RigidBody in an Area node with gravity point increases it's speed continuously

[udit]

Godot version: 3.1.1

Issue description:
Expected behaviour : A RigidBody inside an Area node with gravity and gravity_point enabled, when not influenced by any other force apart from gravity of the area should not gain momemtum/speed over time.

Current behaviour: RigidBody's speed keeps increasing over time.

Steps to reproduce:

1. In Project Settings -> General -> Physics -> 3d, set
   Default Gravity = 0,
   Default Gravity Vector = 0,0,0
   Default Linear Damp = 0
   Default Angular Damp = 0

2. Add Spatial node and add Area node as a child, and set
   Gravity Point = on
   Gravity Vector = 0,0,0
   Gravity = 100
   Linear Damp = 0
   Angular Damp = 0

3. Add CollisionShape as a child of Area
   Set shape as New SphereShape and increase it's radius to a sufficient size like 100

4. Create a new scene and add RigidBody and add a CollisionShape and MeshInstance as childs
   Move the body away from 0,0,0 so it starts orbiting the gravity point.

5. Add a camera to the scene

6. Optional - add script to RigidBody to print velocity to confirm.

Minimal reproduction project: GravityBug.zip

Preview

[harvzor]

Reproduced with the supplied project on 3.2.1.

I'm also having issues with 2D gravity points and orbits not being symmetrical.

[udit]

@harvzor Can you elaborate on orbits not being symmetrical (probably with a minimal reproduction project)? I was working on a 2D project and 2D gravity point physics worked as expected.

[harvzor]

I'm still trying to figure this out.

The only think I've found is that setting gravity_distance_scale to 0.001 causes causes my orbit to (very) slowly get smaller and smaller.

I still have another issue with orbits not being symmetrical. I stripped my project down until the bug was no longer present so I know there's an issue with how I set things up.

[JestemStefan]

I tested provided project and:
Test1: with Area LinearDamp back to default value 0.1 there is no bug. (The rigidbody have stable orbit/amplitude over 15 minutes)

Test2: With any value below 0.1 the bug is present (Tested with 0.099)

Test3: With value over 0.1 (like 0.2, 0.3, 0.4) there is no bug again

So it seems that bug is related to Damping

[Geometror]

This bug is related to Bullet. Switchting to GodotPhysics fixes the problem,
the amplitude increases only very slightly over time (on the 6th decimal place) presumably due to floating point error.
As Bullet will be removed in Godot 4.0 I think this issue can be closed.

[harvzor]

I've managed to recreate the issue with the orbit not being symmetrical: https://github.com/Harvzor/godot-gravity-test

There's also a video on the repo which shows the issue: https://raw.githubusercontent.com/Harvzor/godot-gravity-test/master/godot-gravitytest.webm

I've tried tweaking many random settings, including changing the physics engine to GodotPhysics rather than default (Bullet), but the settings don't seem to make a difference.

Note that the gravity calculation is done here:

godot/servers/physics_2d/godot_area_2d.cpp

Line 305 in afeb757

void GodotArea2D::compute_gravity(const Vector2 &p_position, Vector2 &r_gravity) const {

[bend-n]

This bug is related to Bullet. Switchting to GodotPhysics fixes the problem, the amplitude increases only very slightly over time (on the 6th decimal place) presumably due to floating point error. As Bullet will be removed in Godot 4.0 I think this issue can be closed.

As Bullet is removed now, can this be closed?

[bend-n]

@harvzor Can you create a separate issue?

[harvzor]

My issue is more with unstable orbits rather than the speed of objects increasing, so I think it's fair to say it's a separate issue.

Created a new issue: #68348

Probably this issue can be closed, though I haven't checked myself that GodotPhysics solves it. At least in my projects, acceleration isn't much of a problem.

[udit]

Can confirm this issue does not happen in Godot 4. In Godot 3, it can be fixed by switching to GodotPhysics. Closing this.