Manually set mass about bevy_xpbd

[stargazing-dino]

Thanks for the help and explanation ! Setting the inertia makes a lot of sense and works perfect for my case 🙂

Also haha thank you for the Vector correction. I consider myself still new to rust so I'm just happy it compiled :P but i'll make that change as well

[Jondolf]

Do the planets have colliders? If they do, then the mass properties of the colliders are computed and added on top of the body's existing mass properties, like in rapier. This is documented here.

You can set the mass properties of the colliders to zero by adding ColliderMassProperties::ZERO. With that I am not seeing any mass changing in my examples.

[stargazing-dino]

That worked great for me ! Thank you. I'm enjoying the library :)

[stargazing-dino]

tldr: Nvm, I'm got it :P

I feel like I'm silly for doing this but if I try to do the same thing of setting ColliderMassProperties::ZERO with a RigidBody::Dynamic it doesn't work and my little ball disappears off the screen before I've even added any forces to the ball.

let shape = shape::UVSphere {
    radius: 1.0,
    ..default()
};

commands.spawn((
    PbrBundle {
        mesh: meshes.add(shape.into()),
        material: materials.add(Color::rgb(0.8, 0.7, 0.6).into()),
        transform: Transform::from_xyz(0.0, 0.0, 0.0),
        ..default()
    },
    Collider::ball(1.0),
    ColliderMassProperties::ZERO,
    // RigidBody::Kinematic, // <- Works
    // RigidBody::Dynamic, // <- hops off the screen before I've started my physics stuff 
    Position(Vector::new(14.0, 0.0, 0.0)),
    ExternalForce::default(),
    Mass(1.0),
    Friction::new(6.0),
    Junk {},
));

Again, I'm crazy dumb when it comes to physics anything so feel free to let me know if I'm just doing stuff wrong.

Edit

So I guess it just needs a non-zero density in order to not freak out:

ColliderMassProperties {
    density: 1.0,
    ..default()
},
Mass(1.0),
RigidBody::Dynamic,

Which makes some sense. Anyways, I guess the problem now is that the mass is changing from the 1.0 I set it to to something like 6.4 😭

Edit 2

I guess setting the density to something super low actually fixes it and I get my mass of ~1.0 lol.

ColliderMassProperties {
    density: 0.00001, // honestly, just non-zero prob
    ..default()
},

[Jondolf]

I'm pretty sure the problem isn't actually the density of the collider, it's that you only add Mass and not Inertia as well.

Angular inertia controls how much effort it requires to rotate a body, and when there's zero inertia, there is probably some kind of division by zero happening, which breaks the simulation and causes the bodies to accelerate into infinity (making them disappear).

So I would add something like this:

(
    ColliderMassProperties::ZERO,
    Mass(1.0),
    Inertia(1.0), // In 2D
    Inertia(Mat3::IDENTITY) // In 3D
    // ...other components
)

You can of course set the inertia to whatever you want, as long as it's greater than 0.

Thanks for the issue though, I definitely need to make the docs more explicit about this. I might also just add a warning when the engine detects a dynamic body with zero mass/inertia.

[Jondolf]

Also FYI, it looks like you might be using the crate's math types like Vector. This is fine, but you can also just use the normal types like Vec2 or Vec3. The special math types are there because the crate needs to use different types depending on feature flags like 2d/3d and f32/f64, but in actual usage you already know the types because you have specified the features.