Smoke particle is baked and colliding correctly, but looks different in the render compared to the viewport

[Matthew Halls]

In my project I have tweaked the smoke effect to my liking, and performed a Bake as instructed in the video:

However, when I render the scene (either as an image or an animation), the smoke cloud seems to be a lot smaller than shown in the viewport:

Additionally, if I save the project then close and re-open it, the baked particle effects now resemble the rendered version instead of how they were previously baked:

Deleting the bake and re-baking it returns the particles to the previous style, but the render bug persists. Is this the result of a step that I’ve forgotten to do, or is Blender just playing up?

[spikeyxxx]

Hi Matthew, this looks like this: https://blog.cgcookie.com/forums/topic/just-to-clarify/

Don’t know what exactly is going on, but it seems to be related to the keyframing of the Lifetime of the Particles; somehow the Preview uses a different interpolation between the keyframes then the Render. Or something like that…

[Matthew Halls]

Based on your comment, I modified my scene to better test the issue and I think I’ve made some important observations. The following video has the particle Lifetime keyframed to 55 at frame 50 and 15 at frame 150, under a Constant interpolation type (please excuse the suddenly teleporting rocket at the end):

Note how in the viewport version you get a ‘ring’ of smoke at the transition, while in the rendered version the entire cloud shrinks immediately. This looks to me as though the simulated version of the effect adjusts only the Lifetime value of newly-created particles, while the baked/rendered version adjusts the values of all currently active particles as well.

Unless there’s an option somewhere to change this behaviour I think we’ve found a bug in Blender; the question is whether it’s a product of the baking function simulating the particles differently to the viewport simulation (in my opinion unlikely since the original version of the effect remains – and is made constant by the bake – until the file is closed and reloaded) or in the process by which Blender saves the cache data (I don’t personally know how Blender does this so I can’t say for certain, but I did find this bug report which suggests it’s had other issues with cache saving in the past, even if the exact situation isn’t quite the same).

[spikeyxxx]

I aggree with you that it is probably a bug. If you do not keyframe the Lifetime everything is fine. This might be based on the same issue as in the bug report you found or something completely different, I don’t know… is it a baking thing or a particle system problem?

The strange thing to me is, that most people don’t seem to have this problem!

The particle system is being re-written., so it might get solved ‘automatically’, but I will see if I can re-create the issue with a simple .blend file and report it as a bug.

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