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I'm a noob, sry if I use poor lingo.
I made a sphere using a particle system and it instances a little sphere building block. I then filled the sphere with my little blobs that I want to bounce around wildly inside.

to do this, I:

  • turned off gravity
  • rigid body for the blobs
  • collision modifier for the building block in my sphere particle system.

result: things bounce around as expected, but they fly right THROUGH the sphere. I can't seem to get the blobs to bounce off the sphere instead.

thanks in advance!

enter image description here

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2 Answers 2

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"Collision" is not for rigid bodies. It does not interact with rigid bodies whatsoever. It interacts with soft body and cloth physics only.

To make a collider for rigid bodies, that collider needs to have rigid body physics itself. If you want that collider to not be affected by gravity/collisions/etc, then that collider should either be set to passive (good performance, but not dynamic) or animated (not as good performance, but can be reoriented.)

In the case of the meshes you're showing, these would almost certainly have to be mesh collision (they are not convex) which means poor collision behavior-- lots of missed collision, really bad performance. There may even be some self-intersection, in which case mesh collision is going to have a hard time figuring out inside vs. outside. I would strongly recommend using non-rendering proxy bodies for the actual collision you need, so that you can use sphere or cylinder collision primitives, or some other primitive that roughly represents your rendering shapes.

I can't say how mesh collision interacts with particles-- not everything interacts right with particles, and I haven't tested this-- but if you're saying your particles are rigid bodies, then it sounds like you've already converted them to individual, un-instanced mesh objects, at which point they are no longer particles, so that doesn't matter. But I may be misunderstanding what you're saying, because "blob" and "sphere" seem pretty interchangeable to me, and I don't see any spheres in your image anyways.

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AFAIK, the collision modifier does not transfer to object instance particles: They have their own collision physics. Also, the collision modifier does not directly affect rigid bodies at all. I think what you should do is make a sphere with inverted normals to function as the wall for this system, give it a transparent material, set it as a rigid body, and set the rigid body type to “passive.”

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    $\begingroup$ hey that's a GREAT idea. Thank you! $\endgroup$
    – jhansen6
    Commented Mar 2, 2022 at 23:23
  • $\begingroup$ @jhansen6 I just hope the inverted normal thing works. I don’t remember. Anyway, you’re welcome :-) $\endgroup$
    – TheLabCat
    Commented Mar 2, 2022 at 23:28
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    $\begingroup$ I made the sphere. I went into edit mode and selected my sphere mesh. I went to mesh > normals > recalculate from inside. It looks like ti swapped because the insides of the sphere are orange now. However I run the animation and my objects still fly through! Do I need to apply it somehow after I flip normals? $\endgroup$
    – jhansen6
    Commented Mar 2, 2022 at 23:47
  • $\begingroup$ @jhansen6 No, you should not need to apply anything. I guess that doesn’t work… $\endgroup$
    – TheLabCat
    Commented Mar 2, 2022 at 23:48
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    $\begingroup$ I ended up making a bunch of planes instead in the rough shape of a circle. It worked! thanks for the idea. $\endgroup$
    – jhansen6
    Commented Mar 2, 2022 at 23:57

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