Houdini 20.0 Nodes Dynamics nodes

Fluid Object dynamics node

Attaches the appropriate data for Fluid Objects to an object.

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The Fluid Object DOP creates a Fluid Object inside a DOP simulation. It creates a new object and attaches the data which is needed for it to be used as a Fluid Object.

The geometry object has to have volume to be a valid source for volume-based fluid. For example, volume-based smoke, fire, or liquid will not work on a curve. However, this is not a requirement for particle based fluids.


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.

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.


Each of the fields that define the fluid 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


This is a path to the SOP that will be used to initialize the surface of the fluid. It should be a volume primitive which stores the signed distance to the fluid, such as that generated by the Iso Offset SOP with the Output SDF Volume option.

Velocity SOP Path

The path to the SOP that will initialize the velocity of the fluid. 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 surface SOP, determines if the relative transform between the surface SOP and the DOP simulation should be taken into account.

Closed Boundaries

The velocity field can be clamped to prevent any fluid from entering or leaving the box. If closed boundaries is not set, the velocity on the boundary will be allowed to vary, allowing fluid to leave the box.

X, Y, Z

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

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.

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.



Specifies the elasticity of the object. If two objects of bounce 1.0 collide, they will rebound without losing energy. If two objects of bounce 0.0 collide, they will come to a standstill.

Bounce Forward

The tangential elasticity of the object. If two objects of bounce forward 1.0 collide, their tangential motion will be affected only by friction. If two objects of bounce forward 0.0 collide, their tangential motion will be matched.


Specifies the coefficient of friction for the object. A value of 0 means the object is frictionless.

This governs how much the tangential velocity is affected by collisions and resting contacts.

Dynamic Friction Scale

An object sliding may have a lower friction coefficient than an object at rest. This is the scale factor that relates the two. It is not a friction coefficient, but a scale between zero and one.

A value of one means that dynamic friction is equal to static friction. A scale of zero means that as soon as static friction is overcome the object acts without friction.


Temperature marks how warm or cool an object is. This is used in gas simulations for ignition points of fuel or for buoyancy computations.

Since this does not relate directly to any real world temperature scale, ambient temperature is usually considered 0.



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



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.


BallInTank Example for Fluid Object dynamics node

This example shows an RBD ball being thrown into a tank of liquid.

FillGlass Example for Fluid Object dynamics node

Fills an RBD container with fluid that enters the simulation by being sourced from another RBD object.

FluidFeedback Example for Fluid Object dynamics node

This example shows a ball falling into a tank with feedback. This couples the RBD simulation with the Fluid simulation, causing the ball to float rather than sink.

PaintedGrog Example for Fluid Object dynamics node

This example creates a torus of paint which is dropped on the Grog character. The Grog character is then colored according to the paint that hits him. This also shows how to have additional color information tied to a fluid simulation.

RestartFluid Example for Fluid Object dynamics node

This example shows how to extract part of a fluid simulation and use it to start up a new fluid simulation, possibly with different resolution, location, or size.

RiverBed Example for Fluid Object dynamics node

A simple river bed has a fluid source and fluid sink set up so that liquid rushes down the river.

VariableDrag Example for Fluid Object dynamics node

This example shows how to vary the drag in a fluid simulation. It provides examples of using a specified field to be a high drag zone, of automatically applying drag only to the fluid surface, and of applying negative drag to an area to make the fluid more volatile.

See also

Dynamics nodes