[2D] BallJoint looses momentum

[truchi]

Hello Monsieur,

I am playing with Rapier 2D for fun. I am trying to simulate grappling hooks, I use BallJoint for that. All my colliders have no friction (0.) and full restitution (1.). Top-down, no gravity.

The white ball have initial position [WIDTH / 4., HEIGHT / 2.] and linvel [300., 400.]. The hook is at [WIDTH / 2., HEIGHT / 4.].

Without joint, everything works OK:

https://user-images.githubusercontent.com/9745474/123634811-32a01280-d81b-11eb-8279-c3358c7b7d0e.mp4

With joint, the angvel and linvel slow down, and on impact drop significantly:

let joint = BallJoint::new(point![0.0, 0.0], point![WIDTH / 4., 0.]);
physics
    .joints
    .insert(&mut physics.bodies, pendulum, ball, joint);

https://user-images.githubusercontent.com/9745474/123635417-f1f4c900-d81b-11eb-87cf-0d4f2518cf28.mp4

I think I don't want motor behavior. I'd like a circular trajectory, just like when grappling. Am I doing something wrong? Is this a bug? Should I set angvel with configure_motor_velocity on every impact?

for [x, y, w, h] in WALLS {
    walls.push(
        physics.colliders.insert(
            ColliderBuilder::cuboid(w / 2., h / 2.)
                .friction(0.)
                .restitution(1.)
                .translation(vector![x + w / 2., y + h / 2.])
                .build(),
        ),
    );
}

for &[x, y, r1, r2] in HOOKS {
    hooks.push(
        physics.colliders.insert(
            ColliderBuilder::ball(r1)
                .friction(0.)
                .restitution(1.)
                .translation(vector![x, y])
                .build(),
        ),
    );
}

let ball = physics.bodies.insert(
    RigidBodyBuilder::new_dynamic()
        .translation(vector![WIDTH / 4., HEIGHT / 2.])
        .linvel(vector![300.0, 400.0])
        .build(),
);
physics.colliders.insert_with_parent(
    ColliderBuilder::ball(10.)
        .active_events(ActiveEvents::CONTACT_EVENTS | ActiveEvents::INTERSECTION_EVENTS)
        .friction(0.)
        .restitution(1.)
        .build(),
    ball,
    &mut physics.bodies,
);

// Adding grappling
let pendulum = physics.bodies.insert(
    RigidBodyBuilder::new_static()
        .translation(vector![WIDTH / 2., HEIGHT / 2.])
        .build(),
);
let joint = BallJoint::new(point![0.0, 0.0], point![WIDTH / 4., 0.]);
physics
    .joints
    .insert(&mut physics.bodies, pendulum, ball, joint);

Thanks for the help!

[truchi]

It seems like setting SpringModel doesn't do anything. Shouldn't it, esp. Disabled do something different?

[sebcrozet]

I would recommend switching to the new rope or spring joints. Closing this for now, but please reopen if the rope or spring joint isn’t behaving as expected.