Houdini 20.0 Nodes Dynamics nodes

SOP Solver 2.0 dynamics node

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Since 14.0

The SOP Solver DOP lets the DOP simulation use a SOP Network or chain of SOPs to evolve an object’s geometry over time. At each timestep, the SOP specified in the SOP Path parameter is set up with a number of global parameters accessible with the stamp or stamps expression function. Since these global parameters are modified at each timestep, any SOPs that feed into the output SOP are forced to recook.

The available global parameters are:

ST

Current simulation time being solved for.

TIMESTEP

Length of timestep.

OBJID

The object identifier for the object being solved.

OBJNAME

The name of the object being solved.

DOPNET

This is the full path to the DOP Network that is being solved.

DATANAME

This is the name of the data on the object that is to be solved by the SOP Solver.

DATAPATH

This is the full path to the piece of data within the DOP Network that is being solved by the SOP Solver. It is equivalent to $DOPNET:$OBJID/$DATANAME.

By using an expression like stamps("../OUT", "DATAPATH", "../.:objname/Geometry") in an Object Merge SOP, the output of the previous timestep can be used as the starting point for the next timestep within the SOP Network.

The SOP Solver is itself a SOP network. This means that the SOP Path can point to itself, using '.', to cook its contents as the SOP solving network. The default SOP Solver will contain an DOP import already set up with the appropriate stamp expression, and a Impacts network setup with the usual need to grab the impact data for an RBD object.

When a SOP Solver is applied to multiple objects, each object is solved individually in a separate pass. Likewise, when you dive in and view the SOP network you are not seeing the DOP solve, but instead the network as a normal SOP network. This usually results in the same thing as the default stamps are setup to behave the same in DOP and SOP modes.

Parameters

Use External SOP

Since this node is itself a SOP network, if this toggle is not set the SOP network used for solving is the contained network.

SOP Path

Points to the SOP that is accessed on each timestep.

The global parameter listed above are set on this SOP node, so this is the node that should be used as the first parameter in stamp or stamps expressions to access this information.

Data Name

The name of the Geometry data on the simulation object that should be processed by this solver.

Usually this will be the main geometry for an object, named Geometry, but this is not required.

The type of the data should be Geometry, Scalar Field, Vector Field, or Matrix Field. If it is Geometry, the output of the SOP network is made the new data value. If it is a field type, the first volumes (regardless of name) in the output will be sampled to become the new field values. The fields will not change size or resolution.

Invoke Compiled Block

To run a SOP solver, a copy of the geometry has to be made to ensure the SOPs can still read the original, unmodified, geometry during the solve. This adds a considerable overhead to a SOP solver, however.

If the SOP Path refers to a compiled block, or an Output SOP connected directly to a compiled block, the block can be directly invoked. Doing so allows the copy to be avoided and instead the geometry manipulated in-place.

The compile block’s output becomes the new version of the solved data. An input named “data” will be given the original version solved data.

Note

This only works with processing Geometry, not with fields.

Primary Input

Controls which input gets the original version of the solved data. By default this is “data”.

Note

If you try to load this data using an Object Merge or DOP Import, you will get an empty geometry as it has been removed from the DOP simulation so it can be solved in place.

Num Extra Inputs

It is possible to bind additional inputs to the invoked compile block. If the compile block has a named input matching one of these, it will gain the corresponding geometry when invoked. The normal rules to transform DOP Data into Geometry are performed, so Scalar Fields will be imported as Volumes, etc.

Input Name

The name of the input to bind to. The compile block begin with the matching name will be wired to this data.

Source

Where to gain the geometry from.

This Object’s Data

Data attached to the currently solved object will be used.

Note

You cannot refer to the data being currently solved as it is being solved in-place. Specifically, the data specified by the Data Name parameter and any extra output is not available.

Object’s Data

Data attached to another object in the simulation.

Relationship’s Data (Packed)

A relationship defines a link between this object and many other objects. This brings in all the other objects that match that relationship. The resulting geometry consists of many packed primitives, one per object, that contain the respective data.

Additional primitive attributes are present on each packed primitive to describe the object

objid

Unique integer id of the object.

objname

The name of the object, note DOPs allows duplicate names.

bounce

The Bounce Physical Parameter.

bounceforward

The Bounce Forward Physical Parameter.

friction

The Friction Physical Parameter.

dynamicfriction

The Dynamic Friction Physical Parameter.

temperature

The Temperature Physical Parameter.

pscale

The Thickness of the Surface Collision Parameter.

Solve Metadata

An empty geometry with detail attributes that provide the current status of the solver. This geometry will also contain the variables defined in Stamps.

simtime

Current simulation time.

startsimtime

Simulation time of the start of the current solve step.

endsimtime

Simulation time of the end of the current solve step.

time

Current Houdini time.

timestep

Duration of current solve step.

objid

Unique object id of object currently being solved by this sop solver. This doesn’t exist when solving relationships.

objname

Current name of the solving object. May be duplicate in DOPs. Will be present for relationships.

dopnet

Path of the DOP network currently solving.

datapath

Full path to geometry this sop solver is being asked to process.

dataname

Name of the data this sop solver is being asked to process.

Object Name

Which object to fetch data from.

Relationship

The relationship to use to determine which objects to fetch.

Collider Labels

Objects can have a collider label to provide a hint of how the user wants them to be used by various solvers. The Use Solver Default, Use Volume Collisions and Use Surface Collisions options of the Static Object are such an example. This lets you filter the objects found in a relationship collision by a label in addition to by the relationship type.

Data Name

What data on the object to acquire and convert to geometry.

Fetch Time

When to fetch the data. A SOP Solver solves a window of time, so it may be necessary to get the data either at the start or end of that window.

Num Extra Outputs

It is possible to define a compiled sop network with more than one output node. This is done with the Primary Path. In order to process multiple geometry in-place, these extra outputs can be specified here.

Input Name

The name of the Block Begin Compile SOP that will receive the original version of this geometry.

Data Name

The data on this object to solve with this output.

SOP Path

The path to the compile end node whose results will replace the data. Note this is relative to this node, so often can simply be the name of the node.

SOP Output is in Simulation Space

When extracting the geometry from the SOP at each timestep, turning on this parameter will cause the SOP Solver to transform the geometry from world space to local object space.

NOTE: When data is pulled into SOP using the Object Merge SOP, it is extracted into world space, so this option will often need to be turned on. Alternatively a DOP Transform SOP can be used at the SOP level to change the geometry from one space to another, rather than using this option.

Solve Objects on Creation Frame

Causes the SOP Solver to operate on an object even on the frame in which the object enters the simulation.

If you want to bring an object into a simulation and not modify it on the initial frame, turn this option off.

Stamps

Extra variables can be stamped here and accessed from within the SOP network with stamp expressions. For example, if you stamp a variable called FOO, its value can be queried with stamp("..", "FOO", -1) for a Float variable or stamps("..", "FOO", "") if the variable is of type String.

Note

When used with compiled blocks, these variables are added to the extra Solve Metadata input as detail attributes.

Variable Name

Name of the variable to stamp; this is the identifier that would be used as the second argument in a stamp or stamps expression.

Variable Type

Type of the variable to stamp. Float refers to a numeric value, and can be accessed using the stamp expression function. String variables can hold an arbitrary string of characters, and can be accessed using the stamps expression function.

Float Value

The value to assign to the numeric variable. This value will be returned by a relevant stamp expression from within the SOP network.

String Value

The value to assign to a string variable. This value will be returned by a relevant stamps expression from within the SOP network.

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 $tx + 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.

Solver Per Object

The default behavior for solvers is to attach the exact same solver to all of the objects specified in the group. This allows the objects to be processed in a single pass by the solver, since the parameters are identical for each object.

However, some objects operate more logically on a single object at a time. In these cases, one may want to use $OBJID expressions to vary the solver parameters across the objects. Setting this toggle will create a separate solver per object, allowing $OBJID to vary as expected.

Setting this is also required if stamping the parameters with a Copy Data DOP.

Inputs

All Inputs

Any microsolvers wired into these inputs will be executed prior to this node executing. The chain of nodes will thus be processed in a top-down manner.

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.

Locals

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 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

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.

Examples

DentingWithPops Example for SOP Solver dynamics node

This example combines a number of important DOPs concepts.

  • First, it uses both POP Solver and RBD Solver objects interacting with each other in a bidiretional manner. The RBD object affects the particles, and the particles affect the RBD object.

  • Second, the RBD object atually uses a multi-solver to combine an RBD Solver with a SOP Solver. The RBD Solver controls the motion of the overall object, while the SOP Solver performs the denting of the geometry.

  • Third, the SOP Solver extracts impact information from the RBD Solver to perform the denting. It extracts this information using DOP expression functions.

The end result is a simulation of a torus that is bombarded by a stream of particles. The particles bounce off the torus, and also cause the torus to move. In addition, each particle collision causes a slight denting of the torus.

VisualizeImpacts Example for SOP Solver dynamics node

An example that shows how you can visualize impact data in an RBD simulation by using a SOP Solver to add custom guide geometry to the RBD Objects.

This example has three toruses falling on a grid with green lines showing the position and magnitude of impacts. The force visualization is added as ancillary geometry data to the actual toruses, so the RBD Solver is entirely unaware of the effect. The SOP Solver could also be used as an independent SOP network to extract impact visualization from an RBD Object.

Dynamics nodes