MultibodyJoint behaves incorrectly - does not react to angular momentum changes

[rsk700]

I was trying to use MultibodyJoint and looks like it is simulated incorrectly. I spawned two dynamic RBs, and connected with joint using MultibodyJointSet, on hit Multibody jointed RB does not change its angular momentum, while if using ImpulseJoint for the same simulation it behaves as expected, here is video:

out.mp4

Test code I was using:

// Cargo.toml

// [package]
// name = "multibody-issue"
// version = "0.1.0"
// edition = "2021"

// [dependencies]
// rapier2d = "0.20.0"
// rapier_testbed2d = "0.20.0"


use rapier2d::prelude::*;
use rapier_testbed2d::{Testbed, TestbedApp};

fn main() {
    let app = TestbedApp::from_builders(
        0,
        vec![
            ("Multibody joint - bad", multi_bad),
            ("Impulse joint - ok", impulse_ok),
        ],
    );
    app.run();
}

fn multi_bad(tb: &mut Testbed) {
    let mut bodies = RigidBodySet::new();
    let mut colliders = ColliderSet::new();
    let impulse_joints = ImpulseJointSet::new();
    let mut multibody_joints = MultibodyJointSet::new();

    let ground = bodies.insert(RigidBodyBuilder::fixed().translation(vector![0.0, -2.0]));
    colliders.insert_with_parent(ColliderBuilder::cuboid(0.5, 0.5), ground, &mut bodies);

    let rbt = vector![0.70, 0.0];
    let rb = bodies.insert(RigidBodyBuilder::dynamic().translation(rbt));
    colliders.insert_with_parent(ColliderBuilder::cuboid(0.3, 0.1).mass(0.1), rb, &mut bodies);

    let rb2 = bodies.insert(RigidBodyBuilder::dynamic().translation(rbt));
    colliders.insert_with_parent(
        ColliderBuilder::cuboid(0.05, 0.05).mass(0.1),
        rb2,
        &mut bodies,
    );

    multibody_joints.insert(
        rb,
        rb2,
        RevoluteJointBuilder::new()
            .local_anchor1(point![0.0, 0.0])
            .local_anchor2(point![0.0, 0.0])
            .contacts_enabled(false),
        true,
    );

    tb.set_world(bodies, colliders, impulse_joints, multibody_joints);
}

fn impulse_ok(tb: &mut Testbed) {
    let mut bodies = RigidBodySet::new();
    let mut colliders = ColliderSet::new();
    let mut impulse_joints = ImpulseJointSet::new();
    let multibody_joints = MultibodyJointSet::new();

    let ground = bodies.insert(RigidBodyBuilder::fixed().translation(vector![0.0, -2.0]));
    colliders.insert_with_parent(ColliderBuilder::cuboid(0.5, 0.5), ground, &mut bodies);

    let rbt = vector![0.70, 0.0];
    let rb = bodies.insert(RigidBodyBuilder::dynamic().translation(rbt));
    colliders.insert_with_parent(ColliderBuilder::cuboid(0.3, 0.1).mass(0.1), rb, &mut bodies);

    let rb2 = bodies.insert(RigidBodyBuilder::dynamic().translation(rbt));
    colliders.insert_with_parent(
        ColliderBuilder::cuboid(0.05, 0.05).mass(0.1),
        rb2,
        &mut bodies,
    );

    impulse_joints.insert(
        rb,
        rb2,
        RevoluteJointBuilder::new()
            .local_anchor1(point![0.0, 0.0])
            .local_anchor2(point![0.0, 0.0])
            .contacts_enabled(false),
        true,
    );

    tb.set_world(bodies, colliders, impulse_joints, multibody_joints);
}