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Rigid-bodies

The real-time simulation of rigid-bodies subjected to forces and contacts is the main feature of a physics engine for video-games, robotics, or animation. Rigid-bodies are typically used to simulate the dynamics of non-deformable solids as well as to integrate the trajectory of solids which velocities are controlled by the user (e.g. moving platforms). On the other hand, rigid-bodies are not enough to simulate, e.g., cars, ragdolls, or robotic systems, as those use-cases require adding restrictions on the relative motion between their parts using joints.

Note that rigid-bodies are only responsible for the dynamics and kinematics of the solid. Colliders can be attached to a rigid-body to specify its shape and enable collision-detection. A rigid-body without collider attached to it will not be affected by contacts (because there is no shape to compute contact against).

Creation and insertion​

A rigid-body is created by adding the RigidBody component to an entity. Other components like Transform, Velocity, Ccd, etc. can be added for further customization of the rigid-body. Removing one of these optional components afterwards resets the corresponding property of the rigid-body to its default value.

info

The following example shows several initialization of components to customize rigid-body being built. The input values are just random so using this example as-is will not lead to a useful result.

use bevy::prelude::*;
use bevy_rapier2d::prelude::*;

commands
    .spawn(RigidBody::Dynamic)
    .insert(Transform::from_xyz(0.0, 5.0, 0.0))
    .insert(Velocity {
        linear: Vec2::new(1.0, 2.0),
        angular: 0.2,
    })
    .insert(GravityScale(0.5))
    .insert(Sleeping::disabled())
    .insert(Ccd::enabled());
info

Typically, the inertia and center of mass are automatically set to the inertia and center of mass resulting from the shapes of the colliders attached to the rigid-body. But they can also be set manually.

Rigid-body type​

There are four types of rigid-bodies, identified by the RigidBody component:

  • RigidBody::Dynamic: Indicates that the body is affected by external forces and contacts.
  • RigidBody::Fixed: Indicates the body cannot move. It acts as if it has an infinite mass and will not be affected by any force. It will continue to collide with dynamic bodies but not with fixed nor with kinematic bodies. This is typically used for the ground or for temporarily freezing a body.
  • RigidBody::KinematicPositionBased: Indicates that the body position must not be altered by the physics engine. The user is free to set its next position and the body velocity will be deduced at each update accordingly to ensure a realistic behavior of dynamic bodies in contact with it. This is typically used for moving platforms, elevators, etc.
  • RigidBody::KinematicVelocityBased: Indicates that the body velocity must not be altered by the physics engine. The user is free to set its velocity and the next body position will be deduced at each update accordingly to ensure a realistic behavior of dynamic bodies in contact with it. This is typically used for moving platforms, elevators, etc.

Both position-based and velocity-based kinematic bodies are mostly the same. Choosing between both is mostly a matter of preference between position-based control and velocity-based control.

Note that a fifth type exists, the soft frame, which is reserved to the rigid-bodies Rapier creates to give a frame to the soft-bodies: their pose is computed from the particles of the soft-body, and they aren't meant to be created by hand.

info

The whole point of kinematic bodies is to let the user have total control over their trajectory. This means that kinematic bodies will simply ignore any contact force and go through walls and the ground. In other words: if you tell the kinematic to go somewhere, it will go there, no questions asked.

Taking obstacles into account needs to be done manually either by using scene queries to detect nearby obstacles, or by using the built-in character controller.

Position​

The position of a rigid-body represents its location (translation) in 2D or 3D world-space, as well as its orientation (rotation). Both are stored in the standard Bevy Transform component.

The position of a rigid-body can be set when creating it. It can also be set after its creation as illustrated below.

warning

Directly changing the position of a rigid-body is equivalent to teleporting it: this is a not a physically realistic action! Teleporting a dynamic or kinematic bodies may result in odd behaviors especially if it teleports into a space occupied by other objects. For dynamic bodies, forces, impulses, or velocity modification should be preferred. For kinematic bodies, see the discussion after the examples below.

commands
    .spawn(RigidBody::Dynamic)
    .insert(Transform::from_xyz(0.0, 5.0, 0.0))
/* Change the position inside of a system. */
fn modify_body_translation(mut positions: Query<&mut Transform, With<RigidBody>>) {
    for mut position in positions.iter_mut() {
        position.translation.y += 0.1;
    }
}

In order to move a dynamic rigid-body it is strongly discouraged to set its position directly as it may results in weird behaviors: it's as if the rigid-body teleports itself, which is a non-physical behavior. For dynamic bodies, it is recommended to either set its velocity or to apply forces or impulses.

For velocity-based kinematic bodies, it is recommended to set its velocity instead of setting its position directly. For position-based kinematic bodies, it is recommended to modify its Transform (changing its velocity won’t have any effect). This won't teleport the kinematic body immediately: the modified Transform is used as its next kinematic position, which lets the physics engine compute the fictitious velocity of the kinematic body for more realistic interactions with other rigid-bodies.

Velocity​

The velocity of a dynamic rigid-body controls how fast it is moving in time. The velocity is applied at the center-of-mass of the rigid-body, and is composed of two independent parts:

  1. The linear velocity is specified as a vector representing the direction and magnitude of the movement.
  2. In 3D, the angular velocity is given as a vector representing the rotation axis multiplied by the rotation angular speed in rad/s (axis-angle representation). In 2D, the angular velocity is given as a real representing the angular speed in rad/s.
info

The velocity is only relevant to dynamic rigid-bodies. It has no effect on fixed rigid-bodies, and the velocity of kinematic rigid-bodies are automatically computed at each timestep based on their next kinematic positions.

The velocity of a rigid-body is automatically updated by the physics pipeline after taking forces, contacts, and joints into account. It can be set when the rigid-body is created or after its creation:

/* Set the velocities when the rigid-body is created. */
commands.spawn(RigidBody::Dynamic).insert(Velocity {
    linear: Vec2::new(0.0, 2.0),
    angular: 0.4,
});
/* Set the velocities inside of a system. */
fn modify_body_velocity(mut velocities: Query<&mut Velocity>) {
    for mut vel in velocities.iter_mut() {
        vel.linear = Vec2::new(0.0, 2.0);
        vel.angular = 0.4;
    }
}

Alternatively, the velocity of a dynamic rigid-body can be altered indirectly by applying a force or an impulse.

Gravity​

Gravity is such a common force that it is implemented as a special case (even if it could easily be implemented by the user using force application). The gravity is given by the field RapierConfiguration::gravity of the component RapierConfiguration and can be modified at will. Note however that a change of gravity won't automatically wake-up the sleeping bodies so keep in mind that you may want to wake them up manually before a gravity change.

note

Because fixed and kinematic bodies are immune to forces, they are not affected by gravity.

info

A rigid-body with no mass will not be affected by gravity either. So if your rigid-body doesn't fall when you expected it to, make sure it has a mass set explicitly, or has at least one collider with non-zero density attached to it.

It is possible to change the way gravity affects a specific rigid-body by setting the rigid-body's gravity scale to a value other than 1.0. The magnitude of the gravity applied to this body will be multiplied by this scaling factor. Therefore, a gravity scale set to 0.0 will disable gravity for the rigid-body whereas a gravity scale set to 2.0 will make it twice as strong. A negative value will flip the direction of the gravity for this rigid-body.

This gravity scale factor can be set when the rigid-body is created or after its creation:

/* Set the gravity scale when the rigid-body is created. */
commands.spawn(RigidBody::Dynamic).insert(GravityScale(2.0));
/* Set the gravity scale inside of a system. */
fn modify_body_gravity_scale(mut grav_scale: Query<&mut GravityScale>) {
    for mut grav_scale in grav_scale.iter_mut() {
        grav_scale.0 = 2.0;
    }
}

Forces and impulses​

In addition to gravity, it is possible to add custom forces (or torques) or apply impulses (or torque impulses) to dynamic rigid-bodies in order to make them move in specific ways. Forces affect the rigid-body's acceleration whereas impulses affect the rigid-body's velocity. They are both based on the familiar equations:

  • Forces: the acceleration change is equal to the force divided by the mass: Δa=m−1f\Delta{}a = m^{-1}f
  • Impulses: the velocity change is equal to the impulse divided by the mass: Δv=m−1i\Delta{}v = m^{-1}i

Forces can be added, and impulses can be applied, to a rigid-body when it is created or after its creation. Added forces are persistent across simulation steps, and can be cleared manually.

commands
    .spawn(RigidBody::Dynamic)
    .insert(ExternalForce {
        force: Vec2::new(1000.0, 2000.0),
        torque: 140.0,
    })
    .insert(ExternalImpulse {
        impulse: Vec2::new(100.0, 200.0),
        torque_impulse: 14.0,
    })
    // Needed to read the world-space center-of-mass from `apply_impulse_at_point`.
    .insert(ReadWorldMassProperties::default());
/* Apply forces and impulses inside of a system. */
fn apply_forces(
    mut ext_forces: Query<&mut ExternalForce>,
    mut ext_impulses: Query<&mut ExternalImpulse>,
) {
    // Apply forces.
    for mut ext_force in ext_forces.iter_mut() {
        ext_force.force = Vec2::new(1000.0, 2000.0);
        ext_force.torque = 0.4;
    }

    // Apply impulses.
    for mut ext_impulse in ext_impulses.iter_mut() {
        ext_impulse.impulse = Vec2::new(100.0, 200.0);
        ext_impulse.torque_impulse = 0.4;
    }
}

/* Apply an impulse at a world-space point inside of a system. */
fn apply_impulse_at_point(mut bodies: Query<(&mut ExternalImpulse, &ReadWorldMassProperties)>) {
    for (mut ext_impulse, mprops) in bodies.iter_mut() {
        // The torque impulse is deduced from the world-space center-of-mass of the rigid-body.
        *ext_impulse += ExternalImpulse::at_point(
            Vec2::new(0.0, 100.0),
            Vec2::new(1.0, 2.0),
            mprops.center_of_mass,
        );
    }
}

The ExternalForce component is applied at each timestep until it is modified or removed, whereas the ExternalImpulse component is applied only once and then automatically reset to zero. A force or impulse applied at a specific world-space point can be built with ExternalForce::at_point or ExternalImpulse::at_point: these take the world-space center-of-mass of the rigid-body, which can be read, e.g., from its ReadWorldMassProperties component.

info

Keep in mind that a dynamic rigid-body with a zero mass won't be affected by a linear force/impulse, and a rigid-body with a zero angular inertia won't be affected by torques/torque impulses. So if your force doesn't appear to do anything, make sure that:

  1. The rigid-body is dynamic.
  2. It is strong enough to make the rigid-body move (try a very large value and see if it does something).
  3. The rigid-body has a non-zero mass or angular inertia either because they were set explicitly, or because they were computed automatically from colliders with non-zero densities.

Mass properties​

The mass properties of a rigid-body is composed of three parts:

  • The mass which determines the resistance of the rigid-body wrt. linear movements. A high mass implies that larger forces are needed to make the rigid-body translate.
  • The angular inertia determines the resistance of the rigid-body wrt. the angular movements. A high angular inertia implies that larger torques are needed to make the rigid-body rotate.
  • The center-of-mass determines relative to what points torques are applied to the rigid-body.
note

Zero is a special value for masses and angular inertia. A mass equal to zero is interpreted as an infinite mass. An angular inertia equal to zero is interpreted as an infinite angular inertia. Therefore, a rigid-body with a mass equal to zero will not be affected by any force, and a rigid-body with an angular inertia equal to zero will not be affected by any torque.

Computing the mass and angular-inertia can often be difficult because they depend on the geometric shape of the object being simulated. This is why they are automatically computed by Rapier when a collider is attached to the rigid-body: the collider add its own mass and angular-inertia contribution (computed based on the collider's shape and density) to the rigid-body it is attached to:

// When the collider is attached, the rigid-body's mass and angular
// inertia will be automatically updated to take the collider into account.
commands
    .spawn(RigidBody::Dynamic)
    .insert(Collider::ball(1.0))
    // The default density is 1.0, we are setting 2.0 for this example.
    .insert(ColliderMassProperties::Density(2.0));

Alternatively, it is possible to set the mass properties of a rigid-body when it is created. Keep in mind that this won't prevent the colliders' contributions to be added to these values. So make sure to set the attached colliders' densities to zero if you want your explicit values to be the final mass-properties values.

/* Set the additional mass properties when the rigid-body bundle is created. */
commands
    .spawn(RigidBody::Dynamic)
    .insert(AdditionalMassProperties::Mass(10.0));
/* Change the additional mass-properties inside of a system. */
fn modify_body_mass_props(mut mprops: Query<&mut AdditionalMassProperties>) {
    for mut mprops in mprops.iter_mut() {
        *mprops = AdditionalMassProperties::Mass(100.0);
    }
}

The resulting mass-properties (including the colliders' contributions) can be read from the ReadMassProperties component (local-space mass, center-of-mass and angular inertia), inserted automatically with the RigidBody component, or by adding the ReadWorldMassProperties component (world-space center-of-mass and inverse inertia, taking the locked axes into account) to the rigid-body. The latter isn't inserted automatically because it changes at each step for every moving rigid-body. Both are updated by the physics engine and modifying them has no effect on the simulation. The RapierRigidBodySet (from the ReadRapierContext system parameter) also has helpers taking the rigid-body entity, like RapierRigidBodySet::mass and RapierRigidBodySet::center_of_mass.

Locking translations/rotations​

It is sometimes useful to prevent a rigid-body from rotating or translating. One typical use-case for locking rotations is to prevent a player modeled as a dynamic rigid-body from tilting. These kind of degree-of-freedom restrictions could be achieved by joints, but locking translations/rotations of a single rigid-body wrt. the cartesian coordinate axes can be done in a much more efficient and numerically stable way. That's why rigid-bodies have dedicated flags for this.

/* Lock translations and/or rotations when the rigid-body bundle is created. */
commands
    .spawn(RigidBody::Dynamic)
    .insert(LockedAxes::TRANSLATION_LOCKED);
/* Lock translations and/or rotations inside of a system. */
fn modify_body_locked_flags(mut locked_axes: Query<&mut LockedAxes>) {
    for mut locked_axes in locked_axes.iter_mut() {
        *locked_axes = LockedAxes::ROTATION_LOCKED;
    }
}

Damping​

Damping lets you slow down a rigid-body automatically. This can be used to achieve a wide variety of effects like fake air friction. Each rigid-body is given a linear damping coefficient (affecting its linear velocity) and an angular damping coefficient (affecting its angular velocity). Larger values of the damping coefficients lead to a stronger slow-downs. Their default values are 0.0 (no damping at all).

This damping coefficients can be set when the rigid-body is created or after its creation:

/* Set damping when the rigid-body bundle is created. */
commands.spawn(RigidBody::Dynamic).insert(Damping {
    linear_damping: 0.5,
    angular_damping: 1.0,
});
/* Set damping inside of a system. */
fn modify_body_damping(mut dampings: Query<&mut Damping>) {
    for mut rb_damping in dampings.iter_mut() {
        rb_damping.linear_damping = 0.5;
        rb_damping.angular_damping = 1.0;
    }
}

Dominance​

Dominance is a non-realistic, but sometimes useful, feature. It can be used to make one rigid-body immune to forces originating from contacts with some other bodies. For example this can be used to model a player represented as a dynamic rigid-body that cannot be "pushed back" by any, or some, other dynamic rigid-bodies part of the environment.

Each rigid-body is part of a dominance group in [-127; 127] (the default group is 0). If the colliders from two rigid-bodies are in contact, the one with the highest dominance will act as if it has an infinite mass, making it immune to the contact forces the other body would apply on it. If both bodies are part of the same dominance group, then their contacts will work in the usual way (both are affected by opposite forces with the same magnitude).

For example, if a dynamic body A is in the dominance group 10, and a dynamic body B in the dominance group -20, then a contact between a collider attached to A and a collider attached B will result in A remaining immobile and B being pushed by A (independently from their mass).

info

A non-dynamic rigid-body is always considered as being part of a dominance group greater than any dynamic rigid-body. This means that dynamic/fixed and dynamic/kinematic contacts will continue to work normally, independently from the dominance group they were given by the user.

The dominance group can be set when the rigid-body is created or after its creation:

/* Set dominance when the rigid-body bundle is created. */
commands
    .spawn(RigidBody::Dynamic)
    .insert(Dominance::group(10));
/* Set dominance inside of a system. */
fn modify_body_dominance(mut dominances: Query<&mut Dominance>) {
    for mut rb_dominance in dominances.iter_mut() {
        rb_dominance.groups = 10;
    }
}

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). This is done by looking for collisions along the shapes motion: a rigid-body that moved fast during the timestep casts its colliders from their previous position to their new one, and its position is clamped to the first impact found this way. Its velocities are left untouched and the solver is responsible for preventing penetrations. Since the trajectory is clamped to some intermediate location along its path this technique is commonly called motion clamping.

Rapier applies CCD in two ways:

  • Every fast-moving dynamic rigid-body is swept against the fixed colliders and the soft-bodies of the scene. This is automatic and doesn't need to be enabled: this prevents a falling crate from going through the floor, or a projectile from going through a wall, etc. Note however that this is not enabled fol dynamic rigid-bodies with mesh-like shapes (triangle meshes, polylines, heightfields) as that would be too computationally expensive.
  • A dynamic rigid-body with CCD enabled (aka. a bullet) is swept against the kinematic and dynamic bodies as well. This is more expensive, therefore it is disabled by default and should be reserved to the objects that must not tunnel through moving obstacles. Note that two CCD-enabled objects might still tunel since the CCD resolution does currently not take both continuous motions into account simultaneously.
info

CCD takes action only if the rigid-body is moving fast relative to another collider. Therefore it is useless to enable it on fixed rigid-bodies and on rigid-bodies that are expected to move slowly.

The CCD feature, including the automatic sweeping of the fast dynamic bodies, can be fully disabled by setting the maximum number of CCD substeps to zero in the IntegrationParameters (the RapierContextSimulation::integration_parameters.max_ccd_substeps field, whose default is 1). Larger values let a body resolve several successive impacts within a single timestep, at the cost of additional sweeps.

Per-object CCD can be enabled when creating a rigid-body or after its creation:

/* Enable CCD when the rigid-body bundle is created. */
commands.spawn(RigidBody::Dynamic).insert(Ccd::enabled());
/* Enable CCD inside of a system. */
fn modify_body_ccd(mut ccds: Query<&mut Ccd>) {
    for mut rb_ccd in ccds.iter_mut() {
        rb_ccd.enabled = true;
    }
}

Sleeping​

When a dynamic rigid-body doesn't move (or moves very slowly) during a few seconds, it will be marked as sleeping by the physics pipeline. Rigid-bodies marked as sleeping are no longer simulated by the physics engine until they are woken up. That way the physics engine doesn't waste any computational resources simulating objects that don't actually move. They are woken up automatically whenever another non-sleeping rigid-body starts interacting with them (either with a joint, or with one of its attached colliders generating contacts).

Rigid-bodies are also automatically woken up whenever one of the components of the rigid-body is modified (to apply forces, change its position, etc.) They will not be awaken automatically when changing the gravity though. So you may sometimes want to wake a rigid-body manually by setting the component field Sleeping::sleeping to false: this wakes up the rigid-body as well as the rigid-bodies interacting with it. Setting it to true puts the rigid-body to sleep.

The other fields of the Sleeping component control when the rigid-body falls asleep: it has to move slower than Sleeping::normalized_linear_threshold and Sleeping::angular_threshold during Sleeping::time_until_sleep seconds. A rigid-body can be prevented from ever sleeping by giving it the Sleeping::disabled() component.

Solver settings​

The accuracy of the constraints solver is configured for the whole world by the integration parameters. However, in some cases, part of the simulation might need more fine-grained control. For example, an articulated robot, or a stack involving large mass ratios, might require more solver iterations. This is why a rigid-body can ask for additional solver iterations (either substeps, or internal steps) resulting in the island it belongs to (the bodies it is connected to by contacts and joints) to run with a higher accuracy without hurting the performances of other islands.

Two other settings affect how a rigid-body is integrated. The soft-CCD prediction distance makes the body generate predictive contacts ahead of its own path, which is a cheaper alternative to CCD for the objects that are thin or moderately fast (large values impact the performances badly by increasing significantly the number of collision pairs). Finally, the fast-rotation flag lets the body exceed the angular speed cap, which is enabled by default to keep CCD reliable.

These settings are given by the AdditionalSolverIterations, AdditionalPgsIterations, SoftCcd, and AllowFastRotation components. Removing them restores the default behavior.

/* Give a rigid-body more solver accuracy than the rest of the scene. */
commands.spawn((
    RigidBody::Dynamic,
    // Extra substeps run for the whole island this body belongs to.
    AdditionalSolverIterations(4),
    // Extra internal PGS iterations run per substep for that same island.
    AdditionalPgsIterations(2),
    // Predictive contacts generated up to that distance ahead of the body's path: a cheaper
    // alternative to CCD for slow-but-thin or moderately fast objects.
    SoftCcd { prediction: 0.5 },
    // Let the body exceed the angular speed cap, e.g. for a wheel.
    AllowFastRotation,
));
note

In 3D, the gyroscopic forces of a rigid-body can be disabled as well by giving it the GyroscopicForces::disabled() component. When enabled (the default, even without this component), they give the more realistic behaviors of a spinning solid, e.g., the precession of a spinning top or the Dzhanibekov effect. Disabling them is only recommended if they represent a measurable overhead in your simulation.