Houdini 17.0 Nodes Dynamics nodes

Gas Equalize Volume dynamics node

A microsolver that equalizes the volume of two fields.

On this page

The Gas Equalize Volume DOP is a microsolver used in building larger fluid simulations. The Fluid Solver and Smoke Solver DOPs that allow microsolvers to be added before or after the main solver step to extend or tweak the simulation. Alternatively, advanced users may attempt to build an entire new solver out of microsolvers.

The Gas Equalize Volume DOP adds a constant to an SDF so that its total volume will match a reference field. This can be used to preserve total liquid volume or to get two disparate volumes to match.

Alternatively, it can add a constant to a divergence field to correct the fluid. This mode is more useful for FLIP fluids where the surface field isn’t read by the solver.

Note

If you do not have a goal field, you must set the Scale Goal Volume to 0 as the default goal field of 0 will be interpreted as a 50% opacity fog by the SDF conversion.

Parameters

Two Dimensional

If two dimensional, areas will be computed rather than volumes to ensure the units remain independent of voxel resolution.

Scale Goal Volume

The computed goal volume will be multiplied by this value. This is useful if you want to artificially increase the liquid volume compared to the reference. This should be an SDF.

Add Goal Volume

This constant will be added to the computed and scaled goal volume to form the final volume the SDF will be equalized with. If scale goal volume is 0, this merely becomes the total goal volume.

Max Change

The maximum amount the surface can move in a frame, measured in voxels.

Blend Amount

The rate to change the volume to match the goal volume.

Method

Toggles whether Gas Equalize volume uses the divergence of the fluid (for FLIP fluids) or modifies the surface field of the fluid to keep volume constant (for Level Set fluids).

Delta Step

To estimate how much the volume changes for a given change in SDF value, this sized step (in voxels) is performed to estimate the effective surface area.

Integral Decay rate

When correcting by adjusting the divergence field, the correction may be insufficient. The history of the errors is kept to dynamically control the correction rate. This decay rate controls how quickly the history is lost. A value of 0 means no memory is kept. A value of 0.5 means after 1/24th of a second, half of the memory is lost. A value of 1 never decays the recorded errors, which likely will result in overshooting and oscillations.

Integral Coefficient

The accumulated error is scaled by this coefficient when added back to the divergence field. Larger values will have the accumulated error having a greater effect on the choice of correction values.

Fields

These parameters control what name will be used for the input fields and the internal fields.

Surface Field

The field to offset so that its volume matches the goal, if Use Surface Field is checked.

Goal Field

The target field whose volume will be matched.

Surface Volume

A constant field that is computed that stores the total volume of the surface.

Surface Delta Field

A constant field to store the size of the delta offset field.

Goal Volume

A constant field that is computed that stores the total volume of the goal.

Rolling Integral

A constant field which stores the sum of the error in volume.

Divergence

The divergence field Gas Equalize Volume will tweak to keep volume constant.

Outputs

First Output

The operation of this output depends on what inputs are connected to this node. If an object stream is input to this node, the output is also an object stream containing the same objects as the input (but with the data from this node attached).

If no object stream is connected to this node, the output is a data output. This data output can be connected to an Apply Data DOP, or connected directly to a data input of another data node, to attach the data from this node to an object or another piece of data.

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 the data 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

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

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.

The following examples include this node.

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.

Dynamics nodes