Volumegrid Motion Blur issue w. VDB

[AsgerMortensen]

I have a project, that requires me to render out Houdini Pyro simulations in Vray Maya, and the Vray Volumegrid seems like the obvious choice. However, I am getting some very unexpected results from the produced motion blur.
The cache is a .vdb, and all the velocity data is read in correctly by the Vray Volumegrid in the channels menu, and displayed in the preview. I've attached photos from the vray render, as well as the Mantra render, to illustrate this (I'm only showing the alpha channel, as it best illustrates the issue I'm having). The Mantra render gives me the correct motion blur, while volumegrid gives me a kind of "ghosting" effect on the motion blur, with some obvious artifacts. It is at if the motion blur only effects a small portion of the smoke.
normally this wouldn't be a big issue with volumetrics, but these volumes are moving with very high velocities, and needs the motion blur.

The screengrabs are with and without motion blur from each render engine. It should be named accordingly

Thank you
Asger Mortensen

[Alvaro Letelier]

Hi. I had the same problem with Fire from Houdini rendered in Maya Vray. To get my Vel pass from the VDB I render it using Geometry option. But I quickly realize that it basically converts the whole voxels into geo so your vel pass will not work properly at the comp. So I just did the pass in Houdini scaling my cam and objects accordingly. I think Maya Vray still doesn't support proper Vel pass render with Vray VDB.

[Svetlin.Nikolov]

It would be best if you could attach a scene with a volume grid and a single cache.

Which version of V-Ray are you using? Does this still happen if you try the V-Ray Next beta?

Off the top of my head, it's possible that the velocity y and z channels are swapped, which you could swap manually from the 3rd party channel mapping dialogue, but of course it would be best if this could happen automatically.

Cheers!

[AsgerMortensen]

For some reason I am unable to upload attach cache files, even when zipped. It says *error uploading image*

I am using 3.60.03 for Maya. We do not have the V-ray Next beta installed.

Swapping the y and z channels didn't seem to fix it.

I could send you a wetransfer link with a cache file, if that's okay?

Thank you.

[Svetlin.Nikolov]

Sure, wetransfer works as well. I think we've fixed such an issue a few months back, so if the setup works well with V-Ray Next, will make sure to pick the fix to the V-Ray 3.6 stable nightlies so you can get it too.

Cheers!

[AsgerMortensen]

Here is the download link. Be warned! It is a very ugly sim
I've only given you a few frames of cache data. The frame I rendered for the screenshots was frame 1018.
Let me know if you encounter any issues with the files. I prepped this very quickly

https://we.tl/t-iNXVnjXHdG

[Svetlin.Nikolov]

Hey, got the scene, looking at it right now...

[AsgerMortensen]

Great! I'm very curious

[AsgerMortensen]

Any news on this?

[Svetlin.Nikolov]

Hey,

Negative, I keep getting interrupted with other tasks. At first sight it seems like you might need an experimental moblur algorithm I have stashed. I will expose it through a script parameter so you can try it out and ping you when it's in the stable nightlies, hopefully in a couple of days.

Cheers!

[AsgerMortensen]

Thank you so much, that is great!

[Svetlin.Nikolov]

Okay, I'm finally on it, I hope tomorrow I will be able to finish the change and merge it into the stable V-Ray 3 nightlies for Maya. Will update you when it's done.

Cheers!

[AsgerMortensen]

Thank you so much!

[Svetlin.Nikolov]

Hey, here is the progress so far - still not in the nightlies, but most of the work is done now so it shouldn't be much longer:



EDIT: The shot is from Phoenix for Max, but it would work the same in V-Ray for Maya as the Phoenix shader is shared.

[AsgerMortensen]

Looks great! Can't wait!