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The SOP Merge Field DOP performs calculations on a pair consisting of the destination DOP field and a source SOP volume or VDB. This node supports the majority of the operations that can be performed with Gas Calculate, but doesn’t require the source volume to be imported into a DOP field before merging can be carried out. In addition, SOP Merge Field allows for length-wise comparison of vector fields as well as activity region-aware combining.
The destination field can be a scalar, vector, or a matrix field. A scalar field can be combined with either a volume or a VDB primitive. A vector field can be merged with a triplet of volumes, a triplet of scalar VDBs, or a single vector VDB. Lastly, a matrix field can be combined with 9 volume primitives or 9 scalar VDBs (mixing of types is not allowed).
Result of the operations is stored in this DOP field.
Path to the SOP that contains volume or VDB primitives to be merged.
List of names of the primitives to be merged. If Dest Field is scalar, this has to be a single volume or VDB; if it is a vector field, then the list must consist of three volume names, three scalar VDB names, or one vector VDB name. This list must contain nine volume names or nine scalar VDB names if Dest Field corresponds to a matrix field.
Name of the SOP scalar volume or VDB by which source volume is multiplied before merging with the Dest Field.
Name of the DOP field by which the source volume is multiplied before merging with the Dest Field.
When this option is enabled, negative mask values are interpreted as 0 and positive values are interpreted as 1.
When this option is enabled (along with Absolute), negative mask values are interpreted as 1 and positive values are interpreted as 0.
The merging operation.
Values in the Dest Field will be overwritten by those in the source volume.
Result is the sum of values in Dest Field and source volume.
Result is the difference between values in Dest Field and source volume.
Result is the product of values in Dest Field and source volume.
Values in the Dest Field will be divided by the corresponding values in the source volume.
Result is the larger of the values in Dest Field and source volume. For vector fields, comparison can be performed length-wise by enabling Use Vector Norm.
Result is the smaller of the values in Dest Field and source volume. For vector fields, comparison can be performed length-wise by enabling Use Vector Norm.
Result is the average of values in Dest Field and source volume.
Values in the Dest Field are pushed towards the corresponding entries in source volume, in accordance to settings of Acceleration Guide Strength, Deceleration Guide Strength, and Direction Guide Strength.
Use Vector Norm
If Calculation is set to Maximum or Minimum and destination is a vector field, enabling this option forces length-wise comparison of values (as opposed to component-wise).
Guide Direction Independently
When Calculation is set to
Guide, this option can be enabled to guide
a vector field’s length and direction independently. Guiding is performed
component-wise if this option is disabled.
Acceleration Guide Strength and Deceleration Guide Strength apply to the vector lengths when this option is enabled, and Direction Guide Strength applies to to the vector directions.
Acceleration Guide Strength
Controls how strongly the Dest Field's values are pushed towards the source volume. Applies for the voxels in which the source volume has larger values than the Dest Field.
Deceleration Guide Strength
Controls how strongly the Dest Field's values are pushed towards the source volume. Applies for the voxels in which the source volume has smaller values than the Dest Field.
Direction Guide Strength
When Guide Direction Independently is enabled, controls how strongly the Dest Field's vectors are pushed to align with source volume’s. This option only has an effect with vector fields.
When enabled, only parts of the Dest Field that fall within the active region of the source volume are modified.
Active region for a native Houdini volume is defined to be the area inside its bounding box. VDBs, on the other hand, may have an arbitrary active region. The VDB Visualize Tree SOP can be used to visualize the active voxels of a VDB primitive.
Dest, Source, Post Time Scale
The relevant add and/or multiply operand is scaled according to the current
a is the addition operand and
m the multiplication
t the timestep, the operands are changed as follows.
a = a
m = m
a = a * t
m = m
a = a
m = e ^ (ln(m) * t)
a = a * t
m = e ^ (ln(m) * t)
a = a
m = m / t
Linearly Scale Multiply
a = a
m = m * t
Dest Pre-Add, Dest Pre-Mult
Before performing the Calculation, destination value is set to
A * m + a, where
A is the value of the Dest Field,
m is Dest
a is Dest Pre-Add.
Source Pre-Add, Source Pre-Mult
Before performing the Calculation, source value is set to
A * m + a,
A is the value of the source volume,
m is Source Pre-Mult and
a is Source Pre-Add.
After performing the Calculation, result is changed to
A * m + a,
A is the pre-transformed result,
m is Post-Mult and
Dest, Source, Post Conversion
After the add and multiply operations, the specified conversion is applied.
No conversion is performed.
SDF to Fog
Maps negative values to 1 and positive values to 0 with values lying within the one cell boundary getting linearly interpolated values.
SDF to Surface
Map cells on the zero boundary to 1 and those more than one cell away from the boundary to 0.
Takes the absolute value.
Dest, Source, Post Scale
The scaling factor is applied after conversion. A length scale will multiply by the voxel size, area by the size squared, and volume by the size cubed.
Warn on Missing SOP Primitives
When enabled, this DOP will generate a warning if a named volume to be used for merging cannot be located. When disabled, the merging operation is silently skipped.
Determines if the current solver timestep will be used to apply this node.
If set, the current timestep size will be multiplied by the scale and used for the time increment for this operation. Otherwise, the time scale will specify an absolute fictitious time to integrate by.
By disabling the link between the actual real time and the microsolver time, you can perform operations in a separate, fictitious, time.
The timestep used for this microsolver will be scaled by this amount. This allows one to achieve non-realistic effects, such as parts of the simulation operating at different speeds than other parts.
Similarly, it is useful if a solver needs to be evaluated independently of the main timestep.
Each data option parameter has an associated menu which specifies how that parameter operates.
Use the value from the Default Operation menu.
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.
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
$tx + 0.1 would cause the object to
move 0.1 units to the right on each timestep.
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.
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
* before connecting it to this node. This option makes it
convenient to have all objects feeding into a solver node affect
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.
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.
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
to vary the solver parameters across the objects. Setting this toggle will
create a separate solver per object, allowing
$OBJID to vary as expected.
Any microsolvers wired into these inputs will be executed prior to this node executing. The chain of nodes will thus be processed in a top-down manner.
The operation of this output depends on what inputs are connected to this node. If an object stream is input to this node, the output is also an object stream containing the same objects as the input (but with the data from this node attached).
If no object stream is connected to this node, the output is a data output. This data output can be connected to an Apply Data DOP, or connected directly to a data input of another data node, to attach the data from this node to an object or another piece of data.
This DOP node defines a local variable for each channel and parameter on the Data Options page, with the same name as the channel. So for example, the node may have channels for Position (positionx, positiony, positionz) and a parameter for an object name (objectname).
Then there will also be local variables with the names positionx, positiony, positionz, and objectname. These variables will evaluate to the previous value for that parameter.
This previous value is always stored as part of the data attached to the object being processed. This is essentially a shortcut for a dopfield expression like:
dopfield($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.
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.
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.
In this case, this value is set to the name of the relationship the data to which the data is being attached.
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.
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.
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.
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.
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.