CharacterBody3D sliding along a wall with move_and_slide() has unexpected speed at certain angles

[p3tina]

Tested versions

Reproducible and tested in 4.2.1 stable, 4.3 dev 5, and 4.0 stable.

System information

Godot v4.2.1.stable - Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 1070 (NVIDIA; 31.0.15.5176) - Intel(R) Core(TM) i7-7700K CPU @ 4.20GHz (8 Threads)

Issue description

When a CharacterBody3D slides along a wall with move_and_slide() while rotating and its angle approaches parallelity with the wall, the body will reach an angle where it will first move at half speed and gradually speed up to double speed as the angle changes. It will return to the expected speed as soon as it stops colliding with the wall, or if the body turns back in the direction of the wall.

This issue occurs with both default physics and Jolt physics and regardless of the type of collision shape used on the CharacterBody3D, though the collision shapes do behave slightly differently.

Video of the issue being demonstrated with a cylinder collider on the body:

2024-04-09.10-23-43.mp4

And a video of the issue with a capsule collider, which is more jittery:

2024-04-09.10-30-17.capsule.mp4

Steps to reproduce

Run the test project and slide against the wall while rotating the camera very slightly.

Minimal reproduction project (MRP)

slide test.zip

[rburing]

Thanks for the MRP. This bug is probably somewhere in scene/3d/physics/character_body_3d.cpp.

I'm not very familiar with this higher-level code, but so far I notice at least one thing fishy: CharacterBody3D::move_and_slide resets collision_state before calling _move_and_slide_grounded:

godot/scene/3d/physics/character_body_3d.cpp

Line 84 in 6c57928

collision_state.state = 0;

but CharacterBody3D::_move_and_slide_grounded uses it as the previous state:

godot/scene/3d/physics/character_body_3d.cpp

Line 166 in 6c57928

CollisionState previous_state = collision_state;

so the following boolean can never be true (edit: on the first iteration, so maybe it's okay):

godot/scene/3d/physics/character_body_3d.cpp

Lines 268 to 270 in 6c57928

	// Stop all motion when a second wall is hit (unless sliding down or jumping),
	// in order to avoid jittering in corner cases.
	bool stop_all_motion = previous_state.wall && !vel_dir_facing_up;

If we change it so that collision_state is not reset before calling _move_and_slide_grounded then that boolean does get set to true in the MRP, but there is no "second wall" as referred to in the comment (we were just touching the same wall in the previous tick), so the logic seems to be incorrect.

This is not the whole story though, it will need to be investigated further.