Is this a bug?

[josemendez]

Just as a sanity check, I updated Unity to 2020.1.5f1 and it still happens



This might be a bug, but at the same time - it could be a cool game mechanic. 

Not a “bug” as in an error in the code, rather just constraint fighting as a consequence of tunneling. This is similar to what happens when you use a dynamic attachment (pin) inside a collider: the collision constraint tries to get the particle outside, and the pin constraint takes it back inside. Neither constraint ever "wins", and the result is some dance moves . This is described in the manual for the very specific case of pins+collisions, but can actually happen to other constraint types.

In this scene , the barrels are surface softbodies (no particles filling their volume) so when they interpenetrate each other or a ball, they become interlocked and try to get outside of each other with little success because shape matching and collisions begin undoing each other's work: that’s why they jump around.

Any measure taken to prevent tunneling would eventually fix this: using more substeps, larger particles, etc. Using volume softbodies instead would also make this scene more robust if slightly more expensive, we might just do that in the future.