Houdini 16.5 Nodes Dynamics nodes

Particle Fluid Configure Object dynamics node

Attaches the appropriate data for Particle Fluid Objects to an object.

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The Particle Fluid Configure Object DOP takes a simulation object and attaches the data which is needed for it to be used as a Particle Fluid Object.

This DOP is very similar to the Particle Fluid Object, except that it allows you to explicitly control the creation of the object using another DOP, such as the Empty Object DOP.

Note

If this object contains Input Geometry data, this data will be used as the source geometry to generate particles inside of. Position data is also required to properly transform the Geometry.

Parameters

Particle Separation

This parameter controls the interaction distance between particles in the created Particle Fluid Object.

If the Input Type for this object is set to Surface SOP, then this parameter also controls the number of particles spawned inside of the provided surface. That is, a smaller particle separation results in a greater number of particles and hence a particle-based fluid with higher resolution.

Particle Radius Scale

The radius of the particles is determined by scaling the Particle Separation by this parameter. Setting this value higher will result in more volume in the fluid but less surface detail as it gets smoothed out by the larger particle radius.

Note

In versions prior to Houdini 12, this value was set internally at 2.

Rest Density

The physical density of the particle fluid. This quantity is used by the Particle Fluid Solver DOP to determine how to apply pressure forces to particles in the fluid.

When the density of particles exceeds their rest density, they are pushed apart. Similarly, they are pulled together when their density is less than the fluid rest density.

Viscosity

This parameter is used by the Particle Fluid Solver to control the thickness and resistance to flow of the particle fluid.

A fluid with higher viscosity tends to flow more slowly and appear thicker than one with low viscosity.

Note

This only applies to SPH fluids.

Surface Tension

Controls the magnitude of surface tension forces applied to particles in the fluid by the Particle Fluid Solver. Surface tension forces attempt to pull surface particles more tightly in to the fluid, resulting in a more rounded fluid shape.

Note

This only applies to SPH fluids.

Initial Data

Input Type

Determines how to interpret the SOP geometry specified in SOP Path.

Surface SOP

Use this option to generate particles inside of the specified surface.

The initial separation between particles is determined by the Particle Separation parameter, and so the particle separation also determines the number of particles created.

Particle Field

Use this option to generate a fluid particle at each point in the specified geometry.

This can be used to specify a custom initial distribution for the fluid particles or to resume an existing particle fluid simulation.

It can also be used to combine multiple fluids with different initial conditions. When this option is selected, the particle separation is completely independent of the initial particle distribution. This means that changing the particle separation may substantially alter the results of a simulation.

However, if the Initialize Fluid Attributes toggle is disabled, then the Particle Fluid Object does not create or change any attributes on the imported fluid geometry, and expects those attributes to exist already.

File

Use this option to initialize a fluid simulation directly from a .bgeo file. This can be used to easily reinitialize a simulation from saved geometry data. See the Fluid Geometry File parameter below.

Initial Configuration

This determines how the initial configuration of fluid particles if Input Type is set to Surface SOP.

Grid

Particles are generated on an axis-aligned grid inside of the surface.

Tetrahedral

Particles are generated in a more tightly-packed tetrahedral arrangement inside of the surface.

This can be useful is the fluid needs to settle quickly inside of a container without losing too much of its initial height.

SOP Path

The geometry controlling the initial locations of fluid particles. How this is used depends on Input Type.

Fluid Geometry File

The file to load fluid geometry from when Input Type is set to File.

NOTE: This field expects a file containing geometry extracted from the Geometry field of a particle fluid object.

Tip

When running a long simulation, it is useful to save .bgeo files containing particle fluid geometry at each frame. The simulation can then be restarted from any frame by specifying one of these files in this field.

Use Object Transform

The transform of the object containing the chosen SOP is applied to the geometry. This is useful if the initial location of the geometry is defined by an object transform.

Jitter Seed

When Input Type is set to Surface SOP, a random jitter may be applied to the particles created. This has the effect of making the initial fluid configuration less symmetrical. This parameter is a seed used in the random jitter application.

Jitter Scale

The magnitude of random jitter to apply to each particle.

Initialize Fluid Attributes

This parameter is only meaningful if Input Type is set to Particle Field. In this case, when this parameter is enabled, the DOP will overwrite any existing attributes used by the Particle Fluid Solver DOP (mass, velocity, density, etc.) with new values when it initializes the fluid particles.

Leave this parameter disabled if you wish to initialize a particle fluid object from the particle geometry of an existing particle fluid simulation. This is the case when you are attempting to restart an old simulation, or combine two or more particle fluid objects in to the same object.

Initialize Velocity

When sourcing from a grid of particles, they may already have a velocity. This option lets you override these velocities with your own constant velocity with the Initial velocity parameter below.

Initial Velocity

The initial velocity of the fluid particles created by this DOP.

Initialize Force and Mass

If enabled, add force and mass attributes to Plain particle types. These attributes are always added to SPH and Grain particles.

Particle Type

Plain

The fewest number of attributes. Useful for POPs or FLIP fluids.

SPH

Pressure and other attributes required by SPH fluids.

Grains

Adds attributes to instance a 'grain' to each point so the points can have their own unique collection of spheres. Used by the gas particle forces DOP.

Add Viscosity Attribute

A viscosity attribute is added, but not written to. Its default value is 1 to allow any new particles added to the sim to respect the global viscosity value.

Note

The particle viscosity is usually treated as a multiplier, so 1 means to use the global viscosity value.

Guides

Use this tab to quickly visualize the particle fluid object.

Show Guide Geometry

Enables or disables particle visualization.

Visualization

Selects between Spheres, Sprites, Grain, or Particles visualization of particles.

Sphere visualization stamps a scaled sphere at each point. Sprite will stamp a billboarded sprite. Grain stamps arbitrary geometry on the points.

Scale

This controls the size of the spheres in the guide geometry.

Color

Controls the color of the visualization geometry.

Visualization Type

Instead of a constant color, one of the particle attributes could be visualized.

None

The color is used for all particles.

Speed

The length of the attribute is used. In the case of velocity, this corresponds to the speed.

Direction

The attribute is normalized to a unit sphere and then scaled to fit into the RGB cube, resulting in a spectrum of colors depending on which way the particle is moving.

Value

The attribute’s raw value is used as the color channels.

Visualization Mode

If the attribute is a scalar attribute, or has been turned into a scalar attribute by the Speed visualization type, it can be remapped into a color spectrum.

Visualization Attrib

Which point attribute to visualize as color.

Visualization Scale

Before mapping the visualization range, the attribute is multiplied by this scale.

Detect Range

The minimum and maximum values of the attribute are computed and used for the range. This allows for automatic bounding of the range. The detail attribute vis_range will be set to the computed range.

Visualization Range

This range will be remapped into the 0..1 interval for setting the color or mapping by the Visualization Mode. Using a balanced interval, such as -1..1, is useful for detecting zero crossings of an attribute along with the Two-Tone visualization mode.

Sprite Image

The sprite image to display when Visualization is set to Sprites.

Physical

Use this tab to control general DOPs physical parameters for the particle fluid object.

Bounce

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.

Friction

The coefficient of friction of 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

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.

Collisions

Volume Offset

Controls how far away from collision geometry particle collisions occur.

If Volume Offset is set to 0, collisions occur directly at the boundary of the collision object. If it is set to 1.0, then collisions occur one particle radius away from the collision geometry.

Stored Attributes

Use this tab to select additional attributes to compute and store during the simulation.

Density Field Gradient

Stores the gradient of the fluid density field at each particle position.

This may be useful for identifying particles close to the surface of the fluid, as the magnitude of this vector is larger for particles close to the fluid surface than it is for particles far from the surface.

Pressure Force

Stores the last pressure force vector computed for each particle.

Neighbor Velocity

For each particle, this stores the average velocity of all neighbors of the particle.

By comparing a particle’s velocity with its neighbor velocity, areas of particularly turbulent flow in the fluid may be identified.

Coordinate System

Use this tab to generate a simple coordinate system to be carried along with the fluid. This coordinate system can later be transferred on to the fluid surface. The coordinate system is designed to reinitialize itself over time, and so at all times it stores two different coordinate system as well as a blend attribute to blend between the two. The blend value is stored in the detail attribute "coordinate_transition_state", while the two coordinate systems are stored in the point attributes "coordinate1" and "coordinate2". For each point, if we define the blend value as s and the coordinates as c1 and c2, then a blended coordinate value for that point could be given by s c1 + (1 - s) c2.

Create Coordinate System

Enables or disables the coordinate system on this object.

Coordinate Transition Period

Since any coordinate system on a set of freely moving fluid particles is expected to gradually become incoherent, the coordinate system is designed to periodically reinitialize itself. This specifies the transition period.

Coordinate Transition Length

During each transition period, the coordinate system remains constant for some period of time, and then transitions in to a reinitialized coordinate system over a specified transition length. Use this parameter to control that transition length.

Coordinate Scale

By default, all fluid coordinates are in the range [0,1] and are defined with respect to the initial bounding box of the fluid. Use this parameter to scale the range of each axis from [0,1] to [0,s] where s can be specified.

Override Bounding Box

By default, all particle coordinates are determined with respect to the initial bounding box of the fluid. This bounding box is repeated spatially to accommodate particles that flow out of this bounding box. Use this parameter to define coordinates with respect to a different bounding box.

Minimum Bound

The minimum boundaries of the user-defined coordinate bounding box.

Maximum Bound

The maximum boundaries of the user-defined coordinate bounding box.

Grain

Use this tab to select the grain geometry be instanced at each particle location.

Custom Grain

Enables or disables use of a custom defined grain SOP.

Grain SOP Path

When Custom Grain is enabled, the custom grain SOP path is defined here. Custom grains must be a set of rigidly-connected spheres.

Grain Shape

This options lists a number of default grain shapes.

Grain Radius

Controls the size of the grains.

Sphere Radius

Controls the size of the spheres that comprise the grain.

Inputs

Objects to be Processed

The simulation objects to turn in to Particle Fluid objects by attaching the appropriate data.

Outputs

First

The Particle Fluid Object 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.

See also

Dynamics nodes