Collision detection between moving kinematic and rigid bodies is broken

[e344fde6bf]

Godot version:
3.2.3.stable

OS/device including version:
N/A

Issue description:

Collision detection between a moving rigid body and a kinematic body that uses move_and_collide() will fail to detect a collision if they both move towards the same position on the same frame. See also #46601 for collision detection between two moving kinematic bodies.

Steps to reproduce:

• Create a rigid body that will move towards the origin at 1 unit per a frame (vel=60 at 60Hz physics loop). Position it at x=-60 so at the end of physics frame 60 it will be positioned at the origin.
• Create a kinematic body that is positioned next to the origin. On frame 60 use move_and_collide() such that it will overlap the rigid body at the origin.
• Notice that no collision is detected by the kinematic body, and the rigid body does not react to the kinematic body and ends up overlapping it.

Minimal reproduction project:

test-rigid-kinematic-collision.zip

[Calinou]

I can confirm this on 3.2 08898e9 with both Bullet (MRP default) and GodotPhysics.

Bullet

GodotPhysics

[madmiraal]

Related to #30481 which is the same problem with two KinematicBodies.

[e344fde6bf]

I don't think this is a bug after all, instead it's a result of the update order of rigid bodies.

1. Rigid body positions updated
2. _physics_process() called for nodes

So rigid bodies _physics_process() essential operates on a 1-frame delay. So in the example provided, the kinematic body moves on frame n and sees no collision (because the rigid body is not there yet). The rigid body is stopped on frame n+1 and since its position gets updated at the start of the frame before _physics_process() is called, that's why it ends up being stopped on the kinematic body without the kinematic body seeing a collision.

[Calinou]

@e344fde6bf If this could be better documented, feel free to point out places in the manual or class reference where this could be done 🙂