I've got an animation which contains some force fields and particle systems. I'd like to pause the physics from time to time, allowing an animated camera to get a look at things from various angles.

Is it possible to pause the physics engine while other animations progress normally?

  • $\begingroup$ It would be particularly awesome to selectively pause the physics per object/particle system :) $\endgroup$ – ajwood Apr 8 '15 at 19:21
  • $\begingroup$ All particle systems? (no other kinds of physics sims?) $\endgroup$ – gandalf3 Apr 8 '15 at 19:40
  • $\begingroup$ I'd be happy to get it working with only particle systems. I can imagine wanting the effect for others as well though (especially smoke). $\endgroup$ – ajwood Apr 8 '15 at 19:43
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    $\begingroup$ One hack which I know works for smoke (and probably particles) is copying the cache file a bunch of times for the frame you want to pause, then renaming them so blender will read them as subsequent frames. Of course you'll have to rename the rest of the later frames to account for this. $\endgroup$ – gandalf3 Apr 8 '15 at 20:49

To the best of my knowledge you can't pause the playback of the particles via the physics system.

In order to accomplish this effect, you need to:

  • render the original particle sequence with the standard timing
  • render extra shots of frozen particles with Camera 2 and then edit the shots together in the VSE

In order to do that you need a way to freeze the particles in time so that you can render out the Camera 2 shots.

You can do this by setting up the particles to use a DupliObject, and then using the "Make Duplicates Real" command (CTRL+SHIFT+A) on the correct frames. I found it helpful to also parent the frozen particles to a new parent object.

Here's what I did to accomplish this.

  1. Set up your initial particle system and animation (I chose a 10 particle, 50 frame sequence for the test). You will need to set particle duplication to an object or group of objects. If you just want a small point, use a small sphere)

  2. You may want to bake your particle sim to disk and then switch your particle system to read from external to avoid the simulation changing.

  3. Move the timeline to the frame you want to freeze (let's choose frame 20)

  4. Select the particle emitter and use "Make Duplicates Real" command (CTRL+SHIFT+A)

  5. Snap cursor to emitter and create new object (empty, plane, etc). We'll call this one Frame20Parent.

  6. Hide the emitter. Select new particle duplicates, then parent them to the Frame20Parent object with CTRL+P (keep transformations)

  7. Now, you can render out frames from your Camera 2 animation with the particles frozen in place.

Screen shot of particle emitters and duplicates frozen at frame 20 and 30

Please note: I've spaced out the duplicate groups for clarity, but you would have to keep them in the same location as the original emitter object so the particle locations would match.

If this solution does not work for your project, please provide additional requirements and I can amend the answer.

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  • $\begingroup$ I haven't quite wrapped my head around this yet.. I guess a big trick is going to be getting camera 1 to jump around during the standard timing shot to the locations where camera 2 will be at the start/end of the frozen shots? $\endgroup$ – ajwood Apr 8 '15 at 21:17
  • $\begingroup$ Yes, you'd have to match camera positions if you want the edits to appear seamless. $\endgroup$ – Todd McIntosh Apr 8 '15 at 21:26
  • $\begingroup$ I think this will do the trick for me. It's hackish, but beggers can't be choosers! $\endgroup$ – ajwood Apr 8 '15 at 23:13
  • $\begingroup$ Actually, if you think about it, this is sort of the same as how the original "bullet time" used in The Matrix worked. They shot standard video and then for that one moment where everything freezes and the camera pans around the actor, they used a fixed array of still cameras, and then edited the two shots together. Same sort of idea. $\endgroup$ – Todd McIntosh Apr 9 '15 at 0:38
  • $\begingroup$ Fair enough.. although it still it should be feasible to have a "physics enabled" checkbox to animate :P $\endgroup$ – ajwood Apr 9 '15 at 0:42

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