Houdini 20.5 Nodes Dynamics nodes

Affector dynamics node

Creates affector relationships between groups of objects.

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The Affector DOP creates affector relationships between groups of simulation objects. An affector relationship is what allows some objects to affect other objects.

For example, in an RBD simulation of a stack of bricks, all the bricks should affect each other, and all the bricks should be affected by the ground. The ground, however, should not be affected by the bricks. These relationships can be established by using a Group DOP to create a group for the bricks and another Group DOP to create a group for the ground. An Affector DOP would then be used to say that the ground group affects the bricks group, and that the bricks group affects the bricks group.

An affector relationships where a group affects itself is called a mutual affector relationship, and causes the group involved to become a mutually affecting group of objects. This means that every object in the group affects every other object in the group. When objects are mutually affecting like this, solvers must treat them differently than in one way affector relationships.

Parameters

Activation

When this parameter value is zero, this node does nothing.

This parameter is evaluated only once per timestep, not once for each object. Thus object-specific local variables are not available for use in this parameter or any other in this node.

Number of Relationships

The number of affector relationships this node will create. For each relationship, a pair of parameter values specify the groups involved in the relationship.

Affector Group #

The name of the simulation object group that contains the affector objects.

Affected Group #

The name of the simulation object group containing the objects that will be affected by the affector objects.

If this parameter is the same as the Affector Group parameter, then the specified group becomes a mutually affecting group.

Inputs

First

The objects with groups that will take part in the affector relationships are connected through the single input.

Outputs

First

The exact same set of objects that feed into the input are sent out through the single output.

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.

Examples

SimpleAffector Example for Affector dynamics node

This example shows how to use the Affector DOP to set up a variety of affector relationships between a set of RBD Objects. It also shows how these different affector relationships affect the simulation.

See also

Dynamics nodes