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Rendering with Arnold in CINEMA 4D using the C4DtoA plug-in.
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Arnold Unable to Render Specific X-Particles Emitters/Groups

Message 1 of 12
933 Views, 11 Replies

Arnold Unable to Render Specific X-Particles Emitters/Groups

Problem: I've created and cached a dynamic X-Particles granular fluid sim. When I attempt to render using Arnold, 3 of my 4 emitters render perfectly. The one emitter that will not render has all of the same properties, cache, and tags as the others.


Image: The white particles, emitter "Sand_CK", appear perfectly within the viewport, but do not render.

Background: Previously, I ran into this same problem and attributed it to my setup which switched particles from one group to another mid-simulation - when they switched to the new group, Arnold was unable to render that new group. I accepted this as a limitation and rebuilt my entire simulation (where particles maintain their emitter group), only to find a similar issue. Frustratingly, this was about a week worth of work.

Details: I'm running Windows 10, X-Particles 4.0 Build 824 (Beta), Cinema 4D R21, and C4DtoA 3.04. For reference, the cache took about 28 hours, contains around 4 million particles, and is about 127GB on disk.

Some troubleshooting I've attempted:

  • Viewport: Particles from the emitters show up flawlessly within the C4D viewport
  • Physical Renderer: Particles from the emitter render perfectly using C4D's Physical renderer
  • Older XP: I tried going back to the latest non-beta build of X-Particles, no change.
  • Older Arnold: Older versions of Arnold exhibit the same issue.
  • Older C4D: Opened the same document in R20, same issue.
  • Arnold GPU: Using Arnold GPU has no effect
  • X-P Tag: Virtually all combinations of tweaking/moving/removing the X-Particles Arnold tag has no effect. When switching settings, the IPR reacts instantly (which is odd when tweaking the parameters of over 2 million particles) I've specifically tried rendering geo, points, and adjusting the radius, no change.
  • Materials: Material applied to the emitter seems to have no effect, as Arnold never seems to acknowledge the emitter.
  • Picture Viewer: Picture viewer and IPR produce the same result
  • New Document: Copying the offending emitter, it's group, and the cache object to a new document exhibits the same results: perfect viewport, no render.

From what I can tell, the issue is that Arnold fails to acknowledge the particle emitter entirely - no combination of materials, tags, or settings changes the result because Arnold treats the emitter like it's a null.

Clue: The closest I've gotten to figuring this out is when messing with the X-Particles Groups. In theory, in a cached particle system, the groups should have zero effect on the emitters ability to render - in reality they're required and they act strangely. In my scene, all particles are emitted on frame 001 and were assigned to a group of the same name. Their group numbers are 1, 2, 3 and 5. Some things I've noticed on groups:

  • No Groups, No Render: If I disable the groups, none of the particles emitters render.
  • Groups Don't Correspond: Turning off groups does not line up with the particles which render. Turning off "Sand_Plus" will make it so that "Sand_Pattern" particles no longer renders, for example. Something strange under the hood with group ids?
  • Copy/Pasting Groups Changes Number: If you copy and paste the XP Group Object, it gets assigned a new name and a new Group Number.
  • Changing Number Doesn't Affect Render: Changing the group number of the cached particle system doesn't affect the render.

Any assistance would be very much appreciated. At this stage, I'm worried I might need to switch to a different renderer to finish the project.

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Message 2 of 12

What about a non-beta version of XParticles? Does that still have the problem?

// Stephen Blair
// Arnold Renderer Support
Message 3 of 12

Yep! I went back and tried the latest stable build of XP, same behavior.

In my troubleshooting, no combination of C4D/plugin versions seemed to have any effect on my issue.

Message 4 of 12

Can you make a test scene with just the one problem emitter? And a much smaller number of particles?

// Stephen Blair
// Arnold Renderer Support
Message 5 of 12

@Stephen Blair Thank you for your help!

For troubleshooting, I've created a minimal version of the scene which includes a single frame of the X-Particles Cache. I believe I packaged it up in a way that will work, with the XP Cache linking to the /sim folder. The particles will show up on frame 001, and I've got the larger emitters hidden from viewport for efficiency.

If my issue persists on other machines/setups, a render of frame 001 will look like this (issue being that white "Sand_CK" particles do not render):


Message 6 of 12

Thanks for the scene, I can take a look on Monday. I remember an issue with takes, where group numbers have changed after cloning the scene for the render. Maybe this is something similar, not sure.

Message 7 of 12

It's some kind of a mess with the group numbers as I expected. Trying to fix this.

Message 8 of 12

I've got a potential fix. If you have a valid maintenance, you can contact support(at)arnoldrenderer(dot)com and I can send you a build to test.

Message 9 of 12

Thank you so much for jumping on this! Sending an email now, would love to test it out.

Message 10 of 12


The test build of Arnold that you sent (C4DtoA- perfectly resolved my issue. I tested this both in my test scene and in the full scene file and all particles/emitters are rendering properly.

Side question: I'm far from an expert on X-Particles, so I don't 100% understand the functionality of groups, especially under the hood. If I didn't know any better, I would assume that once a scene has been cached, you could delete the groups and simply render using the cache+emitter. As a general rule, what about the groups makes them a requirement to render the cached emitter? Is there any reason that changing groups, for example, should prevent a render or affect how to textures or random seeds get applied?

Thank you for your help! Excited that I can finish this project out with Arnold.

Message 11 of 12

Glad to hear the fix is working fine. The cache does not contain any Arnold specific data. For instance if you use multiple groups, you can assign different shaders to each group and different shape parameters via Arnold tags (e.g. instancing, visibility etc.). That's why the plugin needs the xpGroup object in the scene when rendering, even if the sim is cached.

Message 12 of 12

The fix is released in C4DtoA

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