Houdini 17.0 Nodes Dynamics nodes

FLIP Configure Object dynamics node

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

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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 desperation, 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 overriden 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 overriden 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

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.

Examples

The following examples include this node.

DensityViscosity Example for FLIP Solver dynamics node

This example demonstrates two fluids with different densities and viscosities interacting with a solid object.

FlipColorMix Example for FLIP Solver dynamics node

This example demonstrates the use of the Flip Solver to mix the colors of a red fluid with a blue fluid to form a purple fluid.

FlipFluidWire Example for FLIP Solver dynamics node

This example demonstrates the use of the Flip Solver and the Fluid Force DOP. The Fluid Force DOP is used to apply a drag force on a wire object according to the motions of a flip fluid. The drag force is only applied at locations where fluid exists in the fluid object.

See also

Dynamics nodes