Dynamic gravity with avian3d

[ctkerns]

Currently Tnua with the avian3d backend uses the gravity resource from Avian implicitly. Changing the gravity resource dynamically can allow for some interesting results:

• Walking on spherical planets both large (Kerbal Space Program) and small (Super Mario Galaxy)
• Walking on walls
• Levitation
• Disabling gravity while in water for swimming controls

The drawback is that since gravity direction is a resource in avian, it is global, so gravity direction changes applied to the character apply to all objects. For some of the cases listed above, this is a complication.

A workaround is to disable gravity in avian for other rigid bodies and make gravity manually using ExternalForce. (Gravity in avian can be scaled per-object but not redirected). But thinking about Server-Authoritative multiplayer and server-side prediction, I think multiple controllers will need to be present in the same ECS World on the server to predict multiple players' movements seamlessly? Is that correct? In this case, the solution will not work.

Ultimately, I've failed to hack gravity for the Tnua controller in Avian. Applying a force will not change how Tnua views up internally.

Edit: It is probably possible to change gravity for each TnuaController directly in the TnuaRigidBodyTracker immediately after the Sensors stage of the pipeline. I will try this next.

[idanarye]

Tnua uses the external forces compnent itself, so whatever solution would need to work.

I think maybe a new component - TnuaOverrideGravity or something like this - that operates in three places:

1. In update_rigid_body_trackers_system it needs to update the tracker's gravity field from that component instead of the global resource.
2. In apply_motors_system it needs to update ExternalForce (after the regular motor sets it) to nullify the actual gravity and add its own gravity.
3. A new system that runs together with apply_motors_system but affects the entities that don't have a TnuaMotor where it does the same. This is so that objects that are not Tnua controlled characters can also have custom gravity.