Soft body collision unstable

[blackears]

Tested versions

v4.5.1.stable.official [f62fdbd]

System information

Godot v4.5.1.stable - Windows 11 (build 26200) - Multi-window, 2 monitors - OpenGL 3 (Compatibility) - NVIDIA T1000 8GB (NVIDIA; 32.0.15.7344) - Intel(R) Core(TM) i7-14700 (28 threads) - 15.69 GiB memory

Issue description

I’m trying out soft bodied objects for the first time. I’ve set up a scene with a static box for the floor, a rigid body ball and two soft bodied objects. When either of my soft bodied objects hit the ground, they shoot off to the sides. I’ve tried adding a physics material to the floor, but this didn’t help.

Most of the values here are set to the defaults. Maybe I've just missed something, but can I get these to act more like squishy pillows? The simulation seems to be blowing up.

Steps to reproduce

Run the attached project. Soft bodies objects will shoot off to the sides.

Minimal reproduction project (MRP)

soft-body-test.zip

[jrouwe]

If you switch to Jolt Physics (Project / Project Settings... / Physics / 3D / Physics Engine / Jolt Physics) it works much better. In my experience the GodotPhysics soft body implementation has the tendency to explode.

Note that beyond setting the 'Pressure Coefficient' (try 100-1000) you don't have a lot of options to try to preserve the volume of these soft bodies. Pressure works with the box, but it doesn't work with the monkey as the mesh is not closed. For better volume preservation, Jolt's tetrahedral volume constraints would need to be exposed in godot.

[Calinou]

godotengine/godot-docs#11423 updates the soft body documentation to add a Jolt Physics recommendation.

[blackears]

Using Jolt works a lot better. The meshes still collapse into puddles, though, even if I use a pressure coefficient of 1000.

I edited the monkey mesh so now it is a manifold without holes:

soft-body-test.zip

[Calinou]

The meshes still collapse into puddles

You probably need to set Linear Coefficient to a higher value. Note that I've noticed that in general, soft body simulation can behave very differently according to the Physics Ticks per Second project setting. Higher values are not always more stable or consistent either.

[blackears]

Increasing the Linear Coefficient that helped keep the cube rigid, but the monkey still collapses. Preserving the tetrahedral volume sounds like it could be useful, so I'd be in favour of exposing it in the editor.

Here's an example with Jolt showing the monkey holding it's shape much better:
https://www.youtube.com/watch?v=MksMh-3RFYw