Houdini 11 Nodes Dynamics nodes

Defines an object’s position, orientation, linear velocity, and angular velocity.

The Motion DOP creates simulation data representing motion. Motion information is a superset of the position information created by the Position DOP. Motion information consists of the position information plus linear and angular velocity values.

## Parameters

Position

Position of the object in world space.

Pivot

The pivot point for the rotation. This is defined in the object’s own space.

Rotation

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

Velocity

Velocity of the object in world space.

Angular Velocity

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 multiplier of 360 will cause the object to rotate once per second. The triple (0, 360, 0) will cause the object to rotate once about the y axis every second.

Parameter Operations

Each data option parameter has an associated menu which specifies how that parameter operates.

 Use Default Use the value from the Default Operation menu. Set Initial Set the value of this parameter only when this data is created. On all subsequent timesteps, the value of this parameter is not altered. This is useful for setting up initial conditions like position and velocity. Set Always Always set the value of this parameter. This is useful when specific keyframed values are required over time. This could be used to keyframe the position of an object over time, or to cause the geometry from a SOP to be refetched at each timestep if the geometry is deforming. You can also use this setting in conjunction with the local variables for a parameter value to modify a value over time. For example, in the X Position, an expression like `\$tx + 0.1` would cause the object to move 0.1 units to the right on each timestep. Set Never Do not ever set the value of this parameter. This option is most useful when using this node to modify an existing piece of data connected through the first input. For example, an RBD State DOP may want to animate just the mass of an object, and nothing else. The Set Never option could be used on all parameters except for Mass, which would use Set Always.
Default Operation

For any parameters with their Operation menu set to Use Default, this parameter controls what operation is used.

This parameter has the same menu options and meanings as the Parameter Operations menus, but without the Use Default choice.

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 modified. If this input is not connected, this node can be used in conjunction with an Apply Data node, or can be used as an input to another data node. All Other Inputs If this node has more input connectors, other data nodes can be attached to act as modifiers for the data created by this node. The specific types of subdata that are meaningful vary from node to node. Click an input connector to see a list of available data nodes that can be meaningfully attached.

## 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.

## Local variables

 channelname This DOP node defines a local variable for each channel and parameter on the Data Options page, with the same name as the channel. So for example, the node may have channels for Position (positionx, positiony, positionz) and a parameter for an object name (objectname). Then there will also be local variables with the names positionx, positiony, positionz, and objectname. These variables will evaluate to the previous value for that parameter. This previous value is always stored as part of the data attached to the object being processed. This is essentially a shortcut for a dopfield expression like: ``` dopfield(\$DOPNET, \$OBJID, dataName, "Options", 0, channelname) ```If the data does not already exist, then a value of zero or an empty string will be returned. DATACT This value is the simulation time (see variable ST) at which the current data was created. This value may not be the same as the current simulation time if this node is modifying existing data, rather than creating new data. DATACF This value is the simulation frame (see variable SF) at which the current data was created. This value may not be the same as the current simulation frame if this node is modifying existing data, rather than creating new data. RELNAME This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP). In this case, this value is set to the name of the relationship the data to which the data is being attached. RELOBJIDS This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP). In this case, this value is set to a string that is a space separated list of the object identifiers for all the Affected Objects of the relationship to which the data is being attached. RELOBJNAMES This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP). In this case, this value is set to a string that is a space separated list of the names of all the Affected Objects of the relationship to which the data is being attached. RELAFFOBJIDS This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP). In this case, this value is set to a string that is a space separated list of the object identifiers for all the Affector Objects of the relationship to which the data is being attached. RELAFFOBJNAMES This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP). In this case, this value is set to a string that is a space separated list of the names of all the Affector Objects of the relationship to which the data is being attached. 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.