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@syracoj THANK YOU for looking into this. Your results are incredibly promising after the disappointing news. Sorry about the time delayed response - European time here.
The ABC you supplied does work, albeit in strange directions. So we're getting vertex color data at least (and the naming doesn't matter, we can tell the mesh what color set holds the velocity data afterwards).
Sample rendering with and without:
above is the graph I used. Not sure if it's correct. We should probably improve the ways we send out data like this.
One thing to try is to set your frame range in to a negative value (-5) and set the start frame of the particle system to -5 as well. That way the motion color stabilizes by frame 1.
It looks like your motion blur effect could be dialed down. Can't remember but in vray is there a way to adjust motion vector source scale? Or perhaps just dialing down the motion blur length would do the trick.
So near, but no cigar quite yet.
Initial tests looked like it was working and that it just needed the velocity tuned down. Unfortunately there's still something not quite right here and I can't put my thumb on where it is. The vectors seem all wrong, but strangely "Almost" right. I know that doesn't help at all, but I've attached a couple of scene files here that will hopefully illustrate where it's going wrong.
Just in case the error was in the VRay I also tried it in Redshift (demo) and it's the same there..
Any clue as to where the velocity data is going wrong ?
We did a test here where we set the bifrost velocity to be constant along x with a value of 10 on a single particle falling under gravity. We then meshed this using bifrostGeoToMaya and exported an ABC w/ the color set. When we added it to Maya w/ Vray and rendered, the motion blur was consistently correct.
we did output velocities for a multi-particle particle system (using the same graph recipe from above) and the moBlur looks correct. We do see some artifacts when new particles are born but we aren't seeing the phenomenon where the motion blur streaks in the opposite direction. I talked to a dev here and he says this can occur with particle systems at birth though.
As far as per object velocity multiplier control is concerned I found one in vray that can be used to dial down the strength of the moBlur. See attached image.
We are tending to lean toward the idea that the velocities in bifrost are correct. We are not dismissing however that there is a potential problem with the Maya mesh that is generated especially when new particles are birthed. Do you absolutely need to use a blobby surface mesh like this? Could you use sphere instances?
Thank you again so much for looking into this Syracoj. It's reassuring to see the effort put into supporting end users. Massively appreciated.
Unfortunately I'm getting completely different results (see attached image) where the streams vary directionality long after birth, and in numerous/random places. We've tuned the velocity multiplier in a whole variety of ways, and the shutter speeds etc all to rule out wild values as the cuplrit. We've similarly done this with both VRay and Redshift to rule out the renderer as culprit. Unfortunately Arnold isn't an option (neither is Redshift at this point) but it was done just to be sure. Blobby meshes are essential to the type of projects we do very often - two of our clients business are "liquid" based industries (Drainage/Flood prevention and industrial Heat Exchange) where being at the mercy of classic fluid simulators are too precarious.
At this point I'm not sure where or how to continue. OpenVDB/SOuP is an avenue we can take but it's a pity, as Bifrost Graph is so close to being everything we needed with the security of future proofing our studio workflow.
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