Houdini 20.5 Nodes Dynamics nodes

Gas Velocity Scale 1.0 dynamics node

Scales fluid velocity based on the fluid’s current speed or a control field.

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Since 19.0

Overview

The Gas Velocity Scale DOP provides controls for either slowing down or speeding up the motion of the simulation.

Dampening the motion

Dampening the motion is useful when you want to slow down a fast rising fireball’s motion over time.

To...Do this

Create dampening effect on a fast rising fireball

  1. Make sure Threshold Field is set to vel as a Vector type.

  2. To see the current speed on a desired frame, click the Compute Range button next to Threshold Range.

  3. Adjust Threshold Range values for the desired range.

  4. Make sure Scale at Min Threshold is set to 1, and lower the Scale at Max Threshold value below 1 a small amount. Small changes to the scale value can have a large impact on the motion.

Motion by another Field

This is useful if you want to simulate very fast moving fluid. For example, the smoke from a suddenly erupting volcano, which rapidly slows down in the atmosphere. Or some fluid in liquid type of motion.

To...Do this

Create fast eruption and sudden halt effect

  1. Set Threshold Field to temperature or flame as a Float type to use this field to control the motion.

  2. Click the Compute Range button next to Threshold Range to see the current Threshold Field values on a desired frame.

  3. Adjust the Threshold Range values for the desired range.

  4. Set Scale at Min Threshold to 0.85 to slow the fluid motion where threshold is at minimum, and set Scale at Max Threshold to 1.25 to increase the fluid motion where threshold is at maximum.

  5. Turn off Apply Scale Below Threshold Min and Apply Scale Above Threshold Max to limit the scaling effect only to the given threshold range.

Tip

You may notice that your motion speeds up indefinitely or halts and stops. To better control where the scale is applied in the simulation domain, turn off Apply Scale Below Threshold Min and Apply Scale Above Threshold Max to limit the scaling effect only to the given threshold range, while make sure that Threshold Range minimum is somewhat larger then 0.
Pyro simulation without Gas Velocity Scale on the left and the flame field used as threshold field on the right.

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.

Velocity Scale

Threshold Field

The type and name of the field to use to affect the velocity scale. This threshold field is mapped from the Threshold Range to Scale at Min Threshold and Scale at Max Threshold to control the motion. To dampen velocities based on the current speed, use the default values. To control the motion based on the given field, change the field to temperature or flame and set the type to Scalar.

Threshold Range

The values of the Threshold Field are mapped from this range to Scale at Min Threshold and Scale at Max Threshold. When Threshold Field is set to Vector, this value represents the length of the given vector field.

Compute Range

Computes the minimum and maximum speed on the current frame.

Scale at Min Threshold

The scale to apply at the minimum threshold given by Threshold Range.

Scale at Max Threshold

The scale to apply at the maximum threshold given by Threshold Range.

Apply Scale Below Threshold Min

You may notice that your motion speeds up indefinitely or halts and stops. To better control where the scale is applied in the simulation domain, turn off this parameter. This will make sure that the scale values will be set to 1 anywhere the Threshold Field's value is less then the minimum of Threshold Range. When it is off, make sure that the minimum of Threshold Range is higher then 0.

Apply Scale Above Threshold Max

You may notice that your motion speeds up indefinitely or halts and stops. To better control where the scale is applied in the simulation domain, turn off this parameter. This will make sure that the scale values will be set to 1 anywhere the Threshold Field's value is higher then the maximum of Threshold Range.

Remap Scale

Use the Scale Range Ramp to map Threshold Range to Scale at Min Threshold and Scale at Max Threshold.

Scale Range Ramp

Controls how the minimum and maximum Scale Range Ramp values are mapped to Scale at Min Threshold and Scale at Max Threshold.

Control Range

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

Compute Range

Computes the minimum and maximum range values of the selected field on the current frame. Use this to get an idea of your maximum field value.

Remap Control Field

Allows the Control Ramp to change how the control field should affect the scale between the minimum and maximum values of Control Range.

Control Field Ramp

Controls how the scale values are mapped between the minimum and maximum values of Control Range.

Bindings

Velocity Field

The vector field to apply the scaling to.

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