Xparticles Render + Vray Render Error

[folklore]

Hello there,
I render with the latest Vray version (6.20.02) and got an issue rendering an Xparticles scene.

The particles are actual geometry. it renders fine with the c4d internal render engine and also the animation in the viewport looks fine.
If I render with Vray, I get a strange render result.

Rendering a range f.e. 25-50 frame, it seems like killed particles still show up in the render and stay there as static geometry.
Rendering only 2 frames, stopping and rendering the next 2 frames and so on, shows the correct animation.

I uploaded 2 short videos, so you understand what I mean.

rendering frame 25-50 (particles are adding up)
https://youtu.be/b2NdnwNEc50

rendering only 2 frames and then another 2 frames and so on ...
https://youtu.be/y9BWzuII4U0

It doesnt matter if I built an xparticle cache or not. motion blur doesnt change a thing alswell.
It seems rendering in running oder is causing a problem, if I render frame 25,then 50, then 26, then 49, it renders fine.
Maybe someone got an idea?

[ivan.vardarov]

Hello,

Could you attach a simple c4d scene with the problem so we can investigate it.
If you prefer to do that privately you can use our contact support form - https://support.chaos.com/hc/en-us/requests/new.

Also what Cinema 4D version are you using?

[folklore]

I am using R21. I attached a simple file here.

[ivan.vardarov]

Thank you for the scene and your detailed report. I've reproduced the issue.

There are a few different things happening here.

- xpGenerator objects - internally they generate multi instance objects which we export as such. However these multi-instance objects don't have their instance ID / particle ID channel populated. When rendering motion blur V-Ray relies on these ID so it can track a given particle between 2 frames (to interpolate it's position etc). So I suggest using VRayParticles object instead - you can read more about it here - https://docs.chaos.com/display/VC4D/V-Ray+Particles - it's Geometry Instancing type is very similar to the xpGenerator object, but it will read the proper IDs channel directly from the Emitter. For rendering perfect spheres (the splashes) - you can also try V-Ray type or Phoenix type in splashes mode.
- Motion Blur export - when we render motion blur, we need information both for the current frame and for other frames (depending on the motion blur settings). During export happens something very similar to what would happen if you move the timeline slider back and forth - X-Particles generates different particles for the same frame. To avoid this problem - always cache the simulation before rendering a scene with motion blur.
- Currently there seems to be a problem with VRayParticles and the velocity channel coming from X-Particles when rendered in Cinema 4D. You can export a vrscene file and render it using V-Ray Standalone. We'll investigate further and try to fix it as soon as possible.