Houdini 16.5 Nodes Dynamics nodes

Gas Resize Fluid Dynamic dynamics node

A microsolver that resizes a fluid to match simulating fluid bounds

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The Gas Resize Fluid Dynamic DOP is a microsolver used in building larger fluid simulations. The Fluid Solver and Smoke Solver DOPs 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 Resize Fluid Dynamic DOP will resize the fields required for different types of fluid simulations according to a reference field. A SOP Solver is used to recalculate the new bounds every timestep. This resizing is done with the Gas Resize Field DOP so it does not affect the actual voxel sampling, just the total number of voxels.

Parameters

Fluid Type

The type of fluid controls what sort of fields need to be resized.

DSD Fire

The following fields are resized in DSD Fire mode: heat, burn, collision, collisionvel, confinement, d, ddot, density, divergence, dt, flamefront, fuel, fuelvolume, kappa, kappastar, pressure, pump, pumpindex, rest, sink, source, sourceindex, temperature, and vel.

Smoke and Pyro

The following fields are resized in Smoke and Pyro mode: collision, collisionvel, density, divergence, pressure, pump, pumpindex, rest, rest2, sink, source, sourceindex, temperature, vel, burn, fuel, and heat.

Fluid

The following fields are resized in Fluid mode: collision, collisionvel, surface, pressure, pump, pumpindex, rest, sink, source, sourceindex, temperature, and vel.

Extra Fields

Any additional fields, such as those you have added with a Scalar Field DOP, can be listed here to ensure they are also resized.

Delay Frames

Specifies the number of frames to wait before starting the resize operation. In order to resize correctly, there must be data of the field specified as a reference.

Bounds

Padding

The computed container bounding box is enlarged by these padding values.

Subtract Threshold

Controls how smooth the bounds should grow or shrink, preventing jitter. A field that expands will always be encapsulated. Shrinking fields will blend toward their new bounds based on the threshold specified. Higher values will result in smoother transitions but larger grid sizes to compute.

Reference Field

Fluid field whose bounds will be computed and used as resize reference. This can be a space delimitted list of fields to use more than one reference field.

Resolution Scale

Controls the sample resolution of the field specified as the Reference Field. A value of 1 samples the field at full resolution.

Field Cutoff

Controls what values trigger resizing. Values in the field less than this cut off will not contribute to the final field size. This avoids very faint smoke from continuing to expand the field.

Track By Object

Includes an external object when computing the new bounds. Sometimes an emitter might move too fast or emit sporadically. Specifying this object to be the Tracking Object prevents the resize operation from excluding fluid calculations outside the original (or initialized) fluid bounds.

Max Bounds

Clamp To Maximum Size

Prevents the box from growing indefinitely. By specifying a bound mode, the fluid will be contained with in a certain volume or space but can grow or shrink within those conditions.

From Object

Maximum bounds derived from geometry.

Initialization Static

Maximum bounds derived from fluid object. When the object is created the size and center are analyzed and stored as init_size and init_center. Every iteration the bounds are compared to the maximum size specified at creation time and adjusted accordingly.

Initialization Dynamic

Maximum bounds derived from the fluid object, but measured against the specified object’s current position. The bounds will grow or shrink to a maximum size relative to the objects position in space. When an object is animated, the bound conditions should travel with the object. This option enables that behavior. If no object is found the bounds are centered or reverted to static.

Manual

Manually define the maximum bound conditions.

Enforce Boundaries

Resizes the specified axis to the maximum allowed bound condition. This is useful when a fluid needs to collide with a wall or roof.

Inputs

First Input

This optional input can be used to control which simulation objects are modified by this node. Any objects connected through this input and which match the Group parameter field will be modified.

If this input is not connected, this node can be used in conjunction with an Apply Data node, or can be used as an input to another data node.

All Other Inputs

If this node has more input connectors, other data nodes can be attached to act as modifiers for the data created by this node.

The specific types of subdata that are meaningful vary from node to node. Click an input connector to see a list of available data nodes that can be meaningfully attached.

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

The following examples include this node.

fieldforce Example for Field Force dynamics node

This example demonstrates the use of the Field Force DOP. It shows how to use a particle system to blow around smoke.

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.

UpresRetime Example for Gas Up Res dynamics node

This example demonstrates how the Up Res Solver can now be used to re-time an existing simulation. The benefit of this is that one can simply change the speed without affecting the look of the sim. On the up-res solver there is a tab called Time. The Time tab offers various controls to change the simulation’s speed.

AdvectByVolume Example for POP Advect by Volumes dynamics node

This example demonstrates how to use POP Advect by Volumes to advect particles using the velocity from a smoke simulation.

ColourAdvect Example for Fluid Source geometry node

This example demonstrates how you can use the Fluid Source SOP to source and advect colours from an additional volume into a smoke simulation.

See also

Dynamics nodes