Houdini 20.5 Nodes Dynamics nodes

Smoke Configure Object dynamics node

Attaches the appropriate data for Smoke Objects to an object.

On this page

The Smoke Configure Object DOP takes a simulation object and attaches the data which is needed for it to be used as a Smoke Object.

This DOP is very similar to the Smoke Object, except it allows you to explicitly control the creation of the object using another DOP, such as the Empty Object DOP. This can be used for more advanced instancing or creating objects every 10 frames.

Parameters

Two Dimensional

One of the divisions of the voxel grid will be forced to one to create a two dimensional field.

Plane

If set to two dimensional, this plane determines which axes remain unaffected.

Division Method

If non square, the specified size is divided into the given number of divisions of voxels. The sides of these voxels may not be equal, however, possibly leading to distorted simulations.

When an axis is specified, that axis is considered authoritative for determining the number of divisions. The chosen axis' size will be divided by the uniform divisions to yield the voxel size. The divisions for the other axes will then be adjusted to the closest integer multiple that fits in the required size.

Finally, the size along non-chosen axes will be changed to represent uniform voxel sizes. If the Max Axis option is chosen, the maximum sized axis is used.

When By Size is specified, the Division Size will be used to compute the number of voxels that fit in the given sized box.

Uniform Divisions

The resolution of the key axis on the voxel grid. This allows one to control the overall resolution with one parameter and still preserve uniform voxels. The Uniform Voxels option specifies which axis should be used as the reference - it is usually safest to use the maximum axis.

Divisions

The resolution of the voxel grid that will be used to calculate the smoke object. Higher resolutions allow for finer detail in both the appearance and in the resulting motion. However, doubling the divisions requires eight times the memory.

Since the substepping should be proportional to the voxel size, doubling the divisions may require double the substepping, resulting in sixteen times the simulation time.

Division Size

The explicit size of the voxels. The number of voxels will be computed by fitting an integer number of voxels of this size into the given bounds.

Size

The size of the voxel grid. The size of each voxel will be this divided by the divisions.

Center

The position in world space of the center of the voxel grid.

Closed Boundaries

The velocity field can be clamped to prevent any smoke from entering or leaving the box.

If closed boundaries is not set, the velocity on the boundary will be allowed to vary, allowing smoke to leave the box.

X, Y, Z

When closed boundaries is set, these select which sides will be closed.

Guides

Each of the fields that define the smoke simulation can be visualized in a number of ways. The help for the Scalar Field Visualization or Vector Field Visualization provides more details about how these work.

Initial Data

Density SOP Path

This is a path to the SOP that will be used to initialize the density subdata.

It should be a volume object, such as that generated by the Iso Offset SOP with the Output Type set to Fog Volume.

Scale

The per-path scale option lets you pre-scale the SOP volumes before they are applied. This is very useful for boosting the initial temperature amounts, for example.

Temperature SOP Path

The SOP to initialize the temperature data with. The temperature field is used by the internal buoyancy forces in the Smoke Solver.

Fuel SOP Path

The SOP to initialize the fuel data with. The fuel field is used by the old combustion model in Smoke Solver.

Velocity SOP Path

The path to the SOP that will initialize the velocity of the smoke. It should be three volume primitives which store the x, y, and z components of the initial velocity field.

Use Object Transform

When sampling the density SOP, determines if the relative transform between the density SOP and the DOP simulation should be taken into account.

Wind Tunnel Direction

The velocity field will be initialized to this constant external value. Furthermore, its end conditions will be set to this value. This can create a wind-tunnel type effect.

A non-zero external direction will allow smoke to leave the box, even if closed boundaries are set, as the boundary velocity will be clamped to the non-zero value.

Border Type

The behavior when the field is sampled outside of its defined box.

Constant

The initial value will be returned.

Repeat

The field will wrap, returning values from the opposite side of the field.

Streak

The value at the edge of the field closest to the sample will be returned.

Add Rest Field

Adds an extra field called “rest” which can be used to store rest positions for shaders.

Scale Rest Res

Scales the resolution of the rest field. Using a lower resolution rest field both reduces memory requirements of the rest field and also stiffens the rest field.

Velocity Sampling

Controls the sampling pattern of the velocity field. If center sampling is used, the faster but less accurate Gauss Seidel iterations will be used by the Project Non Divergent. Also, application of VOP networks can be accelerated as the components of the velocity field will line up. If Face sampling is set the PCG method will be used for projection. The other modes are available for completeness with what the VectorField DOP provides.

Position Data Path

The optional relative path for Position data. This will be used to transform the fluid box, allowing for non-axis aligned fluid sims. A value of ../Position will allow you to attach a Position DOP to your fluid object and thus reorient the fluid.

Fields

While every attempt is made to ensure unused fields have a minimal footprint, for some applications it may be necessary to minimize the number of extra fields created. Each field can be disabled from this list.

Note

The smoke and pyro solvers may expect these fields and stop working if they are missing.

Slice

Slice

Which slice to use. Should be a number between 0 and the number of slices - 1.

Slice Divisions

Number of pieces to cut the volume into along each axis. The total number of pieces, or slices, created will be the product of these numbers. i.e., 2, 3, 4 will create 24 slices.

Overlap Voxels Negative, Positive

Adds a padding on the lower/upper side of the slices. The slices start by dividing space evenly, but then this overlap will cause them to overlap with their neighbors. The field exchange nodes use this overlap to determine what is communicated.

Inputs

First

The simulation objects to turn into Smoke objects by attaching the appropriate data.

Outputs

First

The Smoke object created by this node is sent through the single output.

Locals

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.

See also

Dynamics nodes