# Simulation structures

This page describes all the data structures needed for stepping the simulation as illustrated by the basic simulation example.

## Gravity#

Gravity is represented as a vector. It affects every dynamic rigid-body taking part of the simulation. The gravity can be altered at each timestep (by passing a different vector to PhysicsPipeline::step). Learn more about per-rigid-body gravity modification in the dedicated section.

## Integration parameters#

The IntegrationParameters controls various aspects of the physics simulation, including the timestep length, number of solver iterations, number of CCD substeps, etc. The default integration parameters are set to achieve a good balance between performance and accuracy for games. They can be changed to make the simulation more accurate at the expanse of a bit of performance. Learn more about each integration parameter in the API docs.

## Island manager#

The IslandManager is responsible for tracking the set of dynamic rigid-bodies that are still moving and these that are no longer moving (and can ignored by subsequent timesteps to avoid useless computations). The island manager is automatically updated by PhysicsPipeline::step and can be queried to retrieve the list of all the rigid-bodies modified by the physics engine during the last timestep. This can be useful to update the rendering of only the rigid-bodies that moved:

// Iter on each dynamic rigid-bodies that moved.
for rigid_body_handle in island_manager.active_dynamic_bodies() {
let rigid_body = &rigid_body_set[*rigid_body_handle];
println!("Rigid body {:?} has a new position: {}", rigid_body_handle, rigid_body.position());
}

## Physics pipeline#

The PhysicsPipeline is responsible for tying everything together in order to run the physics simulation. It will take care of updating every data-structures mentioned in this page (except the other pipelines), running the collision-detection, running the force computation and integration, and running CCD resolution.

Usage of PhysicsPipeline::step is illustrated in the basic simulation example. It has two useful methods:

• PhysicsPipeline::step executes one timestep. This is the method you should use most of the time, unless you are trying to use your own containers instead of the predefined RigidBodySet and ColliderSet.
• PhysicsPipeline::step_generic also executes one timestep, but is generic wrt. the rigid-body and collider containers. It is strongly discouraged to use this method unless you understand what is expected from your custom containers (more documentation on this topic will be added in the future). Our bevy_rapier plugin for the Bevy game engine is one example that uses PhysicsPipeline::step_generic to use Bevy's queries as rigid-bodies and collider containers.

## Collision pipeline#

The CollisionPipeline is similar to the PhysicsPipeline except that it will only run collision-detection. It won't perform any dynamics (force computation, integration, CCD, etc.) It is generally used instead of the PhysicsPipeline when one only needs collision-detection.

##### info

Running both the CollisionPipeline and the PhysicsPipeline is useless because the PhysicsPipeline already does collision-detection.

## Query pipeline#

The QueryPipeline is responsible for efficiently running scene queries, e.g., ray-casting, shape-casting (sweep tests), intersection tests, on all the colliders of the scene.

Before it is used, the QueryPipeline needs to be updated in order to take the new colliders' positions into account. The QueryPipeline can be used alone, but it is very common to use the QueryPipeline alongside the CollisionPipeline or PhysicsPipeline:

// Game loop.
loop {
physics_pipeline.step(...);
query_pipeline.update(...);
// Now we can read the results of the physics simulation,
// and we can do advanced scene queries on the colliders.
}

Learn more about scene queries with the QueryPipeline in the dedicated page.

## Rigid-body set#

The RigidBodySet contains all the rigid-bodies that needs to be simulated. This set is represented as a generational-arena, i.e., a vector where each element is indexed using a handle that combines an u32 index and an u32 generation number. This ensures that every rigid-body is given a unique handle. Learn more about rigid-bodies in the dedicated page.

## Collider set#

The ColliderSet contains all the colliders that needs to be simulated. This set is represented as a generational-arena, i.e., a vector where each element is indexed using a handle that combines an u32 index and an u32 generation number. This ensures that every collider is given a unique handle. Learn more about colliders in the dedicated page.

## Joint set#

The ImpulseJointSet contains all the impulse-based joints that needs to be simulated. This set is represented as a generational-arena, i.e., a vector where each element is indexed using a handle that combines an u32 index and an u32 generation number. This ensures that every joint is given a unique handle. Learn more about joints in the dedicated page.

## CCD solver#

The CCD solver is responsible for the resolution of Continuous-Collision-Detection. By itself, this structure doesn't expose any feature useful. So it should simply be passed to the PhysicsPipeline::step method. Learn more about CCD in the dedicated section.

## Physics hooks#

The physics hooks are trait-objects implementing the PhysicsHooks trait. They can be used to apply arbitrary rules to ignore collision detection between some pairs of colliders. They can also be used to modify the contacts processed by the constraints solver for computing forces. Learn more about physics hooks in the dedicated section.

## Event handler#

The event handlers are trait-objects implementing the EventHandler trait. They can be used to get notified when two non-sensor colliders start/stop having contacts, and when one sensor collider and one other collider start/stop intersecting. Learn more about collision events in the dedicated section.