "Scale Opacity" (with the the "Show Invisible Parameters" toggle enabled) and unchech "Invisible".
3. No, it's available in V-Ray 5 as well

Note that enabling it will change the opacity right away - to get to the old result, multiply the Smoke Opacity by 100 to compensate for the unit scaling in the scene (Houdini defaults to meters).

Best regards!

[Dan Andersen]

That's super helpful, thank you