Houdini 20.5 Nodes Dynamics nodes

File Data dynamics node

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The File Data DOP allows a single piece of data to be saved and/or loaded from a file on disk. All simulation data is saved to files with the extension .simdata, regardless of the data type. This is because any data type can be saved or loaded by this one DOP node. Therefore it is a good idea to add a second extension to describe the data type when saving files (such as .Volume.simdata for Volume data).

Parameters

Operation Mode

Indicates the mode of operation for this node.

Automatic

In this mode, the File Data DOP looks for a file that matches the specified name. If one is found, it behaves as if it were in Read mode. Otherwise it behaves as if it were in Write mode.

Note that this mode does not pay any attention to changes made to the network. Hitting the Recook Simulation button will not cause a File Data DOP in this mode to overwrite files that already exist. To force objects in this network to recook and write out updated files, you must delete the existing files on disk.

You can also change the file mode to Write Files to update the simulation, and then change back to Automatic.

Read Files

In this mode the File Data DOP looks for a file with the specified name, and loads it. If no matching file is found, this node behaves as if it were in No Operation node.

The data in that file is attached to the object using the Data Name parameter.

Write Files

In this mode the File Data DOP will write the specified file to disk. Any existing file will be replaced.

The data written to disk is either the data attached by the node on the second input (if there is one) or the data attached to the object with the name specified in the Data Name parameter.

No Operation

In this mode, the File Data DOP behaves as if it were replaced by the node connected to the second input, and does no file operations.

The data connected to the second input is attached to the objects, and any data connected to the third or subsequent inputs will still be attached as subdata to this second input data.

File

The name of the file to which the data should be written. This should usually be an expression involving the data type, and possibly the object identifier and simulation frame number.

This file name is evaluated for each object to which this data is being applied, so it can contain object-specific local variables such as OBJNAME.

Create Intermediate Directories

Create intermediate parent directories for output files as needed.

Load or Save New Data Only

Turning on this option causes this node to load or save files only on the first simulation timestep in which the data was created.

For Read mode, this means a file is only loaded if no data with the specified name currently exists on the object being processed.

In Write mode, this means that a file is written to disk only if the input data was created this timestep.

Data Sharing

Controls the way in which the data created by this node is shared among multiple objects in the simulation.

Data sharing can greatly reduce the memory footprint of a simulation, but at the expense of requiring all objects to have exactly the same data associated with them.

Do Not Share Data

No data sharing is used. Each object has its own copy of the data attached.

This is appropriate for situations where the data needs to be customized on a per-object basis, such as setting up initial positions and velocities for objects.

Share Data Across All Time

This node only creates a single piece of data for the whole simulation. This data is created the first time it is needed, so any expressions will be evaluated only for the first object.

All subsequent objects will have the data attached with the same values that were calculated from the expressions for the first object. It is important to note that expressions are not stored with the data, so they cannot be evaluated after the data is created.

Expressions are evaluated by the DOP node before creating the data. Expressions involving time will also only be evaluated when this single piece of data is created. This option is appropriate for data that does not change over time, and is the same for all objects, such as a Gravity DOP.

Share Data In One Timestep

A new piece of data is created for each timestep in the simulation. Within a timestep though, all objects have the same data attached to them. So expressions involving time will cause this data to animate over time, but expressions involving the object will only evaluate for the first object to which the data is attached.

This option is appropriate for data that changes over time, but is the same for all objects such as a Fan Force DOP, where the fan may move or rotate over time.

Activation

Determines if this node should do anything on a given timestep and for a particular object. If this parameter is an expression, it is evaluated for each object (even if data sharing is turned on).

If it evaluates to a non-zero value, then the data is attached to that object. If it evaluates to zero, no data is attached, and data previously attached by this node is removed.

Group

When an object connector is attached to the first input of this node, this parameter can be used to choose a subset of those objects to be affected by this node.

Data Name

Indicates the name that should be used to attach the data to an object or other piece of data. If the Data Name contains a “/” (or several), that indicates traversing inside subdata.

For example, if the Fan Force DOP has the default Data Name “Forces/Fan”. This attaches the data with the name “Fan” to an existing piece of data named “Forces”. If no data named “Forces” exists, a simple piece of container data is created to hold the “Fan” subdata.

Different pieces of data have different requirements on what names should be used for them. Except in very rare situations, the default value should be used. Some exceptions are described with particular pieces of data or with solvers that make use of some particular type of data.

Unique Data Name

Turning on this parameter modifies the Data Name parameter value to ensure that the data created by this node is attached with a unique name so it will not overwrite any existing data.

With this parameter turned off, attaching two pieces of data with the same name will cause the second one to replace the first. There are situations where each type of behavior is desirable.

If an object needs to have several Fan Forces blowing on it, it is much easier to use the Unique Data Name feature to ensure that each fan does not overwrite a previous fan rather than trying to change the Data Name of each fan individually to avoid conflicts.

On the other hand, if an object is known to have RBD State data already attached to it, leaving this option turned off will allow some new RBD State data to overwrite the existing data.

Inputs

First Input

This optional input can be used to control which simulation objects are modified by this node. Any objects connected through this input and which match the Group parameter field will be affected.

If this input is not connected, this node can be used in conjunction with an Apply Data node, or can be used as input to another data node to create subdata on the data rather than an object.

Second Input

The meaning of this optional input depends on the mode of operation. When in No Operation mode, this node behaves as if it were replaced by the node connected to the first input. So the objects processed by this node will get passed to the node connected to this input to attach data to them.

When in Write mode, the objects being processed by this node are passed to the node connected to this input just as in No Operation mode. In addition, the data attached by this input is the data that will be written to the file on disk. If this input is connected, the Data Name parameter of this node is ignored, and the data name from the input node is used.

When in Read mode, the data connected to this input is ignored. However, the input node is still set as the creator node for the data read from disk, and the Guide Options on the input node are used for drawing guide geometry.

All Other Inputs

Other data nodes can be attached to act as modifiers for the data created or attached by this node.

The specific types of subdata that are meaningful vary from node to node. Whether using this node in Read, Write, or No Operation mode, subdata should be attached to this node rather than the node on the second input. Otherwise the subdata may not be built as expected.

Outputs

First Output

The operation of this output depends on what inputs are connected to this node. If an object stream is input to this node, the output is also an object stream containing the same objects as the input (but with the data from this node attached).

If no object stream is connected to this node, the output is a data output. This data output can be connected to an Apply Data DOP, or connected directly to a data input of another data node, to attach the data from this node to an object or another piece of data.

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