Houdini 20.5 Nodes Dynamics nodes

RBD Packed Object dynamics node

Creates a single DOP object from SOP Geometry that represents a number of RBD Objects.

On this page
Since 13.0

The RBD Packed Object DOP creates a single DOP Object inside the DOP simulation. It takes the geometry from the given SOP Path and uses each primitive that has a transform and a single point to represent an RBD object. This includes primitives such as packed primitives, spheres, and tubes. Each primitive provides the collision geometry for a rigid body, and attributes on the primitive’s point are used to store information such as orientation, mass, and velocity.

The resulting DOP Object represents large numbers of objects in a more efficient manner than the RBD Fractured Object DOP or the RBD Point Object DOP. This object representation is currently only understood by the Bullet Solver.

Note

In Houdini 13, RBD Packed Objects can’t interact with other solvers such as cloth and fluids.

Point Attributes

The Bullet Solver uses several point attributes to store the properties of each piece of a packed object.

Name Type Description
active Integer

Specifies whether the object is able to react to other objects in the simulation. The default active state is set by the Initial Object Type parameter.
age Float

The age of the object, in seconds.
animated Integer

Specifies whether the object’s transform should be updated from its SOP geometry at each timestep. This attribute is only used if the active attribute is set to 0. The default value is set by the Initial Object Type parameter.
bounce Float

The elasticity of the object. If two objects of bounce 1.0 collide, they will rebound without losing energy. If two objects of bounce 0.0 collide, they will come to a standstill. The default value is set by the Bounce parameter.
bullet_add_impact Integer

When enabled, any impacts that occur during the simulation will be recorded in the Impacts or Feedback data. Enabling this option may cause the simulation time and memory usage to increase.
bullet_angular_sleep_threshold Float

The sleeping threshold for the object’s angular velocity. If the object’s angular speed is below this threshold for a period of time, the object may be treated as non-moving.
bullet_ignore Integer

Specifies whether the object should be completely ignored by the Bullet solver. Unlike setting the active attribute to zero, other objects in the simulation will not be able to collide against this object.
bullet_linear_sleep_threshold Float

The sleeping threshold for the object’s linear velocity. If the object’s linear speed is below this threshold for a period of time, the object may be treated as non-moving.
bullet_want_deactivate Integer

Disables simulation of a non-moving object until the object moves again. The linear and angular speed thresholds are used to determine whether the object is non-moving. If the Display Geometry checkbox is turned off, you will see the color of the Guide Geometry change from the Color to the Deactivated Color.
computecom Integer

Specifies whether the center of mass of the object should be computed automatically from the object’s collision shape. The default value is set by the Compute Center of Mass parameter.
computemass Integer

Specifies whether the mass of the object should be computed automatically from the object’s collision shape and density. The default value is set by the Compute Mass parameter.
creationtime Float

Stores the simulation time at which the object was created.
dead Integer

Specifies whether the object should be deleted during the next solve. The POP Kill node can be used to generate this attribute.
deforming Integer

Specifies whether the object’s collision shape should be rebuilt from its SOP geometry at each timestep. The default value is set by the Initial Object Type parameter.
density Float

The mass of an object is its volume times its density. The default value is set by the Density parameter.
friction Float

The coefficient of friction of the object. A value of 0 means the object is frictionless. The default value is set by the Friction parameter.
inertialtensorstiffness Float

A scale factor that is applied to the inertial tensor of the object. A higher stiffness will make the object less likely to spin. The default value is set by the Rotational Stiffness parameter.
inheritvelocity Integer

If non-zero, the v and w point attribute values from the source SOP geometry will override the object’s initial velocity.
life Float

The maximum age of the object, in seconds. When the age attribute exceeds this value, the object will be marked dead.
mass Float

The mass of the object. If computemass is non-zero, this value will be computed automatically by the solver. Otherwise, the default value will be set by the Mass parameter.
name String

A unique name for the object. This value is used by Constraint Networks to identify constraints that should be attached to this object.
orient Quaternion

The orientation of the object around the pivot. When adjusting this attribute, it is not required to update the packed primitive’s transform as it will be updated by the Bullet solver at the end of the timestep.
P Vector

The current position of the object’s center of mass.
pscale Float

The uniform scale of the object around the pivot. This value is multiplied with the scale attribute if it exists. See the scale attribute’s documentation for more details.
pivot Vector

The pivot point that the orientation is applied to. If computecom is non-zero, this will be computed from the object’s collision shape. Otherwise, the value is set from the Center of Mass parameter.
restxform Matrix

The initial transform of the packed primitive. The packed primitive’s current transform is the combination of this transform with the position transform (the transform applied by the simulation). The position transform is defined by P, rest, pivot, orient, pscale, and scale. This attribute is automatically initialized by the solver when a new object is identified and its creationtime attribute is uninitialized.
scale Vector

The scale of the object around the pivot. This value is multiplied with the pscale attribute if it exists. When adjusting this attribute, it is not required to update the packed primitive’s transform as it will be updated by the Bullet solver at the end of the timestep.

The solver automatically updates the collision geometry and the object’s inertial tensor to reflect this scale.

Note

To recompute the object’s mass when the scale is changed, set the computemass point attribute to 1.

Note

Dynamic scaling is not supported for auto-fitted collision shapes (i.e. when Geometry Representation is Box, Capsule, Cylinder, Sphere, or Plane). In such cases you can set the bullet_autofit_valid point attribute to 0 to update the auto-fitted shape, or instead build a suitable bounding proxy in SOPs and use a Geometry Representation of Convex Hull.
v Vector

The linear velocity of the object. The initial velocity is set by the Velocity parameter.
w Vector

The angular velocity of the object, in radians per second. The initial angular velocity is set by the Angular Velocity parameter.

Collision Shape Attributes

The following attributes can be used to customize the collision shape for each object.

Name Type Description
bullet_adjust_geometry Integer

Shrinks the collision geometry to prevent the Collision Padding from increasing the effective size of the object. This can improve the simulation’s performance by preventing initially closely-packed collision shapes from interpenetrating, and also removes the gap between objects caused by the Collision Padding.
bullet_autofit Integer

Specifies whether to use the bounds of the object’s geometry to compute the collision shape when Geometry Representation is Box, Capsule, Cylinder, Sphere, or Plane.
bullet_collision_margin Float

A padding distance between shapes, which is used by the Bullet engine to improve the reliability and performance of the collision detection. You may need to scale this value depending on the scale of your scene. This padding increases the size of the collision shape, so it is recommended to enable Shrink Collision Geometry to prevent the collision shape from growing. This option is not supported for the Concave or Plane geometry representations.
bullet_color Vector

Specifies the color of the shape’s guide geometry.
bullet_deactivated_color Vector

Specifies the color of the shape’s guide geometry if the object is not moving and has been deactivated by the solver.
bullet_georep String

The collision shape used to represent the object. Possible values are convexhull, concave, box, capsule, cylinder, compound, sphere, plane, and none. Setting the collision shape to none will produce a static object with collisions disabled, but constraints can still be attached to the object.
bullet_groupconnected Integer

When Geometry Representation is Convex Hull, specifies whether the solver should create a compound shape that contains a separate convex hull collision shape for each set of connected primitives in the geometry.
bullet_length Float

The length of the Capsule or Cylinder collision shape in the Y direction. If AutoFit is enabled, this value will be computed automatically from the object’s geometry.
bullet_primR Vector

The orientation of the Box, Capsule, Cylinder, or Plane collision shape in the Bullet world. If AutoFit is enabled, this value will be computed automatically from the object’s geometry.
bullet_primS Vector

The size of the Box collision shape. If AutoFit is enabled, this value will be computed automatically from the object’s geometry.
bullet_primT Vector

The position of the Box, Sphere, Capsule, Cylinder, or Plane collision shape in the Bullet world. If AutoFit is enabled, this value will be computed automatically from the object’s geometry.
bullet_radius Float

The radius of the Sphere, Capsule, or Cylinder collision shape. If AutoFit is enabled, this value will be computed automatically from the object’s geometry.
bullet_shrink_amount Float

Specifies the amount of resizing done by Shrink Collision Geometry.

Collision Attributes

The attributes listed below can be used to configure the object’s collision-related behavior.

Name Type Description
activationignore String

If the object is inactive and has a positive min_activation_impulse value, it will not be activated by collisions with any objects that match this pattern. The pattern uses the same syntax as the collisionignore attribute.
activationxformgroup String

For an agent, specifies which of its rigid bodies should be activated when using the min_activation_impulse attribute. The default behavior is to activate all of the agent’s rigid bodies. This attribute is expected to contain the name of a transform group that specifies which joints' rigid bodies should be activated.
collisiongroup String

Specifies the name of a collision group that this object belongs to. This can be used by the collisionignore attribute to ignore collisions against specific groups of objects.
collisionignore String

The object will not collide against any objects that match this pattern. This pattern can use the name of DOP objects, the ID of DOP objects, DOP groups, the collisiongroup point attribute of a packed primitive in an RBD Packed Object, or the collisiongroup_agent point attribute values of an agent primitive. The POP Collision Ignore node can be used to generate this attribute.

  • * will disable all collisions, and an empty string will enable all collisions.

  • * ^object1 will cause the object to only collide against the DOP object whose name is object1.

  • wall* will disable collisions against packed primitives whose collisiongroup attribute begins with wall (or DOP objects whose name begins with wall).

  • object1/wall* will disable collisions against packed primitives from the DOP object object1 whose collisiongroup attribute begins with wall.
min_activation_impulse Float

The minimum impulse from a collision that will cause the object to switch from inactive to active. The activation occurs at the instant that the collision occurs. The activationignore attribute can be used to prevent the object from being activated by collisions with particular objects.
stickycollisionignore String

Sticky collisions will not occur for collisions with any objects that match this pattern. The pattern uses the same syntax as the collisionignore attribute.
minstickycollisionimpulse Float

The minimum impulse from a collision that will cause the object to stick to the other object. The constraint between the two objects is created at the instant that the collision occurs. The stickycollisionignore attribute can be used to prevent the object from being stuck to particular objects.
maxstickycollisionobjects Integer

The maximum number of other objects that this object can be stuck to. A value less than or equal to 0 disables this behavior, allowing the object to be stuck to any number of other objects.
numstickycollisionobjects Integer

Updated by the solver to track the number of other objects that this object was stuck to (limited by maxstickycollisionobjects). This can be used to easily determine whether sticky collisions have occurred for an object, without having to inspect the constraint network geometry.
maxstickycollisionpoints Integer

The maximum number of anchor points (constraints) that can be used to stick this object to another object. Constraints are attached at contact points between the two objects. The default value is 1, allowing a single constraint to be created between each pair of objects.

Speed Limit Attributes

The attributes listed below can be used to clamp the linear or angular speed of specific objects. The clamping is performed during each substep after forces and constraints have been applied (but before positions are updated), so this enforces a hard limit on the object’s speed. If the attribute value is negative, no clamping is performed. For animated or deforming static objects, the speed limit affects the velocities used when resolving collisions. The POP Speed Limit node can be used to generate these attributes.

Name Type Description
speedmin Float

The slowest speed (in units/second) that an object can move. This does not affect stationary objects.
speedmax Float

The fastest speed (in units/second) that an object can move.
spinmin Float

The slowest speed (in radians/second) that an object can rotate. This does not affect non-spinning objects.
spinmax Float

The fastest speed (in radians/second) that an object can rotate.

The attributes listed below can be used to limit the change in the object’s linear or angular velocity when solving constraints and collisions. This does not affect changes to the object’s velocity due to external forces, and is enforced each frame after constraints have been solved and positions have been updated. This can be useful to tame collisions that produce very high linear or angular velocities. If the value is negative, no clamping is performed.

Note

For best results, it is recommended to clamp both linear and angular acceleration simultaneously.

Setting the max acceleration to a very low value can introduce jitter for situations such as objects resting on the ground plane. With such values, the contact constraints cannot change the object’s velocity by enough to counter the acceleration due to gravity.

Name Type Description
accelmax Float

Limits the change in the object’s speed that is caused by enforcing constraints.
angaccelmax Float

Limits the change in the object’s angular speed that is caused by enforcing constraints.

Force Attributes

The attributes listed below can be used to apply forces to specific objects. Nodes such as POP Force and POP Drag can be used to generate these forces.

Name Type Description
airresist Float

Specifies how important it is to match the target velocity (targetv).
drag Float

When an object is dragged by the targetv and airresist attributes, this attribute is used to further scale the drag amount.
dragexp Float

Higher drag exponents cause objects that are far from targetv or targetw to recover faster than those that are already close to the target velocity.
force Vector

Specifies a force that will be applied to the center of mass of the object.
spinresist Float

Specifies how important it is to match the target angular velocity (targetw).
targetv Vector

Specifies a target velocity for the object. This is used in combination with the airresist attribute to compute a force that will move the object towards the desired velocity.
targetw Vector

Specifies a target angular velocity for the object. This is used in combination with the spinresist attribute to compute a torque that will move the object towards the desired angular velocity.
torque Vector

Specifies a torque that will be applied to the object.

Agent Attributes

For agent primitives, a rigid body is created for each transform in its rig that has at least one shape attached to it in the agent’s collision layer. Array attributes are used for properties that can vary for each of an agent’s rigid bodies, and the attributes are named with the _agent suffix. The arrays are indexed using the transform index from the agent’s rig.

The following array attributes are required:

  • bullet_autofit_valid_agent

  • bullet_length_agent

  • bullet_primR_agent

  • bullet_primS_agent

  • bullet_primT_agent

  • bullet_radius_agent

  • bullet_sleeping_agent

  • deactivation_time_agent

  • mass_agent

  • orient_agent

  • pivot_agent

  • trans_agent

  • v_agent

  • w_agent

The following array attributes are optional and can be used instead of the regular point attributes if varying values are needed.

  • active_agent

  • activationignore_agent

  • activationxformgroup_agent

  • bounce_agent

  • bullet_adjust_geometry_agent

  • bullet_collision_margin_agent

  • bullet_color_agent

  • bullet_deactivated_color_agent

  • bullet_shrink_amount_agent

  • collisiongroup_agent

  • collisionignore_agent

  • density_agent

  • force_agent

  • friction_agent

  • inertialtensorstiffness_agent

  • maxstickycollisionobjects_agent

  • maxstickycollisionpoints_agent

  • min_activation_impulse_agent

  • minstickycollisionimpulse_agent

  • numstickycollisionobjects_agent

  • stickycollisionignore_agent

  • torque_agent

The attributes listed below can also be used for agent primitives.

Name Type Description
bullet_updateagentxform Integer

Specifies whether the solver should update the agent’s overall transform (e.g. the P and orient point attributes) based on the root rigid body’s motion. This option is useful for a full ragdoll so that the agent’s point follows its motion, but may cause issues for a partial ragdoll where the agent’s overall transform is already being updated elsewhere (e.g. by the crowd solver).

Reserved Solver Attributes

Below is a list of attributes that are maintained internally by the solver. You should not modify these attributes yourself.

Name Class Type Description
bullet_autofit_valid Point Integer

When AutoFit is enabled, this value stores whether the solver has already computed collision shape attributes such as bullet_length and bullet_primT.
bullet_sleeping Point Integer

Tracks whether the object has been put to sleep by the solver. This occurs after the object and any nearby / constrained objects have been below the Linear Threshold and Angular Threshold for a period of time.

Note

When manually waking an object, the deactivation_time attribute should also be set to zero so that the object isn’t immediately put back to sleep.
deactivation_time Point Float

The amount of time that the object’s speed has been below the Linear Threshold or Angular Threshold. The solver uses this value to disable simulation of objects that have been inactive for a period of time.
found_overlap Point Integer

When the solver sees an object for the first time, it checks whether that object is initially overlapping with any other objects in the simulation and prevents any such pairs of objects from being forced apart. This value is used by the solver to determine whether it has performed this process for the object on a previous frame.
id Point Integer

A unique identifier for the object. This value is used by the solver to identify objects that have been added to or removed from the simulation.
nextid Detail Integer

Stores the id that the solver will assign to the next new object that is added.

Parameters

Creation Frame Specifies Simulation Frame

Determines if the creation frame refers to global Houdini frames ($F) or to simulation specific frames ($SF). The latter is affected by the offset time and scale time at the DOP network level.

Creation Frame

The frame number on which the object will be created. The object is created only when the current frame number is equal to this parameter value. This means the DOP Network must evaluate a timestep at the specified frame, or the object will not be created.

For example, if this value is set to 3.5, the Timestep parameter of the DOP Network must be changed to 1/(2*$FPS) to ensure the DOP Network has a timestep at frame 3.5.

Initial Object Type

Specifies the initial state of the objects. The active, animated, and deforming point attributes can be used to vary these values on a per object basis. For animated and deforming objects, the name point attribute is used to find the matching point from the SOP Path.

Create Active Objects

The objects will be simulated and react to other objects in the simulation.

Create Static Objects

The objects will not move or react to other objects in the simulation.

Create Animated Static Objects

The objects' transforms will be updated on each timestep from the SOP Path, but the objects will not react to other objects in the simulation.

Create Deforming Static Objects

The objects' collision shapes will be rebuilt on each timestep from the SOP Path, but the objects will not react to other objects in the simulation.

Create Deforming Active Objects

The objects will be simulated and react to other objects in the simulation, and their collision shapes will also be rebuilt on each timestep from the SOP Path.

Geometry Source

Specifies the source of the objects' geometry.

SOP

Use the SOP specified by the SOP Path parameter.

First Context Geometry

Use the SOP connected to the DOP network’s first input.

Second Context Geometry

Use the SOP connected to the DOP network’s second input.

Third Context Geometry

Use the SOP connected to the DOP network’s third input.

Fourth Context Geometry

Use the SOP connected to the DOP network’s fourth input.

SOP Path

The path to a SOP (or an Object, in which case the display SOP is used) which will be the geometry for this object.

Overwrite Attributes from SOP

When enabled, specifies a list of point attributes that will be updated on each frame from the SOP Path. The name point attribute is used to find the matching point from the SOP geometry.

Use Object Transform

The transform of the object containing the chosen SOP is applied to the geometry. This is useful if the initial location of the geometry is defined by an object transform.

Display Geometry

Controls if the geometry is displayed in the viewport. Does not reset the simulation when it is changed.

Simulation Geometry

The objects' geometry is displayed in the viewport.

Render Geometry

The geometry specified by the Render SOP Path parameter is transformed to match the simulation geometry and is displayed in the viewport. This is useful for visualizing the results of the simulation when using lower resolution shapes for the simulation geometry.

Render SOP Path

Specifies the SOP geometry to use when Display Geometry is set to Render Geometry.

Initial State

Position

Initial position in world space of the object.

Rotation

Initial orientation of the object. This is in RX/RY/RZ format.

Velocity

Initial velocity of the object.

Angular Velocity

Initial angular velocity of the object. This is the axis of rotation times the rate of rotation. Speed of rotation is measured in degrees per second, so a value of 360 will cause the object to rotate once per second.

Inherit Velocity from Point Velocity

If enabled, the v and w point attributes of the source geometry will override the object’s initial velocity.

Bullet Data

Show Guide Geometry

When turned on, displays a visualization of the object’s collision shape, including the Collision Padding. This is useful for debugging issues with collision detection, but is typically slower than only displaying the object’s geometry.

Color

The color of the guide geometry.

Deactivated Color

The color of the guide geometry if the object is not moving and has been deactivated by the Bullet Solver.

Geometry Representation

The shape used by the Bullet engine to represent the object. The Show Guide Geometry parameter can be used to visualize this collision shape.

Convex Hull

The default shape for the object. The Bullet Solver creates a collision shape from the convex hull of the geometry points.

Concave

The Bullet Solver converts the geometry to polygons and creates a concave collision shape from the resulting triangles. This shape is useful when simulating concave objects such as a torus or a hollow tube. However, it is recommended to only use the concave representation when necessary, since the convex hull representation typically provides better performance.

Box

The bounding box of the object.

Capsule

The bounding capsule of the object.

Cylinder

The bounding cylinder of the object.

Compound

Creates a complex shape consisting of Bullet primitives (including boxes, spheres, and cylinders). You will need to use the Bake ODE SOP.

Sphere

The bounding sphere of the object.

Plane

A static ground plane.

Create Convex Hull per Set of Connected Primitives

When turned on, and Geometry Representation is set to Convex Hull, the Bullet Solver creates a compound shape that contains a separate convex hull collision shape for each set of connected primitives in the geometry.

AutoFit Primitive Boxes, Capsules, Cylinders, Spheres, or Planes to Geometry

When turned on, the object’s Geometry subdata is analyzed instead of using the Position, Rotation, Box Size, Radius, and Length values.

When Geometry Representation is set to Box, Capsule, Cylinder, Sphere, or Plane, use the geometry bounds to create the shape.

Position

The position of the object shape in the Bullet world. This parameter is available when Geometry Representation is set to Box, Sphere, Capsule, Cylinder, or Plane, and AutoFit Primitive Boxes, Capsules, Cylinders, Spheres, or Planes to Geometry is turned off.

Rotation

The orientation of the object shape in the Bullet world. This parameter is available when Geometry Representation is set to Box, Capsule, Cylinder, or Plane, and AutoFit Primitive Boxes, Capsules, Cylinders, Spheres, or Planes to Geometry is turned off.

Box Size

The half extents of the box shape. This parameter is available when Geometry Representation is set to Box, and AutoFit Primitive Boxes, Capsules, Cylinders, Spheres, or Planes to Geometry is turned off.

Radius

The radius of the sphere shape. This parameter is available when Geometry Representation is set to Sphere, Capsule, or Cylinder, and AutoFit Primitive Boxes, Capsules, Cylinders, Spheres, or Planes to Geometry is turned off.

Length

The length of the capsule or cylinder in the Y direction. This parameter is available when Geometry Representation is set to Capsule or Cylinder, and AutoFit Primitive Boxes, Capsules, Cylinders, Spheres, or Planes to Geometry is turned off.

Collision Padding

A padding distance between shapes, which is used by the Bullet engine to improve the reliability and performance of the collision detection. You may need to scale this value depending on the scale of your scene. This padding increases the size of the collision shape, so it is recommended to turn on Shrink Collision Geometry to prevent the collision shape from growing.

This parameter is not available when Geometry Representation is set to Plane.

Shrink Collision Geometry

When turned on, shrinks the collision geometry to prevent the Collision Padding from increasing the effective size of the object.

This can improve the simulation’s performance by preventing initially closely-packed collision shapes from interpenetrating, and also removes the gap between objects caused by the Collision Padding.

When Geometry Representation is set to Box, Capsule, Cylinder, Compound, or Sphere, the radius and/or length of each primitive is reduced by Shrink Amount.

When Geometry Representation is set to Convex Hull, each polygon in the representation geometry is shifted inward by Shrink Amount.

This parameter is not available when Geometry Representation is set to Concave or Plane.

Shrink Amount

The amount of resizing done by Shrink Collision Geometry. By default, this value is equal to the Collision Padding so that the resulting size of the collision shape (including the Collision Padding) is the same size as the object’s geometry.

This parameter is not available when Geometry Representation is set to Concave or Plane.

Add Impact Data

When turned on, any impacts that occur during the simulation is recorded in the Impacts or Feedback data. Turning on this option may cause the simulation time and memory usage to increase.

The Impacts and Feedback data are added as subdata on the DOP object. You can see them in the details view when there is at least one impact during the timestep.

The Feedback data is essentially the same as the Impacts data, but is used when there is mutual interaction in the DOP network between multiple objects that are being solved by different solvers. For example, for an RBD object and FLIP fluid object to mutually interact, the DOP network runs multiple feedback loops to resolve the interactions, and the Feedback data is used to pass impact data between the solvers.

Enable Sleeping

When turned on, disables simulation of a non-moving object until the object moves again. The linear and angular speed thresholds are used to determine whether the object is non-moving. If Display Geometry is turned off, you will see the color of the guide geometry change from the Color to the Deactivated Color.

Linear Threshold

The sleeping threshold for the object’s linear velocity. If the object’s linear speed is below this threshold for a period of time, the object may be treated as non-moving.

Angular Threshold

The sleeping threshold for the object’s angular velocity. If the object’s angular speed is below this threshold for a period of time, the object may be treated as non-moving.

Allow Initial Overlap

When the solver sees an object for the first time, it checks whether that object is initially overlapping with any other objects in the simulation and prevents any such pairs of objects from being forced apart. If this is disabled, initially overlapping objects will be moved apart to resolve the collisions.

Tip

To resolve collisions between initially overlapping objects without adding velocity, ensure that Split Impulse is enabled on the Bullet solver and set the Penetration Threshold to 0.

Min Activation Impulse

The minimum impulse from a collision that will cause the object to switch from inactive to active. The activation occurs at the instant that the collision occurs. The activationignore attribute can be used to prevent the object from being activated by collisions with particular objects.

Physical

Compute Center of Mass

Specifies that the object’s center of mass should be computed automatically from its collision shape on the next frame it is solved.

Inherit Pivot from Point Position

Sets the center of mass to the primitive’s initial point position.

Pivot

Specifies the object’s center of mass.

Compute Mass

Determines if the mass will be calculated automatically from the object’s volumetric representation and glued sub-objects.

Density

The mass of an object is its volume times its density.

Mass

The absolute mass of the object.

Rotational Stiffness

When an object receives a glancing blow, it will often acquire a spin. The amount of spin acquired depends on the shape and mass distribution of the object, known as the inertial tensor.

The Rotational Stiffness is a scale factor applied to this. A higher stiffness will make the object less liable to spinning, a lower value will make it more ready to spin.

Bounce

The elasticity of the object. If two objects of bounce 1.0 collide, they rebound without losing energy. If two objects of bounce 0.0 collide, they come to a standstill.

Bounce Forward

The tangential elasticity of the object. If two objects of bounce forward 1.0 collide, their tangential motion is affected only by friction. If two objects of bounce forward 0.0 collide, their tangential motion is matched.

Friction

The coefficient of friction for the object. A value of 0 means the object is frictionless.

This governs how much the tangential velocity is affected by collisions and resting contacts.

Dynamic Friction Scale

An object sliding may have a lower friction coefficient than an object at rest. This is the scale factor that relates the two. It is not a friction coefficient, but a scale between zero and one.

A value of one means that dynamic friction is equal to static friction. A scale of zero means that as soon as static friction is overcome, the object acts without friction.

Temperature

Temperature marks how warm or cool an object is. This is used in gas simulations for ignition points of fuel or for buoyancy computations.

Since this does not relate directly to any real world temperature scale, ambient temperature is usually considered 0.

Outputs

First

The RBD object created by this node is sent through the single output.

Locals

ST

The simulation time for which the node is being evaluated.

Depending on the settings of the DOP Network Offset Time and Scale Time parameters, this value may not be equal to the current Houdini time represented by the variable T.

ST is guaranteed to have a value of zero at the start of a simulation, so when testing for the first timestep of a simulation, it is best to use a test like $ST == 0, rather than $T == 0 or $FF == 1.

SF

The simulation frame (or more accurately, the simulation time step number) for which the node is being evaluated.

Depending on the settings of the DOP Network parameters, this value may not be equal to the current Houdini frame number represented by the variable F. Instead, it is equal to the simulation time (ST) divided by the simulation timestep size (TIMESTEP).

TIMESTEP

The size of a simulation timestep. This value is useful for scaling values that are expressed in units per second, but are applied on each timestep.

SFPS

The inverse of the TIMESTEP value. It is the number of timesteps per second of simulation time.

SNOBJ

The number of objects in the simulation. For nodes that create objects such as the Empty Object DOP, SNOBJ increases for each object that is evaluated.

A good way to guarantee unique object names is to use an expression like object_$SNOBJ.

NOBJ

The number of objects that are evaluated by the current node during this timestep. This value is often different from SNOBJ, as many nodes do not process all the objects in a simulation.

NOBJ may return 0 if the node does not process each object sequentially (such as the Group DOP).

OBJ

The index of the specific object being processed by the node. This value always runs from zero to NOBJ-1 in a given timestep. It does not identify the current object within the simulation like OBJID or OBJNAME; it only identifies the object’s position in the current order of processing.

This value is useful for generating a random number for each object, or simply splitting the objects into two or more groups to be processed in different ways. This value is -1 if the node does not process objects sequentially (such as the Group DOP).

OBJID

The unique identifier for the object being processed. Every object is assigned an integer value that is unique among all objects in the simulation for all time. Even if an object is deleted, its identifier is never reused. This is very useful in situations where each object needs to be treated differently, for example, to produce a unique random number for each object.

This value is also the best way to look up information on an object using the dopfield expression function.

OBJID is -1 if the node does not process objects sequentially (such as the Group DOP).

ALLOBJIDS

This string contains a space-separated list of the unique object identifiers for every object being processed by the current node.

ALLOBJNAMES

This string contains a space-separated list of the names of every object being processed by the current node.

OBJCT

The simulation time (see variable ST) at which the current object was created.

To check if an object was created on the current timestep, the expression $ST == $OBJCT should always be used.

This value is zero if the node does not process objects sequentially (such as the Group DOP).

OBJCF

The simulation frame (see variable SF) at which the current object was created. It is equivalent to using the dopsttoframe expression on the OBJCT variable.

This value is zero if the node does not process objects sequentially (such as the Group DOP).

OBJNAME

A string value containing the name of the object being processed.

Object names are not guaranteed to be unique within a simulation. However, if you name your objects carefully so that they are unique, the object name can be a much easier way to identify an object than the unique object identifier, OBJID.

The object name can also be used to treat a number of similar objects (with the same name) as a virtual group. If there are 20 objects named “myobject”, specifying strcmp($OBJNAME, "myobject") == 0 in the activation field of a DOP will cause that DOP to operate on only those 20 objects.

This value is the empty string if the node does not process objects sequentially (such as the Group DOP).

DOPNET

A string value containing the full path of the current DOP network. This value is most useful in DOP subnet digital assets where you want to know the path to the DOP network that contains the node.

Note

Most dynamics nodes have local variables with the same names as the node’s parameters. For example, in a Position DOP, you could write the expression:

$tx + 0.1

…to make the object move 0.1 units along the X axis at each timestep.

Examples

ActivateObjects Example for RBD Packed Object dynamics node

This example shows how to modify the “active” point attribute of an RBD Packed Object to change objects from static to active.

AnimatedObjects Example for RBD Packed Object dynamics node

This example shows how to use animated packed primitives in an RBD Packed Object and set up a transition to active objects later in the simulation.

DeleteObjects Example for RBD Packed Object dynamics node

This example shows how to remove objects from the simulation that are inside a bounding box.

EmittingObjects Example for RBD Packed Object dynamics node

This example shows how to use a SOP Solver to create new RBD objects and add them to an existing RBD Packed Object.

ScalingRBDs Example for RBD Packed Object dynamics node

This example demonstrates how to modify the scale of RBD objects during a simulation.

SpeedLimit Example for RBD Packed Object dynamics node

This example shows how to limit the speed of specific objects in the simulation.

See also

Dynamics nodes