Houdini 18.5 Nodes Dynamics nodes

Smoke Object (Sparse) dynamics node

Creates an empty smoke object for a pyro simulation.

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Since 18.0

This DOP creates a smoke object with properly configured fields that can be evolved by a Smoke Solver (Sparse) or Pyro Solver (Sparse). The object will start out empty, and can be populated with smoke or heat using the Volume Source DOP; see pyro sourcing and fluid sourcing for more details.

Parameters

Properties

Enable Sparse Solving

This parameter must be enabled if you intend to perform a sparse smoke simulation; see the sparsity section of the solver’s help card for more information.

Note

Essentially, this option forces certain voxel resolution restrictions on the created fields and creates an active stencil field.

Two Dimensional

Enable this option if you want to perform a two-dimensional simulation.

Note

Two-dimensional smoke simulations cannot be performed in sparse mode.

Voxel Size

The size of each simulation voxel. Smaller voxel size corresponds to a higher resolution simulation (more detail, but slower).

Center

The starting location of the simulation container. If Max Size is turned on, this parameter also specifies where the bounding box is centered.

Note

The simulation container starts out empty and gets resized at initial sourcing. To minimize the cost of this resizing, Center should be roughly in the same area as the initial sources.

Max Size

By default, the container will grow as the simulated smoke and fire inside it evolve. Enabling this option allows you to set a maximum size the container may grow to.

Note

An axis-aligned box of the specified size is positioned at Center to obtain the maximum container size. This region can be visualized by enabling Domain visualization under the Guides tab.

Note

The effective maximum container size will be smaller if the settings under Boundary Conditions cut into this box.

Boundary Conditions

Parameters in this section allow you to close the boundary at a specific level along each axis. A closed boundary acts as a solid impenetrable wall that gas may not enter. It is more accurate and more efficient to close a boundary instead of adding a ground plane collision object to the simulation.

Tip

Enable Domain visualization under the Guides tab to see where each axis will be closed.

Treat X as

Allows you to create a solid wall at a certain coordinate along the X axis. The axis can be kept Open (no wall), be Closed Below (X coordinates less than the specified value are off-limits for the gas) or be Closed Above (X coordinates greater than the specified value are off-limits).

Treat Y as

Allows you to create a solid wall at a certain coordinate along the Y axis. The axis can be kept Open (no wall), be Closed Below (Y coordinates less than the specified value are off-limits for the gas) or be Closed Above (Y coordinates greater than the specified value are off-limits).

Treat Z as

Allows you to create a solid wall at a certain coordinate along the Z axis. The axis can be kept Open (no wall), be Closed Below (Z coordinates less than the specified value are off-limits for the gas) or be Closed Above (Z coordinates greater than the specified value are off-limits).

Creation

Creation Frame Specifies Simulation Frame

Determines if the creation frame refers to global Houdini frames ($F) or to simulation specific frames ($SF). The latter is affected by the offset time and scale time at the DOP network level.

Creation Frame

The frame number on which the object will be created. The object is created only when the current frame number is equal to this parameter value. This means the DOP Network must evaluate a timestep at the specified frame, or the object will not be created.

For example, if this value is set to 3.5, the Timestep parameter of the DOP Network must be changed to 1/(2*$FPS) to ensure the DOP Network has a timestep at frame 3.5.

Number of Objects

Instead of making a single object, you can create a number of identical objects. You can set each object’s parameters individually by using the $OBJID expression.

Object Name

The name for the created object. This is the name that shows up in the details view and is used to reference this particular object externally.

Note

While it is possible to have many objects with the same name, this complicates writing references, so it is recommended to use something like $OBJID in the name.

Solve On Creation Frame

For the newly created objects, this parameter controls whether or not the solver for that object should solve for the object on the timestep in which it was created.

Usually this parameter will be turned on if this node is creating objects in the middle of a simulation rather than creating objects for the initial state of the simulation.

Allow Caching

By preventing a large object from being cached, you can ensure there is enough room in the cache for the previous frames of its collision geometry.

This option should only be set when you are working with a very large sim. It is much better just to use a larger memory cache if possible.

Guides

Each of the fields that define the smoke simulation can be visualized in a number of ways. The help for Multi-Field Visualization, Scalar Field Visualization, and Vector Field Visualization provide more details about how these visualization options work.

Limits of the simulation container can be viewed by enabling Domain visualization. If Max Size is enabled, a box will be drawn in the viewport to show its extent. Additionally, each closed boundary will be displayed, with the drawn arrow pointing towards the side on which the simulation may take place.

Fields

Velocity Sampling

Controls the sampling pattern of the velocity field. Center sampling yields faster simulations, but may give rise to some artifacts (which may be fixed with the help of Hourglass Filtering controls on the solver). Face sampling is slower, but does not suffer from any artifacts.

Wind Tunnel Direction

This parameter specifies the initial state of the velocity field, as well as its default external value. This parameter can be set to endow the smoke with a natural travel direction, as if it were influenced by a strong uniform wind force.

Divergence Sampling

Controls the sampling pattern of the divergence field, which can be used to add expansion/contraction to the velocity field. Center sampling should generally be faster if you are adding expansion from one of the fluid’s fields at simulation time.

When Velocity Sampling is set to Center and all modifications to the divergence field will go through a Volume Source (this includes having Add Expansion disabled on the Pyro Solver), then setting Divergence Sampling to Corner will be faster.

Create Cd and Alpha Fields

When this option is enabled, Cd and Alpha fields will be added to the smoke object, containing the smoke color and color amount, respectively. If the Pyro Solver detects the presence of these fields, it will ensure to evolve them as the simulation advances.

Note

Use Cd as the Diffuse Field in Multi Field visualization to display the smoke with its colors.

Default Color

This parameter specifies the default color of smoke and the initial state of the Cd field.

Default Alpha

Controls the initial (and minimum) value of the Alpha field. Intuitively, the fluid will contain Default Alpha amount of dye of Default Color at the beginning of the simulation.

Outputs

First

The smoke object created by this node.

Locals

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.

See also

Dynamics nodes