Opacity Maps and VDBs

[k_18]

Hi, we have an issue when using opacity maps in conjunction with environment fog and various VDBs. The EnvFog/VDBs render fine together - and so does the opacity map without VDBs (on a LgtMtl with the luminance map going into opacity with multiply by opacity set to on). But when both are visible at the same time the poly plane that the light material is mapped to becomes visible as if not fully transparent. What could be causing this? We tried upping the max transparency levels in the globals but that did not change anything.
Anything on the VDB side that we need to take care of?
Thanks for any help already!

[k_18]

This is to illustrate the problem: In the first image the card shows correctly with environment fog on but no VDBS loaded.
In the second image the VDBs are in and everything renders as expected.
In the third one both the cards as well as the VDBs are set to be visible - and this is where we have the issue: The geometry of the cards becomes visible.

[georgi.zhekov]

Hey,

Is your Volume Grid using Volumetric Geometry mode? If it using just the regular Volumetric option, try swtichting to Geometry mode for the Volume Grid and see if this helps.

[k_18]

If we set the volume to volumetric geometry it does seem to fix it - running a test right now. It does, however, tank the performance to such an extent, that we will need to revert to doing it in post if that is the only way of fixing it in Maya... Any obvious performance settings we might have missed here?

[georgi.zhekov]

Indeed, the geometry mode is quite slower to render, but would solve issues where env fog and volume grid volumes are mixed together. There are a few things that might help speeding things up - you can increase the Volume light cache in the Smoke color options - try something like 0.99 (note that this might produce render artifacts, so if some show up dial it down). You can also try lower values for the Probabilistic shading options in the V-Ray Overrides Volumetrics options. Note that for this option lower is not always faster, so try 1,5,10,15 - it really depends on the scene.

Finally you can also try and increae the step% and the shadow step% in the Volume Grid render options, but these might produce artifacts if you push them too much so use them carefully.

[k_18]

Thanks Georgi, will try. One question : Do I understand correctly that the env fog and volume grid volumes together cause this behaviour and not the opacity map on the card? In that case we might get away without the env fog in that scene...

[georgi.zhekov]

Maybe, can't tell for sure without the scene, but Volume Grid + Env fog won't produce correct result unless the Volume Grid is in Geometry mode.

Aah, another thing you can try as a workaround, that should probably render faster.

Create an empty Volume Grid (this will act as Env Fog), using the Grid rollout options you can specify how large the area to be covered by it.

Go to the Smoke opacity options, set the Based on Option to be based on Smoke and adjust the first curve point. Increasing its value will increase the density of the fog.

This way you can have 1 Volume Grid that will act as a fog and another one that will load the VDBs. These can overlap without the need to use Geometry mode and probably will render faster.

Attatching a screenshot with sample settings.

[k_18]

Thanks a lot, Georgi, will try it out tomorrow first thing.

[k_18]

Thank you so much for the suggested workaround, Georgi. This works like a charm and rendertimes are great!

[georgi.zhekov]

Awesome, glad it's all good now!