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collider_mass_properties

The mass properties of a rigid-body is computed as the sum of the mass-properties manually set by the user for the rigid-body, plus the mass-properties of the colliders attached to it. There are two ways to define the mass-properties of a collider:

  1. The easiest, automatic, way: by giving the collider a non-zero density (the default density is 1.0) or a non-zero mass. This will make sure the other mass-properties like the angular inertia tensor are computed automatically from the collider's shape.
  2. The manual way: by giving an explicit mass and angular inertia to the collider.

It is recommended to use the density-based or mass-based approaches as it will ensure the automatically-computed mass-properties are coherent with the geometric shape. Wrong mass-properties (especially the angular inertia part and center-of-mass location) may lead to odd behaviors. The manual approach is usually useful when modeling real-world objects for which you already know the real-world mass, center-of-mass, and angular inertia tensor.

The mass-properties of a collider can only be set when the collider is created:

let rigidBodyDesc = RAPIER.RigidBodyDesc.dynamic();
let rigidBody = world.createRigidBody(rigidBodyDesc);
// First option: by setting the density of the collider (or we could just leave
//               its default value 1.0).
let colliderDesc = RAPIER.ColliderDesc.cuboid(1.0, 2.0)
    .setDensity(2.0);
// Second option: by setting the mass of the collider.
let colliderDesc2 = RAPIER.ColliderDesc.cuboid(1.0, 2.0)
    .setMass(0.8);
// Third option: by setting the mass-properties explicitly.
let colliderDesc3 = RAPIER.ColliderDesc.cuboid(1.0, 2.0)
    .setMassProperties(0.5, { x: 0.0, y: 1.0 }, 0.3);
// When the collider is attached, the rigid-body's mass and angular
// inertia is automatically updated to take the collider into account.
let collider = world.createCollider(colliderDesc, rigidBody);