Houdini 20.5 Nodes Dynamics nodes

RBD Keyframe Active dynamics node

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The RBD Keyframe Active DOP provides an easy way to have an RBD Object transition from keyframed animation to simulated animation and back again. In addition to performing the transition from active to passive object, this node has parameters to perform the actual position keyframing.

Using Activate Objects

  1. Select the dynamic objects to activate.

  2. Click the Activate Objects tool on the Drive Simulation tab.

Using Deactivate Objects

  1. Select the dynamic objects to deactivate.

  2. Click the Deactivate Objects tool on the Drive Simulation tab.

Parameters

Active Value

Sets the active state of the objects. A value of one or more indicates the object is active and will be animated automatically by the RBD or other Solver attached to the object.

A value less than one indicates the object is passive and will use the keyframed Position and Rotation values to update its position at each timestep.

Use OBJ Path/OBJ Path

Causes the simulated object to get its position information from the specified OBJ node instead of using the Position/Rotate parameters.

Position

When the object is in passive mode, this parameter sets the position of the object in world space.

Velocity is calculated automatically at each timestep using the previous and current position values.

Rotation

When the object is in passive mode, this parameter controls the orientation of the object.

Angular velocity is calculated automatically at each timestep based on the previous and current rotation values.

Set Keyframe with Last Simulated Values

Pressing this button will copy the current position and orientation information for the object into the Position and Rotation parameters of this node.

This makes it easy to do a seamless transition from simulated to keyframed positions and rotations.

Keyframe Last Values

Sets a keyframe in the Position and Rotation parameters using the current values in the Position data of the DOP object being manipulated.

Blend Simulated Values

Specifies a blend factor between the simulated and keyframed values. This is an alternative way to make a smooth transition from simulated to keyframed values.

Instead of jumping immediately from the last simulated position to a particular keyframed position, this blend factor is used to blend the last simulated position with the keyframed position.

Activation

When this value is zero, this node will have no effect on the input objects.

Group

Only the input objects which match the Group filter will be modified.

Inputs

First

The RBD Objects whose active state and position information will be keyframed.

Outputs

First

The exact same set of objects that feed into the input are sent out 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

SimpleKeyActive Example for RBD Keyframe Active dynamics node

This example uses the RBD Keyframe Active node to switch from a keyframed animation to an RBD Solver, and back to keyframed animation. This same animation could be created using a Switch Solver or Blend Solver, but this approach is simpler if the only requirement is switching from keyframed to simulated motion for a few RBD Objects.

Dynamics nodes