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The Group DOP creates simulation object groups based on a combination of a mask string and optional expressions. These groups can be used for two main purposes. First, affector relationships can only be set up between groups of object. See the Affector DOP for more information on affector relationships. Second, object groups can be used to instead of complicated object masks to refer to sets of objects.
Select the dynamic objects to group.
Click the Group tool on the Drive Simulation tab.
Groups are only formed if the Activation Parameter is non-zero. This parameter is evaluated only once per timestep, not once for each object. Object-specific local variables are not available for use in this parameter.
Number of Groups
The number of object groups that this node will create. For each group, a separate set of parameters are used to define which simulation objects will belong to the group.
Group # Name
The name of the new group. Replaces any existing group of the same name.
Group # Mask
A string mask that specifies which objects may belong to the group. This mask may consist of a series of space-separated object names, object ids, or group names.
Wildcard characters ("*" and "?") can be used to match object names. The negation character ("^") can be used preceding a token to exclude objects matching that token from the group.
Group # Expression
Objects which match the Group Mask are then tested against this optional expression.
If the expression, which can contain local variables, evaluates to zero, that object will be excluded from the group. This expression can be used to refine the group membership according to any criteria.
For example, the dopfield expression function could be used to only allow objects with a positive x position remain in the group.
The objects that might be included in one or more of the groups are connected through the single input.
The exact same set of objects that feed into the input are sent out through the single output.
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.
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).
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.
This value is the inverse of the TIMESTEP value. It is the number of timesteps per second of simulation time.
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
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).
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).
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).
This string contains a space separated list of the unique object identifiers for every object being processed by the current node.
This string contains a space separated list of the names of every object being processed by the current node.
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 == $OBJCT should
always be used. This value will be zero if the node does not process
objects sequentially (such as the Group DOP).
This value is the simulation frame (see variable SF) at which the current object was created.
This value is equivalent to using the dopsttoframe expression on the OBJCT variable. This value will be zero if the node does not process objects sequentially (such as the Group DOP).
This is a string value containing the name of the object being processed.
Object names are not guaranteed to be unique within a simulation. However, if you name your objects carefully so that they are unique, the object name can be a much easier way to identify an object than the unique object identifier, OBJID.
The object name can
also be used to treat a number of similar objects (with the same
name) as a virtual group. If there are 20 objects named "myobject",
strcmp($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).
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.
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.
The following examples include this node.
This example shows how to use the Copy Object DOP, in conjunction with a Multi Solver, to automatically break an RBD object in half whenever it impacts another object.
This example uses the Pyro Solver and a Smoke Object which emits billowy smoke up through a turbine (an RBD Object). The blades of the turbine are created procedurally using Copy, Circle, and Align SOPs.
This example combines a number of elements and features of RBD to create a simulation of a paddle wheel being hit by a large number of falling objects.
This example demonstrates features such as resolving penetrations, gluing simple objects together to create more complex objects, grouping of objects, and constraints.
This example demonstrates how to advect curves based on a pyro simulation. An Attribute Wrangle SOP is used to sample the velocity from the volume and apply it to a wire object.