Houdini 16.5 Nodes Dynamics nodes

Copy Objects dynamics node

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The Copy Objects DOP makes copies of the input simulation objects. Unlike the Copy Data DOP, this node does not work using global parameters or re-evaluating the input node. Instead this node makes a number of identical copies of the input simulation objects. The task of making each object unique is left to subsequent DOP nodes.

For example, this node may be followed by an RBD State node which uses the object identifier of the copied objects to give each object a unique position.

Parameters

Activation

This node will create copies of the input objects on any timestep where this parameter evaluates to a non-zero value.

Generally this parameter will contain an expression which will evaluate to one on a particular timestep, and zero all other times. However other arrangements are possible to, for example, create a steady stream of copies of the input objects.

Number of Copies

Sets the number of objects this node should create. Note that this number is independent of the number of input objects. If there are 10 input object, but this parameter value is 2, then only 2 new objects will be created, not 20.

Object To Copy

This parameter controls which input object will be used as the source for the copy operation. This parameter is re-evaluated for each copy made by this node, and so may be an expression that chooses a different input object to copy based on which output object is being generated.

This parameter may match more than one input object (using wildcards or group names). The Object Index parameter is used to decide which among the set of matching objects to copy.

Object Index

This parameter helps control which input object will be used as the source for the copy operation. This parameter is re-evaluated for each copy made by this node, and so may be an expression that chooses a different input object to copy based on which output object is being generated.

The Object To Copy parameter is first used to create a set of matching input objects. The value of this parameter specifies which object within that set to copy.

Tip

To make duplicates of a set of objects, such as the objects produced by an RBD Fractured Object DOP, use an Object To Copy value of *, and an Object Index value of $OBJ. Or to make several copies of each input object, use an Object Index of $OBJ % number_of_input_objects. Then set the Number of Copies to the total number of output objects.

Object Name

Controls the name of the output object. Often this parameter will be simply use the SRCNAME local variable, which is set to the name of the object being copied.

Copy Objects Created By This Node

Normally the only objects available for copying are those in the input to this node. This is useful for creating objects based on a template object as already created objects will not be candidates for copying, simplifying the object index.

For splitting existing objects, however, one may need to recursively split an object that was itself created by this copy object. In this case you need access to both the input objects and the objects created by this node. Note that in this case you will almost always have candidate objects for copying so it is important to set the Activation to only occur on desired frames. With preserve input off and copy own objects off, copying would only occur when template objects are first created, allowing the activation to be left at one.

Preserve Input Objects

If this parameter is off, all input objects are destroyed once the object copies are made. Turning this parameter on passes the input objects untouched through this node.

Solve on Creation Frame

For the newly created objects, this parameter controls whether or not the solver for that object should solve for the object on the timestep in which it was created. Usually this parameter will be turned on if this node is copying objects in the middle of a simulation rather than creating objects for the initial state of the simulation.

Locals

SRCID

The unique ID of the source object.

SRCNAME

The name of the source object.

ST

This value is the simulation time for which the node is being evaluated.

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

This value 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

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

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

TIMESTEP

This value is the size of a simulation timestep. This value is useful to scale values that are expressed in units per second, but are applied on each timestep.

SFPS

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

SNOBJ

This is the number of objects in the simulation. For nodes that create objects such as the Empty Object node, this value will increase for each object that is evaluated.

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

NOBJ

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

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

OBJ

This value is the index of the specific object being processed by the node. This value will always run from zero to NOBJ-1 in a given timestep. This value does not identify the current object within the simulation like OBJID or OBJNAME, just 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 will be -1 if the node does not process objects sequentially (such as the Group DOP).

OBJID

This is the unique object 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.

The object identifier can always be used to uniquely identify a given object. This makes this variable very useful in situations where each object needs to be treated differently. It can be used to produce a unique random number for each object, for example.

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

This value is the simulation time (see variable ST) at which the current object was created.

Therefore, to check if an object was created on the current timestep, the expression $ST == $OBJCT should always be used. This value will be zero if the node does not process objects sequentially (such as the Group DOP).

OBJCF

This value is the simulation frame (see variable SF) at which the current object was created.

This value is equivalent to using the dopsttoframe expression on the OBJCT variable. This value will be zero if the node does not process objects sequentially (such as the Group DOP).

OBJNAME

This is 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 only on those 20 objects. This value will be the empty string if the node does not process objects sequentially (such as the Group DOP).

DOPNET

This is 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 node, 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