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For FLIP fluids, use the FLIP Fluid Object node instead.
This parameter controls the interaction distance between particles in the created Particle Fluid Object. Decreasing this value will decrease the distance between your particles making it more impressive, but may also slow down your simulation, since it will take longer to simulate. Decreasing particle separation means more particles that weigh less, but add up to the same mass per unit area.
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.
In versions prior to Houdini 12, this value was set internally at 2.
Creation Frame Specifies Simulation Frame
Determines if the creation frame refers to global Houdini
$F) or to simulation specific frames (
latter is affected by the offset time and scale time at the
DOP network level.
The frame number on which the object will be created. The object is created only when the current frame number is equal to this parameter value. This means the DOP Network must evaluate a timestep at the specified frame, or the object will not be created.
For example, if this value is set to 3.5, the
Timestep parameter of the DOP Network must be changed to
1/(2*$FPS) to ensure the DOP Network has a timestep at frame
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
The name for the created object. This is the name that shows up in the details view and is used to reference this particular object externally.
While it is possible to have many objects with the same name, this complicates writing references, so it is recommended to use something like
$OBJID in the name.
Solve On Creation Frame
For the newly created objects, this parameter controls whether or not the solver for that object should solve for the object on the timestep in which it was created.
Usually this parameter will be 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.
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.
Use this tab to control the initial configuration of the particle fluid object.
Determines how to interpret the SOP geometry specified in SOP Path.
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.
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.
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.
Use this option to generate particles inside of the specified DOP object geometry
The initial separation between particles is determined by the Particle Separation parameter, and so the particle separation also determines the number of particles created.
This determines how the initial configuration of fluid particles if Input Type is set to Surface SOP.
Particles are generated on an axis-aligned grid inside of the surface.
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.
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.
This field expects a file containing particle fluid geometry; that is, geometry extracted from the Geometry field of a particle fluid object.
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.
The object from which to fetch the data.
You can use group names, object names, object ids, and/or wildcard characters to match an object, and the negation character (^) to eliminate objects from consideration. If multiple objects match, Houdini uses the one with the lowest object ID as the source object.
Source Data Name
Data matching the Source Data Name will be extracted from the source object and provided as the source geometry to control the initial locations of fluid particles.
Use Object Transform
If this toggle is enabled, the transform of the object containing the SOP geometry applied to the geometry. This is useful if the initial location of the geometry is defined by an object transform.
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.
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.
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.
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.
The fewest number of attributes. Useful for POPs or FLIP fluids.
Pressure and other attributes required by SPH fluids.
Adds attributes to instance a 'grain' to each point so the points can have their own unique colleciton of spheres. Used by the gas particle forces DOP.
Add Viscosity Attribute
A viscosity attribute is added, but not written to. It’s 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.
Use this tab to quickly visualize the particle fluid object.
Show Guide Geometry
Enables or disables particle visualization.
Selects between Spheres, Sprites, Grain, or Particles visualization of particles.
Sphere visualization stamps a scaled sphere at each point. Sprite will stamp a billboard image that looks like a sphere. Grain stamps arbitrary geometry on the points.
Controls the color of the visualization geometry.
This controls the size of the spheres in the guide geometry.
The sprite image to display when Visualization is set to Sprites.
Instead of a constant color, one of the particle attributes could be visualized.
The color is used for all particles.
The length of the attribute is used. In the case of velocity, this corresponds to the speed.
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.
The attribute’s raw value is used as the color channels.
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.
Which point attribute to visualize as color.
Before mapping the visualization range, the attribute is multiplied by this scale.
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
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.
Use this tab to control general DOPs physical parameters for the particle fluid object.
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.
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.
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.
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.
For this force to be applied by the particle fluid solvers, the Enable Viscosity Force toggle must be enabled.
Controls the magnitude of surface tension forces applied to particles in the fluid by the Particle Fluid Solver DOP. Surface tension forces attempt to pull surface particles more tightly in to the fluid, resulting in a more rounded fluid shape.
For this force to be applied by the Particle Fluid Solver, the Enable Surface Tension Force toggle must be enabled in the Internal Forces tab of the Particle Fluid Solver.
This only applies to SPH fluids.
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.
Use Point Velocity for Collisions
The local velocity of an affector object is a combination of the angular and linear velocity. However, if the object is deforming and points match frame to frame, the local point velocity can be used as well to estimate the deformation effect.
Use Volume Velocity for Collisions
If an affector object doesn’t have a stable point count, but does have a volume representation, the change in the volume representation can be used as an estimate of deformation velocity.
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.
Stores the last pressure force vector computed for each particle.
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.
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.
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,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.
The minimum boundaries of the user-defined coordinate bounding box.
The maximum boundaries of the user-defined coordinate bounding box.
Use this tab to select the grain geometry be instanced at each particle location.
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.
This options lists a number of default grain shapes.
Controls the size of the grains.
Controls the size of the spheres that comprise the grain.
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.
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).
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.
This value is the inverse of the TIMESTEP value. It is the number of timesteps per second of simulation time.
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
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).
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).
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).
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.
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 == $OBJCT should
always be used. This value will be zero if the node does not process
objects sequentially (such as the Group DOP).
This value is the simulation frame (see variable SF) at which the current object was created.
This value is equivalent to using the dopsttoframe expression on the OBJCT variable. This value will be zero if the node does not process objects sequentially (such as the Group DOP).
This is a string value containing the name of the object being processed.
Object names are not guaranteed to be unique within a simulation. However, if you name your objects carefully so that they are unique, the object name can be a much easier way to identify an object than the unique object identifier, OBJID.
The object name can
also be used to treat a number of similar objects (with the same
name) as a virtual group. If there are 20 objects named “myobject”,
strcmp($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).
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.
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.
This example demonstrates how to couple the Particle Fluid with an RBD object so they both affect each other. The result is a buoyant sphere.