Rigid Body Nodes

The following settings are Rigid Body Nodes, some of which you can access only through the Attribute Editor when the object is selected. For a list of Node commands, see the Bullet Node Command Reference.

Rigid Body Properties

Attributes in the Rigid Body Properties section of the bulletRigidBodyShape node Attribute Editor determine the behavior of dynamic rigid bodies in the simulation.

Body Type

Choose from three body types:

Static Body: does not move
Kinematic RigidBody: hand-animatable
Dynamic RigidBody: moved by rigid body system after initial position set.

Never Sleeps

Activate this so that the dynamic rigid body always participates in the simulation. When this setting is disabled, the rigid body does not participate in the simulation if its linear velocity's magnitude is below 0.8 and angular velocity's magnitude is below 1.0. The velocity thresholds are currently not exposed for user control. The bullet object re-engages if it receives an impact. This is an optimization setting and can help to stop the lasting "simmering" motion of a bullet object.

Mass

Setting the mass to zero for a dynamic rigid body effectively turns it into a kinematic rigid body. That is, the rigid body stops moving due to forces in the physics simulation. This attribute can be animated, and can be useful to lock objects in place.

Center of Mass

Sets the center of mass for the object used as the rotational pivot.

Note: If you change the Center of Mass attribute on an axis for any collider shape other than Hull, the Collider Shape primitive is transformed. To counteract this, add the opposite offset on the same axis in the Collider Shape Offset attributes, under Collider Properties, which you can accessed if you turn off Auto Fit.

Linear Damping

Use this setting to define the clamp value applied to the linear velocity when calculating rolling friction. Because the physical property does not model rolling friction, it uses a dampening system so that objects can come to rest. For example, without a Damping setting, a rolling sphere rolls on forever. It is done by scaling linear velocities at each simulation step by dampening factor for all colliding objects.

A value of 1 retains actual velocity. Any value below 1 decreases velocity, while any value greater increases it. The default setting is 0.01.

Angular Damping

Use this setting to define the clamp value applied to the angular velocity when calculating rolling friction. Because the Physical property does not model rolling friction, it uses a dampening system so that objects can come to rest.

A value of 1 retains actual velocity. Any value below 1 decreases velocity, while any value greater increases it. The default setting is 0.01.

Friction

Use the Friction setting to set the surface resistance of an object. The default setting is 0.5.

Restitution

Sets the speed at which the solve object returns to its original state. Creates "bounce". The default setting is 0.99.

Initial Conditions

This section contains attributes that are applied when the Bullet solver starts simulating.

Initially Sleeping

Activate this so that the dynamic rigid body does not react until something collides with it.

Initial Angular Velocity

Change this setting to modify the angular movement of the object. Use this setting to define the clamp value. Applied to the rigid body's start velocity when calculating rolling friction.

Initial Velocity

Change this setting to modify the initial velocity of the object. This is set at the start frame only.

In World Matrix

Initial position for dynamic rigid bodies, and specified position for kinematic ones.

Glue Shapes

Turn on this option to have control of the Rigid Body Glue constraint, which is useful for scenes with numerous rigid body objects. See Use Glue to Bond Rigid Bodies for more information on using the Glue constraint.

Glue Breaking Threshold

Use this slider to determine how resistant the dynamic constraint is to breakage. This value is relative to the overall scene or object scale.

A threshold value of 100.00 means that the constraint requires much strength to break. A threshold value of 0 means that the constraint breaks immediately, as though no constraint exists at all. Because links are formed at the start frame, once links are broken they do not reform, unless the Glue Breaking Threshold attribute is animated back to 1.

Forces/Impulses

This section contains attributes that control impulses applied to the rigid body every frame. Note that you will often want to animate these values so that they return to zero. Otherwise, in the absence of any counter-acting forces, continuously applying impulses over an entire animation can cause the rigid body to continuously accelerate.

Impulse: Sets the impulse force applied to the object at applied at each frame of simulation.
Impulse Position: Position where impulse force will be applied to the object relative to the center of mass at a specified time.
Torque Impulse: Sets the torque (rotational) impulse force applied to the object at a specified time

Collider Properties

These attributes specify how the shape of the rigid body is represented in the simulation. In general, simpler shapes perform better and are more stable and predictable. The simplest shapes are plane (infinite), box, sphere, cylinder and capsule. Their shape is determined by the following attributes:

Collider Shape Type: Lets you specify the type of object, or the mesh that to use to approximate the solve object. Select Box, Sphere, Capsule, Hull, Plane, Cylinder, Compound and Auto Compound.
Note: It is a good idea to match the shape of your object as closely as possible as the collision behavior is affected by this shape.
Input Mesh: This shows which mesh is connected to rigid body. You cannot edit this field.
Axis: Lets you set an axis for the collider shape approximation.
Length: Lets you set the length of the cylindrical section of a capsule- or cylinder-shaped collider approximation.
Radius: Lets you set the radius of a sphere or cylinder-shaped collider approximation, or the radius of the spherical caps of a capsule-shaped collider approximation.
Extents: Sets the length of the sides of the collider shape approximation.
Refit: Performs a onetime auto-fit on the existing rigid body. Note that the plane collider represents an infinite plane that passes through the position of the object's transform, and is oriented according to its rotation. It extends infinitely in the local X and Z directions.
Auto Fit: Automatically updates the other attributes.
Note: if you change the Autofit option in the channel box, it does not update until you return to the Attribute Editor.

Collision Filters

These attributes let you configure selective collisions. By default, rigid bodies are members of the Default Filter collision group and collide with other members of the Default Filter collision group. The rigid body may be a member of up to 16 groups and can be configured to collide with up to 16 other members of the same group. The collision filter group names can be customized by the Bullet solver's collisionFilters attribute.

See the Bullet Physics Manual at www.bulletphysics.org for more information about the different collider shape types.