Rigid Body Properties

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.

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.

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.

Friction

Use the Friction setting to set the surface resistance of an object.

Restitution

Sets the speed at which the solve object returns to its original state.

Initial Conditions

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

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.

NoteIt 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.

Noteif 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.