Houdini 17.0 Nodes Dynamics nodes

Null dynamics node

Does nothing.

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The Null DOP passes through the objects, data, or relationships that are wired into it unchanged. It can be used as a placeholder to refer to a specific part of a DOP simulation. It can also be used to re-route wires in a more aesthetically pleasing fashion. Finally, it can be used as a generator of a stream of no objects, no data, or no relationships.

For example, to switch between no data and a specific piece of data, a Null DOP could be wired first into a Switch DOP, allowing the selection of 0 to mean nothing.

Inputs

All

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

Outputs

First

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

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.

AnimatedClothPatch Example for Cloth Object dynamics node

This example shows how a piece of cloth that is pinned on four corners. These corners are constrained to the animated geometry.

ClothAttachedDynamic Example for Cloth Object dynamics node

This example shows a piece of cloth attached to a dynamics point on a rigid object.

DensityViscosity Example for FLIP Solver dynamics node

This example demonstrates two fluids with different densities and viscosities interacting with a solid object.

FlipColorMix Example for FLIP Solver dynamics node

This example demonstrates the use of the Flip Solver to mix the colors of a red fluid with a blue fluid to form a purple fluid.

FlipFluidWire Example for FLIP Solver dynamics node

This example demonstrates the use of the Flip Solver and the Fluid Force DOP. The Fluid Force DOP is used to apply a drag force on a wire object according to the motions of a flip fluid. The drag force is only applied at locations where fluid exists in the fluid object.

EqualizeFlip Example for Gas Equalize Volume dynamics node

This example demonstrates how the Gas Equalize Volume dop can be used to preserve the volume in a fluid simulation.

EqualizeLiquid Example for Gas Equalize Volume dynamics node

This example demonstrates how the Gas Equalize Volume dop can be used to preserve the volume in a fluid simulation.

FluidGlass Example for Particle Fluid Solver dynamics node

This example demonstrates how to get a smooth fluid stream to pour into a glass.

SourceVorticlesAndCollision Example for Smoke Object dynamics node

This example demonstrates a simple smoke system using a source, keyframed RBD collision objects, and vorticles.

See also

Dynamics nodes