Houdini 20.0 Nodes Dynamics nodes

Smoke Object dynamics node

Creates an Smoke Object from SOP Geometry.

On this page

The Smoke Object DOP creates an Smoke Object inside the DOP simulation. It creates a new object and attaches the subdata required for it to be a properly conforming Smoke Object.



Two Dimensional

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


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.


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.


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


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.


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 object externally.


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

Solve On Creation Frame

When turned on, newly created objects are solved by the solver on the timestep in which it was created.

This parameter is usually 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.


Creates a smoke container for every point found on either the creation frame or the current frame being processed (using Continuous). By default, the container is created at the point’s center position. The size is defined by the point’s scale vector attribute. If no scale attribute is found, the container will revert to the default Size.

Create Objects From Points

Enables instancing. init_cluster data gets added to every created smoke object. The number given can be controlled through the point’s cluster attribute. This data can be generated using the Cluster Points operator. If the cluster attribute isn’t specified, the point number is taken. This number is used by the Volume Source microsolver to fetch the correct volume information from SOPS.

Override Container Size

Use the point attribute scale to control the size of the container. The center is defined by the point’s position. When disabled or not found, the default parameters Size and Center are available to control the container’s size and position.

Override Division Size

Use point attribute divsize to control the division size for every instanced object. If the attribute is not found, the default Division Size is used.

Number of Objects

If Create Objects From Points is turned off, the number of objects to be created on the creation frame is defined here. The default is one.

Instance Points

The node containing point geometry to instance points on.


Creates a smoke object for every point found on every iteration. So be careful enabling this. By default, instancing occurs only once for every point found on the creation frame. When objects need to be created on the fly, use Continuous. Make sure to only have points present when an object needs to be created, such as on impact, and delete/remove the point afterward. This can be done using a timeshift operation to compare the point’s existence to the frame before.


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.


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

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.


The initial value will be returned.


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


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.


Uses faster but less accurate Gauss Seidel iterations.


Use the slower but more accurate PCG method.

The other choices in the menu are only included because they are provided by the Vector field DOP.

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.


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.


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


The Heat field is manipulated by Smoke Solvers and Pyro Solvers. You can stamp the heat field with the current source mask or burn field. It then moves passively with the fluid and decays linearly over time.

It can be thought of as “Time since this voxel was added to the system”, but it starts at 1 for just added and falls to 0 after a user specified number of seconds.



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.



The Smoke object created by this node.



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.


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).


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.


The inverse of the TIMESTEP value. It is the number of timesteps per second of simulation time.


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.


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).


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).


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).


This string contains a space-separated list of the unique object identifiers for every object being processed by the current node.


This string contains a space-separated list of the names of every object being processed by the current node.


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).


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).


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).


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.


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.


2dfluid Example for Smoke Object dynamics node

Demonstrates exporting a 2d fluid into COPs where it can be saved to disk as a sequence of image files to then be used as texture maps, displacement maps, etc.

DelayedSmokeHandoff Example for Smoke Object dynamics node

This example shows a way to turn an RBD into smoke a certain number of frames after the RBD object has hit something.

Open CL smoke Example for Smoke Object dynamics node

Demonstrates a simple Open CL accelerated smoke sim that can be used as a starting point for building optimized GPU accelerated smoke sims. See the Use OpenCL parameter on the Smoke solver.

For fastest speeds, the system needs to minimize copying to and from the video card. This example demonstrates several methods for minimizing copying.

  • Turns off DOPs caching. Caching requires copying all the fields every frame. Useful if you want to scrub and inspect random fields, not if you want maximum speed.

  • Only imports density to SOPs. This means copying only one field from the GPU to CPU each frame.

  • Saves to disk in background. This gives you the best throughput.

  • Uses a plain Smoke solver.

Displaying the simulated output in the viewport requires a GPU → CPU → GPU round trip, but this is required in general to support simulating on a card other than your display card.

RBDtoSmokeHandoff Example for Smoke Object dynamics node

This example shows a way to turn an RBD object into smoke. It uses multiple different colored smoke fields inside the same smoke object.

SourceVorticlesAndCollision Example for Smoke Object dynamics node

This example demonstrates a simple smoke system using a source, keyframed RBD collision objects, and vorticles.

rbdsmokesource Example for Smoke Object dynamics node

A ghostly tetrahedron bounces around a box, its presense shown by its continuous emission of smoke.

See also

Dynamics nodes