Houdini 20.5 Nodes Dynamics nodes

Gas OpenCL Enforce Boundary dynamics node

Uses OpenCL to perform boundary enforcement for fluid fields.

On this page
Since 19.5

The Gas OpenCL Enforce Boundary DOP takes care of boundary enforcement for fluid fields, using collider’s surface and velocity VDBs. In particular, this node overwrites Velocity Field values inside the collider to match its velocity; additionally, any listed Field will be reset inside the collider.

Note

If the object has an attached position data, this DOP will respect its translation.

Parameters

Collider Type

Controls the mode of operation for this DOP.

Individual Volumes

In this mode, the collider is represented by the current Surface VDB and Velocity VDB in the Collider Geometry. If Instance Geometry is enabled, the same collider is instanced at its points.

Packed Sets

This mode treats Collider Geometry as a library of colliders, each element being a named packed primitive containing its signed distance field. Each point in Instance Geometry intances a particular collider, identified by the name. Velocity VDB is not exposed, as every piece is assumed to be rigid.

You can use the Pyro Source Pack SOP to generate a library of colliders from rigid body pieces, by setting its Input parameter to Rigid Body Pieces.

Field

List of fields to enforce boundary on: voxels that fall inside a collider will be set to 0.

Velocity Field

Name of the velocity vector field. Velocity values inside the collider will be changed to reflect the collider’s motion. The collider’s velocity is read from Velocity VDB (or assumed to be 0 if not available). if Instance Geometry is being used, linear and angular velocities of the instance (v and w point attributes, respectively) are also incorporated.

Wind Velocity

Ambient velocity of the wind. This operator compensates for wind by subtracting it from values written to the Velocity Field.

Collider Geometry

Name of the geometry data on the simulation object containing the collider. If Collider Type is Individual Volumes, this geometry should include the Surface VDB and Velocity VDB. When Collider Type is set to Packed Sets, this geometry must contain packed primitives for the pieces.

Surface VDB

Name of the signed distance VDB for the collider. If Collider Type is Individual Volumes, this VDB must be present in Collider Geometry; when working with Packed Sets, this is the name of the signed distance field inside each packed piece.

Velocity VDB

Name of the velocity VDB for a deforming collider. If not present, 0 velocities are assumed inside the object.

VDB Name Suffix

This string will be appended at the end of each Surface VDB and Velocity VDB to construct the actual names of these fields. Only used when Collider Type is Individual Volumes.

Instance Geometry

If Collider Type is set to Individual Volumes and this is enabled, the collider will be instanced at every point of the specified geometry data. P (position), orient (unit quaternion for orientation), and pivot (pivot location for rotations) point attributes on the instance geometry affect the placement of each instance. v and w (linear and angular velocities) attributes are also incorporated into the collider’s velocity.

If Collider Type is set to Packed Sets, this geometry must contain instance points for the packed sets. In addition to the placement attributes listed above, each instance point must have a name attribute to identify its piece.

Locals

channelname

This DOP node defines a local variable for each channel and parameter on the Data Options page, with the same name as the channel. So for example, the node may have channels for Position (positionx, positiony, positionz) and a parameter for an object name (objectname).

Then there will also be local variables with the names positionx, positiony, positionz, and objectname. These variables will evaluate to the previous value for that parameter.

This previous value is always stored as part of the data attached to the object being processed. This is essentially a shortcut for a dopfield expression like:

dopfield($DOPNET, $OBJID, dataName, "Options", 0, channelname)

If the data does not already exist, then a value of zero or an empty string will be returned.

DATACT

This value is the simulation time (see variable ST) at which the current data was created. This value may not be the same as the current simulation time if this node is modifying existing data, rather than creating new data.

DATACF

This value is the simulation frame (see variable SF) at which the current data was created. This value may not be the same as the current simulation frame if this node is modifying existing data, rather than creating new data.

RELNAME

This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).

In this case, this value is set to the name of the relationship to which the data is being attached.

RELOBJIDS

This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).

In this case, this value is set to a string that is a space separated list of the object identifiers for all the Affected Objects of the relationship to which the data is being attached.

RELOBJNAMES

This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).

In this case, this value is set to a string that is a space separated list of the names of all the Affected Objects of the relationship to which the data is being attached.

RELAFFOBJIDS

This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).

In this case, this value is set to a string that is a space separated list of the object identifiers for all the Affector Objects of the relationship to which the data is being attached.

RELAFFOBJNAMES

This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).

In this case, this value is set to a string that is a space separated list of the names of all the Affector Objects of the relationship to which the data is being attached.

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