motion blur with changing topology

[braveRabbit]

I am currently trying to get a nParticle based fluid mesh with changing topology rendered with motion blur but so far all fails.

The release notes for VRay 3.6 state that this is possible somehow but I cannot seem to find the right way.

Currently I am caching the meshed result to an alembic file.
"Velocity Per Vertex" is active for the particle shape node and the particle system has already been cached with subframes.
The export to the alembic file also has relative samples per frame checked on and when importing the resulting cache the mesh has animated subframes - working correctly.

Though rendering with motion blur doesn't do anything and also nothing when trying to output a velocity render pass.

I also tried to use the alembic cache as the source for a VRay proxy as well as exporting the fluid mesh as a VRay proxy mesh directly (with all the motion blur options enabled).

Obviously rendering a mesh with changing topology should work (according to the release notes) and it seems that I am missing some important step or setting.

Is there anyone able to shed some light onto this?

Thanks.

[shaio]

Hi,
I'll try to reproduce the issue. I suspect that this might be an issue with the alembic proxy. If possible sharing a scene would also be very helpful in checking the issue.

[braveRabbit]

Hi Ivan,

thanks for looking into this.

Here's a link to a Maya project with all the necessary files and two screenshots of settings

Here's the list of files and their info in the order of interest:

alembic_with_motionBlur.ma
vRayProxy_with_motionBlur.ma

These files each use the alembic file /cache/alembic/finalLiquid.abc.
The first uses the alembic directly, the other through a VRayProxy node.
The alembic just has been used and everything is at the default settings.
Motion blur is enabled as a render setting override.

The finalLiquid.abc has been created from the scene particle_setup.ma with the settings from alembicCacheSettings.png. Relative samples are created for quarter frames.

particle_setup.ma

This is the scene which has been used to create the alembic cache.
The particles are driven by another alembic file (/cache/alembic/meshSource.abc) and the particle blobbies output to a mesh.
The particle node has "Velocity per Vertex" enabled (and the vertex colors in the file alembic_with_motionBlur.ma show that it's working).
The particles have been cached with the settings from particleCacheSettings.png. Subsamples are set to quarter frames.

As mentioned before, maybe there is a step in the process which I am missing but that's the reason why I have all necessary files included.

Thanks in advance. Cheers.

[shaio]

Great, I'll take a look at the scene and will get back to you with some more information. Thank you for the feedback and the scene.

[shaio]

Hi,
I've reported the issue in our system and will let you know when it's fixed or if I find a workaround for the it.

[braveRabbit]

Thanks for looking into it and logging the issue.

In case you or anyone else is interested my current workaround is:
- with all particle attributes in place a particle cache is created with subsamples at quarter frames (0.25)
- the resulting cache is then used to create the blobby mesh
- "velocity per vertex" is enabled on the particle shape node
- the blobby mesh gets written to an alembic cache with relative samples at subframes (-0.25, 0.25) (also the color sets option must be enabled for the export in order to write the velocity information)
- when rendering the alembic mesh an extra texture render element with a user color node attached is used to render the velocity color per vertex to an openEXR channel (the user color is set to velocityPV - the name of the color set)
- use the velocity render element in nuke for a vector blur

Cheers,
Ingo

[shaio]

Hi,
you can put the colorSet (velocityPV in your case) name under the Motion Vector Color Set field inside the meshShape. Then if you increase the "Velocity multiplier" in the Alembic proxy paremeters" section of the vraymesh node then motion blur should became visible.

[braveRabbit]

Thanks Ivan, I will check that.