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I've been playing with smoke and fluid simulators lately and I noticed that sometimes you can cache simulations while playing (Alt + A) the animation (but it only works at a very low fps) and sometimes you can't.

What is the expected behavior of this feature? I found it really useful for tuning properties, but sometimes the only way to see the simulation is by using the bake button instead of playing.

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2 Answers 2

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Caching is done with any type of physics (except for fluid simulation) while playing animation (files are being written to disk while animation is played) and everything will be cleared once any changes of settings are made. Note that you'll be able to edit settings of physics simulations on the fly, and previous cache will be cleared once animation playback started again (new values will be initialized). It will not start if animation playback was started after starting frame of simulation.
Because of amount of computations done in real time, caching will last longer and speed of animation might be quite slow.

Caching

  • As animation is played, each physics system writes each frame to disk, between the simulation start and end frames. These files are stored in folders with prefix blendcache, next to the blend-file. Note that for the cache to fill up, one has to start playback before or on the frame that the simulation starts.
  • The cache is cleared automatically on changes - but not on all changes, so it may be necessary to free it manually e.g. if you change a force field.
    Cache and Bake

Baking will pre-calculate animation taking into account every option set up and animation will not be changed unless bake freed or deleted. Once simulation is baked it is impossible to change most settings of objects used; meshes changes won't be taken into account and old results will be played instead.

However, no computations will be done in real time and animation playback will be faster.

Baking

  • The system is protected against changes after baking.
  • If the mesh is changed the simulation is not calculated anew.
    Cache and Bake

Note also that baking physics simulations can be different in options depending on type of simulation, so several settings will depend on what you bake.

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  • $\begingroup$ thank you for the in-depth answer. I believe that was exactly the problem: starting after the first frame of simulation and maybe making changes that won't trigger the refresh. $\endgroup$
    – arvere
    Jul 21, 2016 at 17:16
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Baking calculates every single frame, just like when rendering an animation. Caching is that it remembers the data so it can replay it. This means that caching is better for doing edits.

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  • $\begingroup$ ok, so I may have misunderstood both concepts. But still, I can't figure why sometimes it simulates when I hit alt+a and sometimes it doesn't.. $\endgroup$
    – arvere
    Jul 20, 2016 at 15:55

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