Houdini 20.5 Nodes Dynamics nodes

Merge dynamics node

Merges multiple streams of objects or data into a single stream.

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The Merge DOP takes any number of separate streams of objects or data and merges them into a single stream. Data and objects cannot be merged into a single stream. All inputs must be object streams, or all inputs must be data.

Merging objects does not imply any relationship between those objects. However, for convenience, the Affector Relationship parameter can be turned on to create affector relationships between the different streams of objects. The same functionality can be achieved with Group and Affector DOPs, but many more nodes would be required. Objects with the Intangible Value set will not have a relationship created by a merge node.

When merging data, any connection to this node’s output will be treated as if every input of this node is connected to it. This is useful when dealing with digital assets which provides only a single data input but to which you may wish to attach several pieces of data.

Similarly, this node can be used to allow a digital asset to output several pieces of data through a single data output. This node also provides a convenient visual method of grouping several pieces of data into a single stream for easier wiring.

Note

Bypassing this node disables the relationship calculations and only processes the first input.

Parameters

Activation

When this parameter value is zero no relationships will be added. When it is one, the Affector Relationship will control the addition of relationships. In any case, the objects are merged into a single stream.

Affector Relationship

If this node is merging streams of simulation objects, this parameter sets up affector relationships between the objects.

No Change

No new affector relationships are created between the input objects.

Left Inputs Affect Right Inputs

Given a series of inputs 1, 2, 3, and so on, the objects connected to input 1 become affectors for the objects on inputs 2, 3, and so on. The objects at input 2 become affectors of the objects at input 3 and beyond. Using this option will cause the input objects to be solved in the order in which they are connected to this node.

Mutual

All objects on all input streams become mutual affectors.

Relationship

When merging streams of objects, this determines what sort of relationship should be created between the streams. Useful choices are:

None

No affector relationship is created - the same as setting No Change in the Affector Relationship field.

Constraint

Used internally to define two objects that have a constraint between them.

Pump

Affected objects will set their local velocities to match the velocity of the source object. Applies to fluid objects.

Sink

Affected objects will delete their volume where it contacts the affectors. Applies to fluid objects.

Group

Used internally to make objects part of the same group.

Collide

Affected objects will respond to collisions from affector objects.

Target

A place holder affector for user defined effects. Used by the Gas Target Forces.

Source

Affected objects will use the affector objects as sources for operations such as creating liquid or smoke density.

Empty

Enforces a particular solve order - affected objects will be solved after affector objects - but no other intrinsic meaning.

This is useful when SOP Solvers refer to other objects creating a dependency that isn’t visible to the DOP Engine.

Inputs

All

All the objects or data connected to the inputs of this node are fed out through the single output.

Outputs

First

All the objects or data connected to the inputs of this node are fed 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.

See also

Dynamics nodes