Houdini 20.5 Nodes Dynamics nodes

Apply Data dynamics node

Attaches data to simulation objects or other data.

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The Apply Data DOP attaches one or more pieces of data to a set of simulation objects, or to another piece of data.

The effect of attaching data to an object varies depending on the way in which the data is attached, and the solver attached to the object. For example, force data (Fan Force, Gravity Force) can be attached to an object to influence the motion of the object.

Geometry data may be attached to give a form to the object so it can be used in collision detection with other objects. Position or Motion data can be attached to directly control the way an object moves through space.

Although data is primarily attached to an object for use by the object’s solver, there are no limitations on what kind of data can be attached to an object. Any data that a solver doesn’t have a specific use for is simply ignored.

This ability to attach arbitrary data to an object can be used to hold state information that may be used elsewhere in the hip file. It also means that an object can be switched from one solver to another even if those solvers have different requirements on the data attached to the object.

The effect of attaching data to other data varies depending on the way in which the data is attached, and the solver attached to the object. Generally data attached to other data modifies the behavior or the parent data in some way.

For example, Noise subdata can be attached to most force data to add a random factor to the generated force. Or solver data can be attached to a Switch Solver or Blend Solver to specify the solvers to switch or blend between.

Parameters

Activation

If this parameter is zero, the objects passing through this node are not altered in any way. No data is attached.

This parameter is evaluated for each object passing through this node, so this parameter may use object specific local variables to enable or disable the operation of this node on a per-object basis.

Group

Only objects matching the Group string have data attached to them by this node. The Group string may be a series of space separated object group names, object names, or object ids. Wildcards, “?” and “*”, can be used to match object names by pattern.

Any token can be preceded by the negation character, “^”, in which case the objects matching that token are explicitly excluded. Only objects which match the Group parameter and evaluate the Activation parameter to a non-zero value are modified by this node. This parameter is only meaningful when attaching data to objects.

Inputs

First

The first input to this node supplies the objects or data that will have new data attached to them.

Subsequent

The remaining inputs to this node are the data that are to be attached to the objects or data on the first input.

Outputs

First

The objects or data input to this node are sent through the single output, but with their new data attached.

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.

Dynamics nodes