Houdini 20.5 Nodes Dynamics nodes

FLIP Configure Object dynamics node

Attaches the appropriate data for Particle Fluid Objects to become a FLIP based fluid.

On this page
Since 11.0

The FLIP Configure Object DOP takes a particle fluid object and attaches the data which is needed for it to be used as a FLIP based fluid.

Parameters

Division Size

The FLIP Fluid Solver creates a volume grid to perform velocity projection on. The size of this grid does not have to match the actual particle sizes.

If the division size is less than the particle size, unnecessary work will be done.

A division size greater than the particle size will smear neighboring particles together, resulting in a smoother, stabler, and faster simulation. The recommended division size is twice the particle separation, so there will be approximately eight particles per grid cell.

If you halve the division size, you create eight times the work.

Collision Division Size

The division size for the collision-related fields.

It can be useful for the collision fields to be higher resolution than the other simulation fields, especially when protoyping simulations at low resolutions, since the FLIP Fluid Solver takes into account fractional collision voxels when solving.

If this value is set as a fraction of the overall Division Size, be careful to restore it when running high resolution simulations. Another option is to choose a division size that provides sufficient collision detail by visualizing the collision field, then leave it at that fixed value, independent from any changes to the overall division size.

Density

The mass density of the fluid. This value is stored in the object’s density field and can be manipulated in the Volume Velocity input of the FLIP solver, or overridden with particle attributes by using its Density tab.

Viscosity

The dynamic viscosity of the fluid. The value of the Viscosity parameter depends on the scale of the particles. At the default scale, you will need values around 1000 for a thick fluid, and around 10000 for a doughy fluid. See the discussion of viscosity in the user guide.

This value is stored in the object’s viscosity field and can be manipulated in the Volume Velocity input of the FLIP solver, or overridden with particle attributes by using its Viscosity tab.

Enable Viscosity must be set on the Viscosity tab of the FLIP solver for this setting to have any effect.

Add Divergence Field

Instead of always making the local particle velocities divergent free, one can instead introduce artificial divergence or convergence using the divergence field. This field can also be manipulated by using the Divergence tab of the FLIP Solver.

Use External SOPs

This option will initialize the surface and/or velocity volumes from SOP volumes. This is useful when using the Narrow Band option in the FLIP Solver because it will no longer be necessary to fill the entire volume with particles to initialize the simulation.

Closed Boundaries

Controls if the particles should be allowed to go ballistic or hit an invisible glass wall when the maximum bounds is reached, as specified in the Volume Limits tab of the FLIP Solver. Turning this checkbox on makes it easier to setup water tank style simulations.

Guides

Each of the fields that define the FLIP fluid 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.

Inputs

First

The particle fluid objects to turn into FLIP fluids by attaching the appropriate data.

Outputs

First

The Particle Fluid object with FLIP data 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