Incorrect inertia with custom center of mass

[Lemonymous]

Godot version

v4.0.stable.mono.official [92bee43ad]

System information

Windows 10, Forward+, NVIDIA GeForce GTX 1050 Ti (31.0.15.1694)

Issue description

When a RigidBody2D has center_of_mass_mode = RigidBody2D.CENTER_OF_MASS_MODE_CUSTOM, and a custom center_of_mass as well as inertia set to 0; the rigid body's auto computed inertia can be incorrect.

A video of the attached mrp - Both boxes have the same size polygon, and inertia set to 0. So their inertia should be calculated to the same amount. However, the right box rotates slower.

https://user-images.githubusercontent.com/10490534/222751892-74e9f5da-ce99-4f81-8e8b-247e71447bcd.mp4

Steps to reproduce

• Create a RigidBody2D
• Add a CollisionPolygon2D with a polygon not centered around 0,0 - i.e vertices: (100, 100), (200, 100), (200, 200), (100, 200)
• Set the RigidBody2D's center_of_mass_mode = RigidBody2D.CENTER_OF_MASS_MODE_CUSTOM
• Set center_of_mass to some non 0,0 Vector2. (150, 150) in this example.
• Leave inertia at 0
• either
  • Print out the rigid body's inertia with 1.0 / PhysicsServer2D.body_get_direct_state(body.get_rid()).inverse_inertia)
  • Collide the object with another object
• Observe that the inertia is higher than if the same polygon was constructed around 0,0, and that the rigid body rotates much slower when colliding, than a body with the same polygon constructed around 0,0.

Minimal reproduction project

This project shows two boxes with the same size polygon side by side. The left box's polygon is constructed around 0,0 The right box's polygon is constructed around 150,150

• Unpause the game with space to cause the boxes to fall and collide with another box.
• Observe that the right box will rotate much slower, and that the printed out inertia for the right box is much higher than the left box.

test center of mass.zip

[Lemonymous]

Does this have something to do with it? https://github.com/godotengine/godot/blob/61d2c855114c824f5ca27ded0a1fa71cc7b21134/servers/physics_2d/godot_body_2d.cpp#L70

Should it not check if the body has a custom center of mass?

[Lemonymous]

Possibly related to issue #55148

[EGA-SUPREMO]

This can be troublesome, my game features a destructible terrain, and when a new polygon is divided, the function Geometry2D.clip_polygons() returns a polygon with a high offset, this can be workaround with transform2d or something, but still isnt ideal