POP Solver - Houdini online help

Parameters

POP Network

POP Path	Points to the POP Network which is used to process the points in the object’s Geometry data at each timestep.
Ensure All Points Are Particles	Tells the solver to check the object’s geometry to ensure that all points are part of particle primitives. POP Nodes will only process points that are part of particle primitives. Any points that are not part of particle primitives are added to a new particle primitive with various attributes set according to the following parameter values.
Inherit Velocity	Turning on this option causes the solver to not only look at the velocity attribute data on the object’s geometry to determine its velocity. It also looks at the Position data on the object, and adds any velocity and angular velocity components to the velocity of each point. This option is very useful to ensure continuity when handing off an object to the POP Solver from another solver (such as RBD).
New Particle Generator	If a new particle primitive is created due to the Ensure All Points Are Particles parameter, the generator attribute of the new parameters is set to this value. POPs use the generator attribute to determine which POP Node was the source for this primitive, and therefore which route through the POP Network the particles will take.
New Particle Lifespan	If a new particle primitive is created due to the Ensure All Points Are Particles parameter, the lifespan of the particles are set to this value. This measures how long in seconds that a particle will live before it dies of old age. A value of -1 will cause the particles to never die of old age.

Standard

Initial State	Geometry file on disk to use as initial state of the POP simulation.
Random Seed	Provides a seed value for all random number generation performed by the POP Network.
Oversampling	How many times to cook the POP Network for each DOP simulation timestep.
Max # of Particles	Controls the maximum number of particles that can exist in the POP simulation at any given moment. A value of 0 means particles can always be birthed.
Remove Unused Points	Remove unused (killed) points from the object’s geometry.

Input Geo

SOP 1	Path to a SOP to use as the First Context Geometry.
SOP 2	Path to a SOP to use as the Second Context Geometry.
SOP 3	Path to a SOP to use as the Third Context Geometry.
SOP 4	Path to a SOP to use as the Fourth Context Geometry.

Collisions

Stop Particles	The parameters in this tab provide filters to apply to all DOP objects which are affectors of the POP Object. Any objects which match this filter cause particles to stop when involved in a collision. The filters in this tab are applied in order, so the first filter which matches a given object determines the behavior of that object.
Kill Particles	Any objects which match this filter cause particles to be killed when involved in a collision.
Continue Particles	Any objects which match this filter cause particles to continue unaffected when involved in a collision.
Stick Particles	Any objects which match this filter cause particles to stick to them when involved in a collision. Sticking the particles to the object is achieved using constraints.
Bounce Particles	Any objects which match this filter cause particles to bounce off them when involved in a collision. The bounce attribute of the particle is used to determine the amount of energy preserved in the collision.

Solver

Geometry Name	This specifies which geometry data will be used as the particle system by the POP Solver. Usually this will be Geometry as that represents the standard geometry of a node. However, any other data of GeometryCopy type may be specified and manipulated by the particle network. One can thus update marker particles in a fluid simulation or vorticle particles by using the appropriately named geometry data.
Do Cook First	The normal POP order of cooking is Reap, Apply Rules, Prep, and then Cook. This works for POPs as it allows accelerations to be seen before they are applied. However, when chaining multiple POP networks together it means the suppression rules need to be set on the next network rather than the current one. Setting Do Cook First changes the cook order to Prep, Cook, Reap, Apply Rules. Note that since DOP Forces are set outside the cook invocation, setting Do Cook First will disable Dop Forces. However, in the case of particle fluids, this is likely the desired behaviour in any case.
Prep Particles	Before cooking particles they need to be reset from their old values. The Prep Particles stage of the cook process zeros out the accelerations and torques and updates the state flags based on the age of the particles. If you want multiple POP solvers to accumulate forces, one must disable the particle in one of the solvers to allow the acceleration to be passed from one solver to the next.
Cook POPs	Determines if the POP network will be executed for the particles. The POP network adjusts particle accelerations according to the POP nodes. Cooking does not update the velocity or position to reflect timestepping, however.
Apply Rules	After the new accelerations and torques have been calculated by the pop network, they need to be integrated over time to calculate the new velocities and positions. The Apply Rules stage does this. It also handles speed limits and up vector adjustments.
Suppress *	These options suppress individual portions of the Rule update. If Apply Rules is off, everything is suppressed. If it is on, one can turn of specific sections.
Reap Particles	For efficiency particles flagged for deletion are all deleted in a single reap pass. This stage can be enabled with this option. Note Particles that are marked for deletion are removed from the simulation, so even if their deletion is disabled, they will still stop simulating.
Dop Forces	Enables the processing of DOP level forces. If one is chaining multiple POP Solvers together, one only wants to apply the forces once to avoid double counting.

Parameter Operations

Each data option parameter has an associated menu which specifies how that parameter operates.

Use Default	Use the value from the Default Operation menu.
Set Initial	Set the value of this parameter only when this data is created. On all subsequent timesteps, the value of this parameter is not altered. This is useful for setting up initial conditions like position and velocity.
Set Always	Always set the value of this parameter. This is useful when specific keyframed values are required over time. This could be used to keyframe the position of an object over time, or to cause the geometry from a SOP to be refetched at each timestep if the geometry is deforming. You can also use this setting in conjunction with the local variables for a parameter value to modify a value over time. For example, in the X Position, an expression like `$positionx + 0.1` would cause the object to move 0.1 units to the right on each timestep.
Set Never	Do not ever set the value of this parameter. This option is most useful when using this node to modify an existing piece of data connected through the first input. For example, an RBD State DOP may want to animate just the mass of an object, and nothing else. The Set Never option could be used on all parameters except for Mass, which would use Set Always.

Default Operation

For any parameters with their Operation menu set to Use Default, this parameter controls what operation is used.

This parameter has the same menu options and meanings as the Parameter Operations menus, but without the Use Default choice.

Make Objects Mutual Affectors

All objects connected to the first input of this node become mutual affectors.

This is equivalent to using an Affector DOP to create an affector relationship between “*” and “*” before connecting it to this node. This option makes it convenient to have all objects feeding into a solver node affect each other.

Group

When an object connector is attached to the first input of this node, this parameter can be used to choose a subset of those objects to be affected by this node.

Data Name

Indicates the name that should be used to attach the data to an object or other piece of data. If the Data Name contains a “/” (or several), that indicates traversing inside subdata.

For example, if the Fan Force DOP has the default Data Name “Forces/Fan”. This attaches the data with the name “Fan” to an existing piece of data named “Forces”. If no data named “Forces” exists, a simple piece of container data is created to hold the “Fan” subdata.

Different pieces of data have different requirements on what names should be used for them. Except in very rare situations, the default value should be used. Some exceptions are described with particular pieces of data or with solvers that make use of some particular type of data.

Unique Data Name

Turning on this parameter modifies the Data Name parameter value to ensure that the data created by this node is attached with a unique name so it will not overwrite any existing data.

With this parameter turned off, attaching two pieces of data with the same name will cause the second one to replace the first. There are situations where each type of behavior is desirable.

If an object needs to have several Fan Forces blowing on it, it is much easier to use the Unique Data Name feature to ensure that each fan does not overwrite a previous fan rather than trying to change the Data Name of each fan individually to avoid conflicts.

On the other hand, if an object is known to have RBD State data already attached to it, leaving this option turned off will allow some new RBD State data to overwrite the existing data.

Inputs

First Input

This optional input can be used to control which simulation objects are modified by this node. Any objects connected through this input and which match the Group parameter field will be modified.

If this input is not connected, this node can be used in conjunction with an Apply Data node, or can be used as an input to another data node.

All Other Inputs

If this node has more input connectors, other data nodes can be attached to act as modifiers for the data created by this node.

The specific types of subdata that are meaningful vary from node to node. Click an input connector to see a list of available data nodes that can be meaningfully attached.

Outputs

First Output

The operation of this output depends on what inputs are connected to this node. If an object stream is input to this node, the output is also an object stream containing the same objects as the input (but with the data from this node attached).

If no object stream is connected to this node, the output is a data output. This data output can be connected to an Apply Data DOP, or connected directly to a data input of another data node, to attach the data from this node to an object or another piece of data.

Local variables

channelname	This DOP node defines a local variable for each channel and parameter on the Data Options page, with the same name as the channel. So for example, the node may have channels for Position (positionx, positiony, positionz) and a parameter for an object name (objectname). Then there will also be local variables with the names positionx, positiony, positionz, and objectname. These variables will evaluate to the previous value for that parameter. This previous value is always stored as part of the data attached to the object being processed. This is essentially a shortcut for a dopfield expression like: dopfield($DOPNET, $OBJID, dataName, "Options", 0, channelname) If the data does not already exist, then a value of zero or an empty string will be returned.
DATACT	This value is the simulation time (see variable ST) at which the current data was created. This value may not be the same as the current simulation time if this node is modifying existing data, rather than creating new data.
DATACF	This value is the simulation frame (see variable SF) at which the current data was created. This value may not be the same as the current simulation frame if this node is modifying existing data, rather than creating new data.
RELNAME	This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP). In this case, this value is set to the name of the relationship the data to which the data is being attached.
RELOBJIDS	This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP). In this case, this value is set to a string that is a space separated list of the object identifiers for all the Affected Objects of the relationship to which the data is being attached.
RELOBJNAMES	This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP). In this case, this value is set to a string that is a space separated list of the names of all the Affected Objects of the relationship to which the data is being attached.
RELAFFOBJIDS	This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP). In this case, this value is set to a string that is a space separated list of the object identifiers for all the Affector Objects of the relationship to which the data is being attached.
RELAFFOBJNAMES	This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP). In this case, this value is set to a string that is a space separated list of the names of all the Affector Objects of the relationship to which the data is being attached.
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:

$positionx + 0.1

…to make the object move 0.1 units along the X axis at each timestep.

Example files

MatchShape

$HFS/houdini/help/examples/nodes/dop/popsolver/MatchShape.cmd

Load | Launch

This is an example of using the POP Solver with its Shape Matching capabilities. A POP Object is created from a polygonal teapot, and the POP Solver then applies forces to maintain this shape as the particles fall on a ground plane and are squished by an RBD Object.

Usages in other examples

Example name	Example for
FieldForceSmoke	Field Force dynamics node	Load \| Launch
popswithrbdcollision	RBD Point Object dynamics node	Load \| Launch
DrainExample	Particle Fluid Emitter dynamics node	Load \| Launch
VolumeSource	Particle Fluid Emitter dynamics node	Load \| Launch
DentingWithPops	SOP Solver dynamics node	Load \| Launch
PressureExample	Particle Fluid Solver dynamics node	Load \| Launch
PopFlow	Particle Fluid Solver dynamics node	Load \| Launch
ParticlesAndCloth	Particle Fluid Solver dynamics node	Load \| Launch
TankDrain	Particle Fluid Sink dynamics node	Load \| Launch
DrainExample	Particle Fluid Sink dynamics node	Load \| Launch
PressureExample	Particle Fluid Object dynamics node	Load \| Launch
PopFlow	Particle Fluid Object dynamics node	Load \| Launch

POP Solver dynamics node