Houdini 20.5 Nodes Dynamics nodes

Gas Disturb 2.0 dynamics node

Adds fine detail to a smoke simulation by applying “disturbance” forces to a velocity field.

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Since 18.0

The disturbance operator applies noisy forces to the velocity field, mimicking the effects of localized environmental change. This localized change in momentum cancels itself out over time, preserving the simulation’s general motion and overall shape. It is effective, however, at introducing small-scale details to the simulation.

Gas Disturb DOP has two methods for generating the noise: Continuous and Block-Based.

In both modes, the disturbance force adjusts magnitude of the applied force in an effort to keep the effect similar at different resolutions. In Continuous mode, Strength controls the expected variance of the integrated force over a region of size Reference Scale. The number of composited layers in Block-Based mode depends on block sizes and the voxel size (since blocks smaller than a single voxel cannot be represented). However, the DOP compensates for any truncation (when the actual number of layers is less than value of Max Octaves).

There are additionally two ways to apply the generated random force. By default, the force is exerted linearly: the disturbance is simply added to the Disturb Field. When Rotational Force is enabled, values in Disturb Field are instead rotated towards the randomly-generated vectors, without change of length. When applied to a velocity field, this corresponds to redirecting the momentum while maintaining the kinetic energy. Rotational application of disturbance is especially useful in fire simulations, where it can be used to add character to the flames.

Note

You can make this node apply forces that are very similar to the previous version of this DOP. To this end, use block-based mode with Max Octaves set to 1; you also need to mutliply Disturbance value from the old node by 24 to get comparable Strength.

Parameters

Time Scale

A scaling factor for time inside this solver. 1 is normal speed, greater than 1 speeds up the simulation, while less than 1 slows it down.

Rotational Force

When enabled, the values in Disturb Field are rotated towards the randomly-generated directions without changing their length.

Settings

Strength

Controls strength of the applied force.

Note

When Rotational Force is enabled, Strength governs how fast the vectors are rotated towards their goal.

Threshold Range

When this option is turned on, value of the Threshold Field is mapped from this range to 0-1 and applied as a multiplier on top of Strength.

Note

When the Threshold Range is enabled, voxels whose Threshold Field value falls beyond the first value of the range will not get disturbance applied.

Mode

Controls the nature of the generated random vectors.

Continuous

Every voxel gets an independently-assigned random vector.

Block-Based

The random pattern is formed by composing several layers of blockwise-constant noise.

Reference Scale

Variance of the aggregated noise field over a region of this size will be equal to Strength when Mode is set to Continuous. Provides a scale for normalizing the force against voxel size. A larger value for this parameter will increase magnitude of the applied force.

Tip

You should set this parameter to a reasonable value for your scene scale and leave it there; Strength can be used to more finely control how much force is applied.

Base Block Size

Controls size of the biggest blocks in the generated noise pattern when Mode is set to Block-Based.

Pulse Length

Length of time (in seconds) that the noise pattern is held fixed; only applies when Mode is set to Block-Based.

Note

The noise pattern changes on every frame in Continuous mode.

Lacunarity

The ratio of block sizes between successive noise layers. Value of 2, for example, means that the first layer has blocks that are twice the size of the second layer; the second layer will in turn have blocks that are twice as large as the next layer. This parameter is only applicable in Block-Based mode.

Roughness

The ratio of amplitudes between successive noise layers. Value of 0.5, for instance, means that the second layer will have half the amplitude of the first one. This parameter is only applicable in Block-Based mode.

Tip

Lower Roughness values will better preserve the block structure in the generated noise, whereas higher ones (nearing or exceeding 1) will yield more chaotic patterns resembling white noise.

Max Octaves

The maximum number of noise levels to compose in Block-Based mode.

Control

Control Field

When enabled, the force exerted is scaled by the content of this field.

Control Range

Map from this range of values in the control field.

Remap Control Field

Enables or disables the control field ramp.

Control Field Ramp

The ramp’s vertical axis is strength of the effect and the horizontal axis is the value in the control field.

Visualization

Visualize Strength

This option can be enabled to visualize strength of the applied force at different points in space.

Note

The strengths will also be saved into the Visualization Field.

Mode

Determines how the visualization will appear in the viewport.

Plane

Viewport will show a color-coded cutout of the strength field.

Smoke

Viewport will show a fog volume whose denser areas correspond to regions of higher applied strength.

Plane Orientation

Orientation of the cutting plane for Plane visualization.

Plane Position

Relative position of the cutting plane inside the bounding box.

Color Mapping

Controls how strength values are mapped from the Guide Range to colors.

Guide Range

Range of strength values that gets mapped into the 0-1 range before color conversion. The final visualization colors are controlled by Color Mapping.

Smoke Density

Multiplier on density of the fog volume representing force strength.

Bindings

Disturb Field

The vector field to apply the disturbance forces to.

Threshold Field

This field’s values are used for thresholding purposes (when Threshold Range is enabled).

Visualization Field

When Visualize Strength is enabled, the amount of applied force at each voxel is stored in this scalar field.

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.

Advanced

Use OpenCL

Use the OpenCL device to accelerate computations.

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