Houdini 20.5 Nodes Dynamics nodes

Gas Shred dynamics node

Applies a Shredding Force to the velocity field specified.

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Since 12.0

Creates and applies a shred force. The shred force rips apart a velocity field (default is velocity) based on the gradient of a field (default is temperature) and can greatly improve the look of fire. Shredding also introduces streaks if applied properly. This shred field is modulated by the Control Field and lookup ramps provided. This controls where and with what magnitude the shred force shows up, so you can ensure it occurs only in the regions of the sim you want.

Tip

Since shredding works on the gradient of the field specified as temperature, less diffusion of that field (by default temperature) results in a stronger, more noticeable effect. When temperature diffuses more, the gradient becomes less dynamic. Resulting in bigger streaks. The higher the grid resolution, the more detailed the gradient becomes.

Parameters

Visualize Shredding

Visualizes the shred vector field that will be applied to the specified velocity field.

Modify Temperature Through Shredding

Shifts the temperature values (of the field defined as temperature) before calculating the gradient. Because the operation repeats itself every timestep, a shift in force can be enforced making the result more dynamic.

Time Scale

Specifies a scale factor that relates DOP time to the simulation time for this microsolver. A value greater than one means the simulation time advances faster than the DOP time. A value less than one causes the simulation to appear to run in slow motion relative to the DOP time. Several expression functions such as doptime exist for converting from global times to simulation times and vice versa.

Scale

Amount of shred force applied to specified velocity field.

Temperature Threshold

Defines the point where the velocity will be squashed or stretched. Based on the field defined as temperature. Values lower then the Threshold will be stretched, values higher then the threshold will get squashed. The right value depends on the amount of temperature to be found in the simulation. The lighter the simulation, the lower the value should be.

Threshold Width

Defines the width of the blend area where values get squashed or stretched. Creating a neutral zone around the Threshold value. A value of 0 results in a harsh squash, stretch effect. Higher values result in smoother streaks.

Squash

How much to squash the velocity field based on the calculated gradient, for values sampled higher then the Temperature Threshold value.

Stretch

How much to stretch the velocity field based on the calculated gradient, for values sampled lower then the Temperature Threshold value.

Clip Gradient

Controls the maximum slope allowed from the calculated gradient. When a lot of temperature is present the gradient’s magnitude will turn out too strong.

By setting a limit, the influence will be capped at the value set. This ensures that a more balanced shred effect can be achieved. The correct value should correspond to the maximum shred value found in the area where you want the shredding to have the most influence, most often this is not a the core but somewhere higher up. Since the core is probably the hottest part in your simulation, the influence there will be capped to the value given.

Temp Influence Rate

If Modify Temperature Through Shredding is enabled, this values controls how fast the field defined as temperature is allowed to shift places before the gradient is calculated (on which to base shedding).

Control Settings

Specifies where shredding has any influence and to what degree.

Control Field

When enabled, the force is scaled by the field specified.

Control Influence

How much influence the Control Field has on the previously calculated force. A value of 0 disables the control field.

Control Min Max

Remaps (normalizes) the incoming Control Field values based on the specified minimum and maximum. The output is always in a range of 0 to 1.

Remap Control Field

Use the Control Field Ramp to remap the control field’s (normalized) values.

Visualization

For all visualization options, refer to the Scalar Field Visualization and Vector Field Visualization node help.

Bindings and Temporary Fields

Clear Temporary Fields

Zeros the temporary fields when done to reduce memory usage.

Shred Temperature Gradient Field

The name of the temporary fields to store intermediate steps needed. These will be created with a Gas Match Field so it does not need to exist ahead of time.

Velocity Field

Used for sampling information from, and apply shred force to.

Temperature Field

Field used as a base for the gradient. The gradient is used to drive the shred force.

Stencil Field

A scalar field to use as a stencil for where to perform this node’s computations. Voxels whose stencil value strictly exceeds 0.5 will have the operation applied, while the rest will be left unchanged.

Note

If a stencil field isn’t provided or does not exist, the operation will be performed everywhere.

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 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.

Dynamics nodes