Emitter particles glitch on some frames (no physics involved)

I have a problem with particle system that developed seemingly randomly (everything worked ok before). I have a ground mesh with no modifiers that I generate grass particles from. Emitter type, no physics, live and static during whole animation (created at first frame). Particles are chosen from group with pick random and use count. For distribution density I use vertex group of emitter object. Particle count is 200 000.

Problem is that when rendering the animation, particle locations glitch on some random frames, lets say about every 20th frame is glitchy. It seems that some particles remain in the same position and some are distributed differently. Rendering the scene from another angle produces glitching particles on different random frames. Re-rendering the exact same file fixes the problem for some frames, others produce glitch with about the same frequency. I render on Amazon EC2 linux vm-s, Blender version 2.79.

Has anyone had similar problem and can suggest any solutions?

Looks like a bug to me, same file must produce same result but doesn’t. My current guess is that the amount of particles produces some undefined behavior, previously I used 100K ones and that didn’t glitch. I’ll try changing the amount and re-rendering to see if problem disappears. EDIT: it didn’t help, glitched maybe even more.

I feel like I’ve heard this one before, try searching forum, maybe it had to do with mismatched blender versions on local/Amazon. What happens locally only, low res, low sample?

Ok, thanks for the idea, I’ll see of I can dig something out. First quick searches didn’t give me threads related to similar problems.

Blender versions do not march, thats true, on AWS I had 2.79a-rc and locally 2.79. I’ll try to make new AMI and see if matching versions remove the problem.

Local rendering is kind of difficult because the whole scene is very memory hungry, I run out of RAM easily even on cpu render (on 32GB system). I haven’t tried yet if glitch is produced locally in stripped down scene, I’ll try to do that.

EDIT: updating Blender version and creating new AMI didn’t help, still glitches. I’ll now try to break down the grass object, maybe it has something to do with complexity. I have other grass emitters in the scene that have the same parameters but emit smaller amounts of particles, these don’t glitch.

EDIT: breaking the object into smaller parts didn’t help when particle system is the same on all pieces, glitches on practically every frame. Next I’ll try to use unique particle system for each grass emitter part.

So, I have tried all of these:

  • new Blender version;
  • smaller amount of particles;
  • breaking object into pieces;
  • give each piece separate unique particle system;

None of these helped, more pieces and smaller amount of particles gave way more glitching. So currently it definitely looks like a bug, but I’m still not sure what circumstances cause it. Because some particles remain in the same place it might be the pick random function and/or use count or their combination.

EDIT: nope, it isn’t the pick random and use count either. Still glitches.

You need to strip it down and test locally. You must determine if this is a blender bug, or an Amazon config problem.

Looks like I found the problem area and temporary solution: particles rendered from group glitch, particles from single object do not.

I’ll see if I can reproduce the problem locally.

Hmm, I kind of doubt you can reproduce it locally, only because I use group all the time. But if you can it would be a bug.

I can now pretty surely confirm (after re-rendering a few glitchy shots) that object type particles don’t produce this glitch what group based ones did.

I will put up a local render today and see what comes out.

EDIT: local render did not produce glitch during 100 frames, so it seems like problem related to either Amazon EC2 ubuntu linux instance or that blender linux version. As object type particles don’t glitch it probably isn’t related to random scatter in general. Maybe something related to how objects in group are handled… but really, I’m out of ideas. Good thing that object particles seem to work correctly so if anyone sees this problem, try object particles as a workaround.

Photox, thank you for your thoughts!