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The Constraint DOP is used to describe the constraints of a simulation object. The Constraint DOP itself does not contain any of the information about where or how an object is constrained. That information must be attached as subdata to the constraint.

A constraint consists of one Relationship and two Anchor nodes. The Anchor nodes control how the current position or orientation of the object is calculated, and how the goal position or orientation is calculated. The Relationship controls what the solver should do to make the anchor values equal.

This node has been built into several higher level digital assets such as the RBD Pin Constraint. These assets combine the Constraint DOP with the required Relationship and Anchor data to create some of the more common constraints for a particular type of simulation object. But in spite of their differences, all of these assets simply create one or more Constraint data with one Relationship and two Anchors as subdata.

Parameters

Show Guide Geometry

Enables the display of guide geometry in the viewport to represent this constraint.

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 relationship. If this parameter is an expression, it is evaluated for each relationship.

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

Group

When used in conjunction with an Apply Relationship node, this parameter is used to determine which relationships are acceptable to this node.

If the relationship is not recognized as part of this group, this data will not be attached to the relationship.

Data Name

Indicates the name that should be used to attach the data to the relationship.

Unlike the case of attaching data to simulation objects or other data, the Data Name here is not very important because a relationship can only have a single piece of data attached to it at a time. This Data Name can thus be used simply to help identify the purpose of the relationship.

Affected Objects

If objects are connected to the first input of this node, this parameter specified the set of input objects that will be the affected objects in the relationship.

If used in conjunction with an Apply Relationship node, this parameter is not used because that node defines the actual relationship.

Affector Objects

If objects are connected to the first input of this node, this parameter specified the set of input objects that will be the affector objects in the relationship.

Relationship Name

If objects are connected to the first input of this node, this parameter specifies the name of the relationship. If this parameter value matches the name of an existing relationship, that relationship will be replaced.

Unique Relationship Name

If objects are connected to the first input of this node, this parameter can be turned on to ensure that a unique relationship name is generated by this node.

This ensures that no existing relationship will be overwritten by the one defined by this node.

Make All Objects Mutual Affectors

If objects are connected to the first input of this node, turning this parameter on causes the union of the affector and affected object parameters to be placed into both the affector and affected object lists for the relationship.

This is often used as a shortcut to specifying the same list of objects in both the Affector and Affected Objects parameters.

Inputs

First Input

This optional input can be used to control which simulation objects may be involved in the relationship created by this node.

If this input is not connected, this node can be used in conjunction with an Apply Relationship node to create many relationships at the same time.

All Other Inputs

If this node has more input connectors, data nodes can be attached to act as modifiers for the relationship described 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 additional relationship created by this node).

If no object stream is connected to this node, the output is a relationship data output. This data output can be connected to an Apply Relationship node to create many relationships at once.

Locals

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.

Examples

The following examples include this node.

LookAt Example for Anchor: Align Axis dynamics node

This example shows how to build a Look At Constraint which keeps a teapot pointed at a bouncing ball. It shows how to build constraints out of anchors and constraint relationships.

DegreesOfFreedom Example for RBD Solver dynamics node

This example demonstrates the use of the Constraint Type parameter on the RBD Constraint node. This parameter controls the number of degrees of freedom the constrained object has.

Dynamics nodes