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Maniphest T88206

Rigid body "compound parent" child, not working
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Description

System Information
Operating system: Linux-5.4.0-72-generic-x86_64-with-glibc2.17 64 Bits
Graphics card: GeForce GTX 1060 6GB/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 460.73.01

Blender Version
Broken: version: 2.92.0, branch: master, commit date: 2021-02-24 16:25, hash: rB02948a2cab44

Short description of error
A low mesh object that is part of a "compound parent", behaves unpredictable when set to "mesh", instead of "convex hull", in Rigid Body panel "shape" option.

Exact steps for others to reproduce the error
Open attached .blend file and play animation to run simulation.

JAR compound.blend22 MBDownload

Best regards

Related Objects

Event Timeline

Alex Batalha (B4T) created this task.May 11 2021, 7:53 PM
Richard Antalik (ISS) added subscribers: Sebastian Parborg (zeddb), Richard Antalik (ISS).May 12 2021, 7:02 PM

This is documented that mesh collisions are more unstable than convex hull. See https://docs.blender.org/manual/en/dev/physics/rigid_body/properties/collisions.html#collisions

I don't think this is a bug.
From what I read in D5797, compound shape was meant allow you to parent cylinder object and sphere object to simulated object as cylinder and sphere are convex. your object is concave.

When I tried to create example file it immediately exploded, so I may be wrong.
@Sebastian Parborg (zeddb) Do I understand compound shape correctly? would you consider this a bug?

Sebastian Parborg (zeddb) closed this task as Archived.May 12 2021, 8:00 PM

@Richard Antalik (ISS) I don't think this is a bug from the information given, no.

As you pointed out mesh objects behave more unstable than other shapes and this example has small mesh shapes colliding with other mesh shapes.
So nearly a worst case scenario.

Compound shapes are as you also pointed out meant to combine multiple simpler shapes into a big one.
For example here you would use the "Sphere" and "Capsule" shape to create the lollipop ball and stick respectively.

In general you never want to have mesh objects collide with other mesh objects.

@Alex Batalha (B4T) Basically, if you want this classified as a bug, then you have to prove to me in someway that this is not a limitation of the physics library we use:
https://github.com/bulletphysics/bullet3

If you have cpp programming skills, then you could modify the examples files that comes with the bullet project and recreate this in there.
If you manage to get the same scenario working nicely in there with mesh on mesh collisions, then you have proven that blender is doing something wrong.