[Bullet] Sharp change in damp function when passing 1.0 mark with distro packaged Bullet (no BT_USE_OLD_DAMPING_METHOD)

[stylemistake]

Godot version

3.4.stable

System information

Arch Linux, GLES3, Radeon RX550

Issue description

I opened my project in new version of Godot (3.4), and it made all RigidBodies much stiffer to control, and experimentally I have figured out that linear_damp and angular_damp settings were to blame.

This is how I would describe the current (3.4) damp response:

It should be a consistent response, a continuous curve without any sudden jumps, and it is a game breaking regression.

Jump happens at around total 1.0 mark, which is project dependent (if project settings has 0.1, then it's going to jump at 0.9 in rigidbody settings).

Steps to reproduce

1. Create a RigidBody
2. Write a script that applies a consistent torque to it
3. Try at 0.89 angular_damp, measure terminal angular velocity
4. Try at 0.91 angular_damp, measure terminal angular velocity

It is project settings dependent, since 0.1 is the default damp and all numbers combine (in our case 0.1 + 0.9) to create the final damp number.

Minimal reproduction project

No response

[Calinou]

@stylemistake Is this in 2D or 3D? If 3D, can you reproduce this with the GodotPhysics engine by changing the 3D physics engine in the Project Settings? (The default in 3D is Bullet.)

[stylemistake]

Disregard my previous comment, this is only reproducible with the Bullet engine.

[Calinou]

cc @pouleyKetchoupp

I remember seeing changes related to Bullet drag damping, but I thought these weren't backported to 3.x.

[madmiraal]

The original fix was backported in #39084.

[akien-mga]

Also worth noting that Bullet was updated to 3.17 for Godot 3.4, which may have impacted this.

[madmiraal]

@stylemistake Please provide an MRP, because I'm unable to reproduce this issue. It works fine for me using both Bullet and Godot physics.
Bullet physics:

Godot physics:

Sample project: 55599.zip

[akien-mga]

Arch Linux

Are you using the official Godot binary, or one packaged by Arch? This can impact the Bullet configuration.

My guess is that you might be using a distro packaged Bullet which doesn't define BT_USE_OLD_DAMPING_METHOD like we do for our vendored version:

godot/modules/bullet/SCsub

Line 208 in d394cbc

env_bullet.Append(CPPDEFINES=["BT_USE_OLD_DAMPING_METHOD", "BT_THREADSAFE"])

That would also be a problem in my Fedora and Mageia packages, so I guess we might need to prevent unbundling Bullet, or document this as a caveat for distro packages.

Maybe upstream Bullet would be amenable to making this a runtime Boolean option instead of a compile time define, so that we could still use distro packages by setting a runtime flag.

[stylemistake]

@madmiraal Thank you for making a test project, and this is how it behaves in my environment:

output_00002.mp4

distro packaged Bullet which doesn't define BT_USE_OLD_DAMPING_METHOD like we do for our vendored version

@akien-mga Can confirm, PKGBUILD for Godot unbundles Bullet and doesn't account for these defines.

Once I've changed it to use the bundled version of bullet, problem seems to be gone, and it's an easy change as well, slightly annoying that it has to be bundled.

[stylemistake]

Nothing to fix here, so I'm closing this.