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collider_restitution

Restitution controls how elastic (aka. bouncy) a contact is. The elasticity of a contact is controlled by the restitution coefficient. A restitution coefficient set to 1 (fully elastic contact) implies that the exit velocity at a contact has the same magnitude as the entry velocity along the contact normal: it is as if you drop a bouncing ball and it gets back to the same height after the bounce. A restitution coefficient set to 0 implies that the exit velocity at a contact will be zero along the contact normal: it's as if you drop a ball but it doesn't bounce at all.

note

The friction and restitution coefficients are both managed in very similar ways: with the CoefficientCombineRule or with contact modification. The paragraph bellow is almost identical to the paragraph about friction.

Each collider has its own restitution coefficient. This means that when two colliders are in contact, we need to apply a rule that combines the restitution coefficients of these two colliders into a single coefficient that will be used for the contact. This rule is described by the CoefficientCombineRule enum:

  • CoefficientCombineRule.Average: the average of the two coefficients is used for the contact.
  • CoefficientCombineRule.Min: the minimum among the two coefficients is used for the contact.
  • CoefficientCombineRule.Multiply: the product of the two coefficients is used for the contact.
  • CoefficientCombineRule.Max: the maximum among the two coefficients is used for the contact.

By default, the Average rule is used. Each collider can be given its own restitution combine rule. When two colliders are in contact, we need to select one of their combine rule. The following precedence is used: Max > Multiply > Min > Average.

For example if one collider with the Multiply restitution combine rule is in contact with a collider with the Average restitution combine rule, then the Multiply rule will be applied for the restitution coefficient of this contact (i.e. the coefficients of both colliders will be multiplied to obtain the coefficient used by the contact).

info

The CoefficientCombineRule system exists to cover a wide variety of use-cases efficiently. If this is not flexible enough, it is possible to get full control over the selection of restitution coefficients for each contact point using contact modification. For example, contact modification allows the simulation of colliders with non-uniform restitution coefficients.

The restitution coefficient and restitution combine rule can both be set when the collider is created or after its creation:

/* Set the restitution coefficient and restitution combine rule
  when the collider is created. */
let colliderDesc = RAPIER.ColliderDesc.ball(0.5)
    .setRestitution(0.7)
    .setRestitutionCombineRule(RAPIER.CoefficientCombineRule.Min);
let collider = world.createCollider(colliderDesc);
/* Set the restitution coefficient and restitution combine rule
   after the collider creation. */
collider.setRestitution(0.7);
collider.setRestitutionCombineRule(RAPIER.CoefficientCombineRule.Min);