Chaos Scatter issue with VRayVelocity Render Element

[rocco_schult]

Hello All,
i think i have come across a bug when using Chaos Scatter with the VRayVelocity Render Element.
I had to plant a lot of trees along a road where the camera is traveling through. For maximum adaptability in comp, motionblur was supposed to be applied in comp via the VRayVelocity RE. However it seems as if the velocity value for all Chaos Scatter objects is multiplied by a whopping factor of ~1.200.
I created a simplified scene showing what i mean (images attached). The trees on the left are scattered with Chaos Scatter and the trees on the right are placed by hand.
As you can see, the vector motion blur completly freaks out. Reducing the Velocity chan mult for the instanced model object and/or the Chaos Scatter object did not work.
Rendering motion blur "in camera" works fine though.

We are using VRay 6 Hotfix 3 for 3ds Max 2022. I rendered out 16 bit half float .EXRs and applied the vector mb in blackmagic fusion.

Cheers & thanks in advance for you help!

[hermit.crab]

Could you send the scene so we can take a look? Mention the forum thread in the message.

[hermit.crab]

Thanks for the provided scene. The issue is reproducible - it seems there's a problem with ChaosScatter and the unclamped VRayVelocityRE. I've logged it (internal bug-tracker id: VMAX-12989) for developer investigation.

[rocco_schult]

Cool! Thank you!

[valentin_alek]

I'm having the same issue, how did you resolve that one ?

[Joelaff]

Since hermit.crab said it had to do with Unclamped velocity I wonder if you could increase your Max Velocity in the render element settings to keep all the velocity values between 0-1, and just increase the blur in the post. With 32bit float this should make no quality difference. Even 16bit int would give you 65536 blur levels.

More info here under Suggested Workflow:
https://docs.chaos.com/display/VMAX/...gestedWorkflow