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Weird behavior with particle collision
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System Information
Windows 7 + Linux

Blender Version
Broken: 2.69 + 2.70

Short description of error
Particles only collides correctly 1. time, next loop has weird wavy "collision" (attached blend + video)

Exact steps for others to reproduce the error
Setup basic particle emitter and collision plane, adjust so particles bounce off a little of the plane. Play animation and see correct/expected result. When the animation restarts the particle collision is wrong, and will stay that way until a setting (either particle or collision) is changed, then the collision is correct again, but only on the first play through. Baking particles does not work as it is not the expected result.



Event Timeline

Ejner Fergo (ejnersan) set Type to Bug.
Ejner Fergo (ejnersan) created this task.
Ejner Fergo (ejnersan) raised the priority of this task from to Needs Triage by Developer.
Lukas Toenne (lukastoenne) claimed this task.

This is an issue with the point cache interpolation. By default it uses a very coarse step minimum of 10 frames. It's supposed to store a higher density of samples if particles move more, but that doesn't seem to help much here. With colliding particles the interpolation generally doesn't work very well ...

Simple workaround for this is to set "Cache Step" to 1 in the Cache properties panel for particles. Ultimately we need to refactor the whole point cache anyway, but this should solve the problem for now.

Thanks Lukas, that was it.