Weird Sphere Movement in Bullet Physics

LewsTh3R1n

Hi all,

I have been testing out Godot and it has been great so far. I am trying to make a simple project where the user balances a ball on a board. The ball moves according to the tilt of the board (player controls the board).

The ball movement is fine when I use Godot physics. But something weird happens when I enable Bullet physics and the ball moves with a lot of stutter and even seems to teleport small distances. With Bullet With Godot Physics

Godot versions tested: 3.2.3.stable.flathub, 3.2.3.stable.official

I have been scratching my head over this for a while now - it must be something simple, but I am sorry, I just can't figure it out. Would appreciate any help.

Thanks :)

Calinou

Try upgrading to Godot 3.2.4rc3 after making a backup of your project; some physics bugs have been fixed there.

LewsTh3R1n

Thank you for the suggestion. I tried that just now - still no luck.

ball moving weirdly I made the ball much smaller to exaggerate the effect.

TwistedTwigleg

How big is the ball, what node(s) are the green plane, and how are you rotating the plane? My hunch is what is happening is collision tunneling, where the plane moves and the ball goes fast enough (or its small enough) that it doesn't see the plane until the ball is inside it, causing the physics issue.

LewsTh3R1n

The Ball is a RigidBody and the plane is a KinematicBody. Both of these nodes "have" a CollisionShape and a CSGSphere as children.

how are you rotating the plane? I am rotating it based on an input angle using scene.rotate_object_local()

I have tried the ball at various sizes and the issue does become much more pronounced at very small sizes. At bigger sizes, it seems, the issue occurs only when the plane is moved.

In GodotPhysics, this issue does not happen. There is a different issue of the ball not starting at all and sometime clipping through the plane in GodotPhysics. (... the clipping issue happens in Bullet as well).

I will look more into collision tunneling. Thanks for the pointer.

LewsTh3R1n

So, I was doing something stupid... :/

The RigidBody's (ball's) coordinate system was off-center from that of its children. So, the collisionshape and the rigidbody's positions did not coincide.

It seems to work better now after fixing this.

Wolfe

@LewsTh3R1n said: So, I was doing something stupid... :/

The RigidBody's (ball's) coordinate system was off-center from that of its children. So, the collisionshape and the rigidbody's positions did not coincide.

It seems to work better now after fixing this. It's awfully easy to inadvertently move or rotate a node accidentally just by clicking in the scene preview when you think you're on the mouse pointer tool or think you have nothing selected. It's made me kind of paranoid about double-checking transforms that are supposed to be (0, 0, 0) up and down my scene trees. You're not the only one. :)

Megalomaniak

Do note that you can lock the selectability of a nodes children from the viewport header. Might help.