Advanced collision-detection

Collision-detection is a two-steps process. First the broad-phase detects pairs of colliders that are potentially in contact or intersecting. Second, the narrow-phase processes all these pairs in order to compute contacts points and generate collision events. Based on these points, the constraints solver computes forces that may generate contact force events. All the pairs detected by the broad-phase can be accessed from the physics World class.

Collision and contact force events#

Collision events are triggered when two colliders transition between intersecting or not. Transitioning between having one contact point to having more than one contact points is not reported an event.

In addition, after forces are computed by the constraints solver, contact force events may be generated between two colliders. There events can be collected by optionally passing an instance of EventQueue to World.step(eventQueue). The colliders involved in each event is identified by its handle.

Events can be collected by passing an instance of EventQueue to World.step(eventQueue). The colliders involved in each event is identified by its handle.

let eventQueue = new RAPIER.EventQueue(true);
world.step(eventQueue);
eventQueue.drainCollisionEvents((handle1, handle2, started) => {
/* Handle the collision event. */
});
eventQueue.drainContactForceEvents(event => {
let handle1 = event.collider1(); // Handle of the first collider involved in the event.
let handle2 = event.collider2(); // Handle of the second collider involved in the event.
/* Handle the contact force event. */
});
warning

Collision events (resp. contact force events) are only generated between two colliders if at least one of them has the ActiveEvents.COLLISION_EVENTS flag (resp. ActiveEvents.CONTACT_FORCE_EVENTS flag) in its active events.

Generally, the user isn’t interested in contact force events unless the force magnitudes exceed some threshold. In order to skip low-force events, the engine will compute the sum of the magnitude of all the contacts between the two colliders and only trigger a contact force event if that magnitude is larger than the threshold set with Collider.setContactForceEventThreshold (defaults to 0) for any of the two colliders with the ActiveEvents.CONTACT_FORCE_EVENTS flag enabled.

The contact graph#

The contact graph (encapsulated by the World class) can be read in order to determine whether two specific non-sensor colliders are in contact, or to determine all the non-sensor colliders in contact with one particular non-sensor collider. Contact points and contact normals will also be provided when a contact exists.

The contact geometry (contact points, contact normal, penetration depth, etc.) can be read from the contact manifolds stored in a contact pair:

  1. Each contact pair may contain multiple contact manifolds. Each contact manifold represents a set of contacts sharing the same contact normal.
  2. Each contact manifold contains the list of geometric contacts detected by the narrow-phase.
  3. Each contact manifold also contains a list of contacts that were processed by the constraints solver for force calculation (aka. the solver contacts). These solver contacts are a subset of the contacts detected by the narrow-phase, expressed in a way that is more efficient for the constraints solver to process.

All the geometric contact data are expressed in the local-space of the colliders. The solver contacts are expressed in world-space.

info

There will always be only up to one contact manifold between two colliders with convex primitive shapes. If one collider has a shape composed of several pieces (trimesh, polyline, heightfield, or compound shape) then there will be multiple contact manifolds, one for each piece that may result in an actual contact.

world.contactsWith(collider, (otherCollider) => {
// This closure is called on each collider object potentially
// in contact with `collider`.
});
world.contactPair(collider1, collider2, (manifold, flipped) => {
// Contact information can be read from `manifold`.
});

The intersection graph#

The intersection graph can be read in order to determine whether two specific colliders (assuming at least one of them is a sensor) are intersecting, or to determine all the colliders intersecting one particular collider (assuming at least one collider of each pair is a sensor). The intersection graph contains one graph edge for each pair of colliders such that:

  1. At least one of the collider is a sensor.
  2. And they are close enough so the broad-phase considers they have a chance to be intersecting.

Each such edge contains one boolean indicating if the colliders are actually intersecting or not:

warning

Keep in mind that intersection tests are performed between two colliders only if at least one of the colliders is a sensor. If they are both non-sensor colliders then they will be involved in the contact graph instead of the intersection graph.

world.intersectionsWith(collider, (otherCollider) => {
// This closure is called on each collider potentially
// intersecting the collider `collider`.
});
let intersections = world.intersectionPair(collider1, collider2);

Physics hooks#

Physics hooks are user-defined callbacks used to change the behavior of the physics simulation. In particular, they can be used to filter contacts (in a more flexible way than collision groups and solver groups).

Contact and intersection filtering#

Sometimes, collision groups and solver groups are not flexible enough to achieve the desired behavior. In that case, the contact filtering hooks let you apply custom rules to filter contact pairs and intersection pairs:

  • For each potential contact pair (between two non-sensor colliders) detected by the broad-phase, if at least one of the colliders involved in the pair has the bit ActiveHooks.FILTER_CONTACT_PAIRS enabled in its active hooks, then PhysicsHooks.firterContactPair will be called. If this filter returns null then no contact computation will happen for this pair of colliders. If it returns a SolverFlags value then the narrow-phase will compute contact points.
  • For each potential intersection pair (between a sensor colliders and another collider) detected by the broad-phase, if at least one of the colliders involved in the pair has the bit ActiveHooks.FILTER_INTERSECTION_PAIRS enabled in its active hooks, then PhysicsHooks.filterIntersectionPair will be called. If this filter returns false then no intersection computation will happen for this pair of colliders. If it returns true then the narrow-phase will test whether or not they are intersecting.

When PhysicsHooks.filterContactPair returns a not-null value, it needs to provide a set of solver flags for this contact pair. These solver flags indicate what happen with the contacts of this contact pair afterwards:

  • If the returned solver flags contains the SolverFlags.COMPUTE_IMPULSES bit, then the constraints solver will compute forces for these contacts. If this bit is not included in the returned flags, then no contact force will be computed for this pair of colliders.
note

Right now there is no solver flags other than SolverFlags.COMPUTE_IMPULSES. Other flags may be added in the future.

Continuous Collision Detection#

Continuous Collision Detection (CCD) is used to make sure that fast-moving objects don't miss any contacts (a problem usually called tunneling). See the rigid-body CCD section for details.