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 a RigidBodyBuilder that is based on the builder pattern: each of its methods returns a new builder with the corresponding property set, so they can be chained. The builder is obtained from one of the static methods of the RigidBody class. Then it needs to be inserted into the physics world with PhysicsWorld.add_body (or directly into its RigidBodySet with world.rigid_bodies.insert), which returns the RigidBodyHandle identifying the new rigid-body.

info

The following example shows several setters that can be called to customize the rigid-body being built. The input values are just random so using this example as-is will not lead to a useful result.

import rapier3d as rp

# The world that will contain our rigid-bodies.
world = rp.PhysicsWorld()

# Builder for a fixed rigid-body.
_ = rp.RigidBody.fixed()
# Builder for a dynamic rigid-body.
_ = rp.RigidBody.dynamic()
# Builder for a kinematic rigid-body controlled at the velocity level.
_ = rp.RigidBody.kinematic_velocity_based()
# Builder for a kinematic rigid-body controlled at the position level.
_ = rp.RigidBody.kinematic_position_based()
# The properties of the builder can also be given as keyword arguments.
_ = rp.RigidBody.dynamic(translation=(0.0, 5.0, 1.0), gravity_scale=0.5)
# Builder for a body with a status specified by an enum.
rigid_body = (
    rp.RigidBody.new_body(rp.RigidBodyType.DYNAMIC)
    # The rigid body translation.
    # Default: zero vector.
    .translation((0.0, 5.0, 1.0))
    # The rigid body rotation, as a scaled rotation axis.
    # Default: no rotation.
    .rotation((0.0, 0.0, 5.0))
    # The rigid body position. Will override `.translation(...)` and `.rotation(...)`.
    # Default: the identity isometry.
    .position(rp.Isometry3((1.0, 3.0, 2.0), rp.Rotation3.from_scaled_axis((0.0, 0.0, 0.4))))
    # The linear velocity of this body.
    # Default: zero velocity.
    .linvel((1.0, 3.0, 4.0))
    # The angular velocity of this body.
    # Default: zero velocity.
    .angvel((3.0, 0.0, 1.0))
    # The scaling factor applied to the gravity affecting the rigid-body.
    # Default: 1.0
    .gravity_scale(0.5)
    # Whether or not this body can sleep.
    # Default: True
    .can_sleep(True)
    # Whether or not CCD is enabled for this rigid-body.
    # Default: False
    .ccd_enabled(False)
    # All done, actually build the rigid-body.
    .build()
)
# Insert the rigid-body into the world.
rigid_body_handle = world.add_body(rigid_body)

All the properties are optional. The only calls that are required are RigidBody.fixed(), RigidBody.dynamic(), RigidBody.kinematic_velocity_based(), RigidBody.kinematic_position_based(), or RigidBody.new_body(body_type), to initialize the builder. Each of them also accepts the builder properties as keyword arguments. Calling .build() to actually build the RigidBody is optional: PhysicsWorld.add_body accepts the builder as well, and its optional colliders argument attaches a list of colliders to the new rigid-body in the same call.

Once inserted, the rigid-body is accessed with world.rigid_bodies[handle]. The returned RigidBody is a view of the rigid-body stored in the world: modifying its properties (e.g. rigid_body.linvel = (1.0, 0.0, 0.0)) modifies the simulated rigid-body directly, and a RigidBody built but not inserted yet is copied by the insertion (so modifying it afterwards has no effect on the world). The rigid-body is removed from the world, together with its colliders and the joints attached to it, with PhysicsWorld.remove_body.

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 RigidBodyType enumeration:

RigidBodyType.DYNAMIC: Indicates that the body is affected by external forces and contacts.
RigidBodyType.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.
RigidBodyType.KINEMATIC_POSITION_BASED: 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.
RigidBodyType.KINEMATIC_VELOCITY_BASED: 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.

The type of a rigid-body can be read, or modified after its creation, with the RigidBody.body_type property (it can also be tested with the is_dynamic, is_fixed, and is_kinematic properties).

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). Its translational part is represented as a vector (Vec3, though a tuple of three floats is accepted as well) and its rotational part as a unit quaternion (Rotation3). Both are combined into a pose (the Isometry3 type).

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.

# Set the position when the rigid-body is created.
rigid_body = (
    rp.RigidBody.dynamic()
    # The rigid body translation.
    # Default: zero vector.
    .translation((0.0, 5.0, 1.0))
    # The rigid body rotation, as a scaled rotation axis.
    # Default: no rotation.
    .rotation((0.2, 0.0, 0.0))
    # The rigid body position. Will override `.translation(...)` and `.rotation(...)`.
    # Default: the identity isometry.
    .position(rp.Isometry3((1.0, 2.0, 3.0), rp.Rotation3.from_scaled_axis((0.2, 0.0, 0.0))))
    # All done, actually build the rigid-body.
    .build()
)

# Set the position after the rigid-body creation.
rigid_body = world.rigid_bodies[rigid_body_handle]
# Setting these properties automatically wakes the rigid-body up.
rigid_body.translation = (0.0, 5.0, 1.0)
rigid_body.rotation = rp.Rotation3.from_scaled_axis((0.2, 0.0, 0.0))
assert rigid_body.translation == rp.Vec3(0.0, 5.0, 1.0)
assert rigid_body.rotation.scaled_axis == rp.Vec3(0.2, 0.0, 0.0)

rigid_body.position = rp.Isometry3((1.0, 2.0, 3.0), rp.Rotation3.from_scaled_axis((0.0, 0.4, 0.0)))
assert rigid_body.position == rp.Isometry3(
    (1.0, 2.0, 3.0), rp.Rotation3.from_scaled_axis((0.0, 0.4, 0.0))
)

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 use the special methods:

RigidBody.set_next_kinematic_rotation
RigidBody.set_next_kinematic_translation
RigidBody.set_next_kinematic_position (for both at once)

These methods will let the physics pipeline compute the fictitious velocity of the position-based kinematic body for more realistic interactions with other rigid-bodies. These methods won't immediately modify the position of the kinematic body itself. The position of the kinematic body will be automatically set to these values during the next physics pipeline update (the pending pose can be read with the RigidBody.next_position property).

platform_handle = world.add_body(rp.RigidBody.kinematic_position_based(translation=(0.0, 1.0, 0.0)))
platform = world.rigid_bodies[platform_handle]

# Move the platform up by 0.01 at each step.
for _ in range(10):
    next_translation = platform.translation + rp.Vec3(0.0, 0.01, 0.0)
    platform.set_next_kinematic_translation(next_translation)
    # The position isn't modified until the next step.
    assert platform.next_position.translation == next_translation
    world.step()

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:

The linear velocity is specified as a vector representing the direction and magnitude of the movement.
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.
rigid_body = (
    rp.RigidBody.dynamic()
    # The linear velocity of this body.
    # Default: zero velocity.
    .linvel((1.0, 3.0, 4.0))
    # The angular velocity of this body.
    # Default: zero velocity.
    .angvel((3.0, 0.0, 0.0))
    # All done, actually build the rigid-body.
    .build()
)

# Set the velocities after the rigid-body creation.
rigid_body = world.rigid_bodies[rigid_body_handle]
# Setting these properties automatically wakes the rigid-body up.
rigid_body.linvel = (1.0, 3.0, 4.0)
rigid_body.angvel = (3.0, 0.0, 0.0)
assert rigid_body.linvel == rp.Vec3(1.0, 3.0, 4.0)
assert rigid_body.angvel == rp.Vec3(3.0, 0.0, 0.0)

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 PhysicsWorld.gravity property (initialized by the gravity argument of the PhysicsWorld constructor, and zero by default) or as an argument to the PhysicsPipeline.step method, 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. They can all be woken up at once with PhysicsWorld.wake_up_all.

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.
rigid_body = (
    rp.RigidBody.dynamic()
    # Divide by 2 the strength of gravity for this rigid-body.
    .gravity_scale(0.5)
    .build()
)

# Set the gravity scale after the rigid-body creation.
rigid_body = world.rigid_bodies[rigid_body_handle]
# Setting this property automatically wakes the rigid-body up.
rigid_body.gravity_scale = 0.5
assert rigid_body.gravity_scale == 0.5

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: $\Delta{}a = m^{-1}f$
Impulses: the velocity change is equal to the impulse divided by the mass: $\Delta{}v = m^{-1}i$

Forces can be added, and impulses can be applied, to a rigid-body after it has been created. Added forces are persistent across simulation steps, and can be cleared manually.

rigid_body = world.rigid_bodies[rigid_body_handle]

# The rigid-body is woken up, unless `wake_up=False` is given.
rigid_body.reset_forces()  # Reset the forces to zero.
rigid_body.reset_torques()  # Reset the torques to zero.
rigid_body.add_force((0.0, 1000.0, 0.0))
rigid_body.add_torque((100.0, 0.0, 0.0))
rigid_body.add_force_at_point((0.0, 1000.0, 0.0), (1.0, 2.0, 3.0))

rigid_body.apply_impulse((0.0, 1000.0, 0.0))
rigid_body.apply_torque_impulse((100.0, 0.0, 0.0))
rigid_body.apply_impulse_at_point((0.0, 1000.0, 0.0), (1.0, 2.0, 3.0))

The forces and torques added with RigidBody.add_force, RigidBody.add_torque, and RigidBody.add_force_at_point are accumulated until they are reset with RigidBody.reset_forces and RigidBody.reset_torques. Their current sum is given by the RigidBody.user_force and RigidBody.user_torque properties. The impulses, on the other hand, modify the velocity of the rigid-body immediately. The points given to add_force_at_point and apply_impulse_at_point are expressed in world-space.

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:

The rigid-body is dynamic.
It is strong enough to make the rigid-body move (try a very large value and see if it does something).
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.
The rigid-body is awake (by waking it up manually with RigidBody.wake_up or keeping the wake_up argument to its default value True).

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:

rigid_body_handle = world.add_body(rp.RigidBody.dynamic())
# The default density is 1.0, we are setting 2.0 for this example.
collider = rp.Collider.ball(1.0).density(2.0)
# When the collider is attached, the rigid-body's mass and angular
# inertia is automatically updated to take the collider into account.
world.add_collider(collider, parent=rigid_body_handle)

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 mass-properties when the rigid-body is created.
rigid_body = (
    rp.RigidBody.dynamic()
    .additional_mass(0.5)
    # Sets both the mass and angular inertia at once.
    .additional_mass_properties(
        rp.MassProperties(local_com=(0.0, 1.0, 0.0), mass=0.5, principal_inertia=(0.3, 0.2, 0.1))
    )
    .build()
)

# Set the mass-properties after the rigid-body creation.
rigid_body = world.rigid_bodies[rigid_body_handle]
# The rigid-body is woken up, unless `wake_up=False` is given.
rigid_body.set_additional_mass_properties(
    rp.MassProperties(local_com=(0.0, 1.0, 0.0), mass=0.5, principal_inertia=(0.3, 0.2, 0.1))
)

The RigidBodyBuilder.additional_mass method only adds a mass (the angular inertia being scaled accordingly, based on the shapes of the colliders), whereas RigidBodyBuilder.additional_mass_properties specifies the full mass-properties (mass, center-of-mass, and principal angular inertia) given as a MassProperties. After the creation of the rigid-body, they are modified with RigidBody.set_additional_mass and RigidBody.set_additional_mass_properties respectively.

The resulting mass-properties (including the colliders' contributions) can be read with the mass, local_center_of_mass (in the local-space of the rigid-body), center_of_mass (in world-space), and mass_properties properties of the RigidBody. They are updated automatically by the physics engine but, if you need them right after modifying the colliders or the additional mass-properties of a rigid-body (without waiting for the next timestep), they can be updated manually with rigid_body.recompute_mass_properties_from_colliders(world.colliders).

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.

The locked axes are given by a LockedAxes value combining, with the | operator, the TRANSLATION_LOCKED_X, TRANSLATION_LOCKED_Y, TRANSLATION_LOCKED_Z, ROTATION_LOCKED_X, ROTATION_LOCKED_Y, and ROTATION_LOCKED_Z flags (or TRANSLATION_LOCKED and ROTATION_LOCKED to lock all the translations or all the rotations at once). It is set with RigidBodyBuilder.locked_axes when the rigid-body is created, or with the RigidBody.locked_axes property afterwards. Alternatively, the enabled_translations and enabled_rotations builder methods and properties take a tuple of three booleans indicating, for each axis, if the corresponding translation or rotation is allowed.

# Lock translations/rotations when the rigid-body is created.
rigid_body = (
    rp.RigidBody.dynamic()
    # Prevent translations along all axes, and rotations around all axes.
    .locked_axes(rp.LockedAxes.TRANSLATION_LOCKED | rp.LockedAxes.ROTATION_LOCKED)
    # Only enable rotations around the X axis.
    .enabled_rotations((True, False, False))
    .build()
)

# Lock translations/rotations after the rigid-body creation.
rigid_body = world.rigid_bodies[rigid_body_handle]
# Setting these properties automatically wakes the rigid-body up.
rigid_body.locked_axes = rp.LockedAxes.TRANSLATION_LOCKED | rp.LockedAxes.ROTATION_LOCKED
# Only enable rotations around the X axis.
rigid_body.enabled_rotations = (True, False, False)

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 the damping coefficients when the rigid-body is created.
rigid_body = rp.RigidBody.dynamic().linear_damping(0.5).angular_damping(1.0).build()

# Set the damping coefficients after the rigid-body creation.
rigid_body = world.rigid_bodies[rigid_body_handle]
rigid_body.linear_damping = 0.5
rigid_body.angular_damping = 1.0
assert rigid_body.linear_damping == 0.5
assert rigid_body.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 the dominance group when the rigid-body is created.
rigid_body = rp.RigidBody.dynamic().dominance_group(10).build()

# Set the dominance group after the rigid-body creation.
rigid_body = world.rigid_bodies[rigid_body_handle]
rigid_body.dominance_group = 10
assert rigid_body.dominance_group == 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 PhysicsWorld.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 is created.
rigid_body = rp.RigidBody.dynamic().ccd_enabled(True).build()

# Enable CCD after the rigid-body creation.
rigid_body = world.rigid_bodies[rigid_body_handle]
rigid_body.ccd_enabled = True
assert rigid_body.ccd_enabled

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).

However, a sleeping rigid-body won't respond to any user action. This is why it is possible to wake-up the rigid-body manually with RigidBody.wake_up() or PhysicsWorld.wake_up(handle) (if their strong argument is True, the default, the rigid-body is guaranteed to stay awake for several timesteps, otherwise it may fall asleep again immediately). Setting the pose, the velocities, the gravity scale, the type, or the locked axes of a rigid-body through its properties always wakes it up. Some methods take an additional wake_up boolean argument that, if True (the default), ensures that the rigid-body wakes up before the action takes place. For example:

RigidBody.add_force(force, wake_up=True) will wake-up the rigid-body before adding the force.
ImpulseJointSet.remove(joint, wake_up=True) (resp. MultibodyJointSet.remove) will wake-up the two rigid-bodies attached by the removed joint.
ColliderSet.remove(collider, islands, bodies, wake_up=True) will wake-up the rigid-body the removed collider is attached to (PhysicsWorld.remove_collider always does).

Unless you want to achieve special effects, it is recommended to keep the default value True of the wake_up argument. One example of case where setting the argument of wake_up to False makes sense is to simulate a custom constant gravity with RigidBody.add_force(force, wake_up=False). This will result in the force being added to the rigid-body, but will allow the rigid-body to fall asleep if it reaches a dynamic equilibrium.

Whether a rigid-body is sleeping is given by the RigidBody.is_sleeping property, and it can be put to sleep manually with RigidBody.sleep(). A rigid-body can be prevented from ever sleeping with RigidBodyBuilder.can_sleep(False), or created already asleep with RigidBodyBuilder.sleeping(True). The velocity thresholds and the delay before the rigid-body falls asleep are given by the linear_threshold, angular_threshold, and time_until_sleep attributes of the RigidBodyActivation returned by the RigidBody.activation property. Keep in mind that this property returns a copy: the modified RigidBodyActivation must be assigned back to RigidBody.activation to take effect.

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 additional_solver_iterations, additional_pgs_iterations, soft_ccd_prediction, and allow_fast_rotation methods of the RigidBodyBuilder. After the creation of the rigid-body, they can be read and modified with the RigidBody properties of the same names.

# Give a rigid-body more solver accuracy than the rest of the scene.
rigid_body = (
    rp.RigidBody.dynamic()
    # Extra substeps run for the whole island component this body belongs to.
    .additional_solver_iterations(4)
    # Extra internal PGS iterations run per substep for that same component.
    .additional_pgs_iterations(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.
    .soft_ccd_prediction(0.5)
    # Let the body exceed the angular speed cap, e.g. for a wheel.
    .allow_fast_rotation(True)
    # Gyroscopic forces give more realistic behaviors, e.g. the precession of a spinning top.
    .gyroscopic_forces(True)
    .build()
)

note

The gyroscopic forces of a rigid-body can be disabled as well with RigidBodyBuilder.gyroscopic_forces(False), or by setting its RigidBody.gyroscopic_forces_enabled property to False after its creation. When enabled (the default), 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.

User-data

Each rigid-body can be given a user-defined data: a Python int in the range of an unsigned 128-bits integer (i.e., from 0 to 2**128 - 1). This integer can have any value and is never used/modified by the physics-engine. This can for example be useful to store an index (or any integer identifier) referencing your own data associated to the rigid-body, or to add some custom data for custom contact filtering/modification.

This user-data can be set when the rigid-body is created or after its creation with the RigidBody.user_data property:

# Set the user-data when the rigid-body is created.
rigid_body = rp.RigidBody.dynamic().user_data(42).build()

# Set the user-data after the rigid-body creation.
rigid_body = world.rigid_bodies[rigid_body_handle]
rigid_body.user_data = 42
assert rigid_body.user_data == 42

Creation and insertion​

Rigid-body type​

Position​

Velocity​

Gravity​

Forces and impulses​

Mass properties​

Locking translations/rotations​

Damping​

Dominance​

Continuous collision detection​

Sleeping​

Solver settings​

User-data​

Creation and insertion

Rigid-body type

Position

Velocity

Gravity

Forces and impulses

Mass properties

Locking translations/rotations

Damping

Dominance

Continuous collision detection

Sleeping

Solver settings

User-data