[Enchancement] Implement per-collider properties

[MuscleChicken]

Currently, the physical properties of a rigid body (density, friction, restitution, collision layer) are tied to the body itself. It would be nice to have these properties available on the collision shapes so that bodies can be composed of multiple collision shapes with different properties.

One big advantage of rapier is that it supports per-collider properties https://docs.rs/rapier3d/latest/rapier3d/geometry/struct.Collider.html

In editor, this could be done through a RapierCollisionShape node which includes export fields for density, body skin, collision layer, and physics material.

[Ughuuu]

How would it work though in regards to the parent rb? Would it update the rb mass/density/friction/etc.?

[MuscleChicken]

I imagine the rb's friction/restitution would be unchanged, and the attached RapierCollisionShape would override them. In a collision for example, if a rb has a high friction, but the collider has a low friction, then the low friction would be applied.

In regard to the mass properties, maybe the mass/inertia of a child collider would be added on to the mass of the rb. For example, if the rb has a mass of 1kg with an attached RapierCollisionShape with a density of 1 kg/m^3 and volume of 1m^3, then the rb's mass would be 2kg.

But maybe you have a better idea.

[Ughuuu]

I wonder about this if godot has any proposal about it.

About the mass adding up, idk, sounds ok but i wonder what people would expect to happen. So mass of rb would be total mass (sum), not density (how it is now). Then, each collider has a mass of 1/count. But if you override mass of 1 ofa collider to 10, it adds 10 to total mass, but also subtracts 1/count that it already had.

About the friction, sounds quite odd, about it being override. Eg areas have multiple ways of setting friction that then the body can override. Same probl should be for colliders.

Either way, i would probably want a proposal on godot side first, to see if they ever do this, how they would do it so we aren't too far away.(eg maybe they instead go the density route?)