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RBD initial frame interpenetration

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louis dunlevy
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Hi!

If you make a bunch of RBDs with say the RBD Point Object and they are all horribly interpenetrating right from the creation frame, is there a way of getting them to push each other apart? Just running the solver doesn't seem to work so I'm a bit baffled… looks like I'm missing a button!

Louis
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craig
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I assume you've tried setting the “Resolve Penetration” parameter in the RBD Solver to 1 or more…

I notice in the attached that it only partly works if you just display the solver. If you add gravity, then it works perfectly. I'm going to ask why this is…

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interpenetrate.hip (44.2 KB)

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louis dunlevy
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Yeah, look at this modified file though using points instancing

I've put Resolve Penetration up to 5 and left Gravity on etc etc but no banana…. objects are getting trapped within other objects.

Turning down SubContact Passes does improve matters but only partially.
L

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interpenetrate_instance.hip (142.8 KB)

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louis dunlevy
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I'm actually getting worse results on another sim with Resolve Penetration. Stuff jumping around as it tries to fix interpenetration.
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craig
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It turns out that for things like spheres, it is possible for the interpenetration to be resolved by allowing them to lie on top of each other. This has been logged as a bug, but in the meantime, here is a novel workaround. The key is to make sure that no points from one object can be inside the volume of another, and this solves that by adding points to the interior of each sphere.

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interpenetrate_instance_fix.hip (77.2 KB)

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louis dunlevy
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Thanks!

That's a novel soloution indeed. I do note however that if the “Resolve Penetration” parameter in the RBD Solver is set to a number lower than 3 it's quite unstable at the beginning. I've been having this behaviour in the middle of larger sims.

Good workaround though! I'll give it a whirl..
Louis
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craig
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I do note however that if the “Resolve Penetration” parameter in the RBD Solver is set to a number lower than 3 it's quite unstable at the beginning.

That's not surprising. When you push one object out of the way of another, you potentially create another intersection that wasn't there before. So you have to do another round, to get that new intersection sorted. But then - whoops! - I've moved it into a new object. Etc.

You can see this at the end of simulations too, where you have a pile of objects lying on top of each other. Each frame, they jostle each other around a bit, creating new work for the next frame…
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