Support slippery ground

[idanarye]

[idanarye]

This may be implemented as part of #11.

[dror-g]

Hi, small problem that might be related to this - Rapier collider friction seems to be ignored / overridden.
If using rounded colliders like ball or a capsule on its side - entities with such colliders can climb on top one another.
Biggest effect on this behaviour is basis acceleration. the default of 60 gives snappy response, but is also strong enough to climb a rounded shape.
Setting the sensor to a shape, even rounded, rather than the default ray makes it even worse as the entity can walk past the "edge" of the rounded entity it's on even further.
When setting tilt_offset_angacl to a low value like 100 or 10, one entity will roll off another, but it still won't slide off as expected from a frictionless sphere.
Perhaps the problem is the StandingOn property detection?

Reducing acceleration is an easy workaround but characters will be not as snappy.
Of course one could avoid setting AI entities to walk to the same exact point (that's how I get "stacking"), or use a very tall collider perhaps...

But if there's a way to respect friction / slippery colliders that would be great.
Thanks!

[idanarye]

Rapier's collider friction is neither ignored nor overridden - it simply does not take effect to begin with, because the character's collider is floating above the ground, not touching it.

I don't have a clear design for this feature yet, but one thing is for certain - it'll have to involve reading the collider entity and the normal from the ray/shape cast, and deciding based on that. This means there are two big decisions to make:

1. Should Tnua read the physics backend's collider friction parameters?
2. Should this be done inside Tnua's systems, or inside the user control system (which also has access to the ray/shape cast data, and which can alter the TnuaBuiltinWalk basis using that information)?

I'm leaning toward not relying on the physics backend's collider friction parameters, and through implementing this in the user control system (and provide helpers, of course) - but I'm not sure what exactly I want it to pass to the basis...