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The Gas Seed Markers DOP is a microsolver used in building larger fluid
simulations. The 
Fluid Solver and 
Smoke Solver DOPs that allow
microsolvers to be added before or after the main solver step to extend
or tweak the simulation. Alternatively, enterprising people may attempt
to build an entire new solver out of microsolvers.
The Gas Seed Markers DOP creates marker particles, or surfels, along the boundary of a signed distance field. These can be used to correct a fluid solution to help preserve volume and maintain high detail on boundaries.
The created surfels have the Cd attribute which signifies if they are
inside or outside surfels. Inside surfels are red, outside are green.
They also have a pscale attribute which provides their distance to the
surface clamped to between 0.1 and 0.5 of the voxel size.
Parameters ¶
Surfels
The geometry data which will have surfels added to it. Any existing surfels will be taken into account in the ensuing update. Surfels that are too far outside will be deleted and voxels with too few surfels will have additional ones added.
Surfels Group
An optional point group to specify which surfels to consider when counting existing surfels within a voxel.
Surface
The signed distance field to seed the marker surfels around. The zero isocontour and an approximately three cell distance on other side of it will be seeded with surfels.
Collision Field
A signed distance field representing collision geometry. Any inside surfel that is inside this field will be deleted.
Source Field
A signed distance field representing a fluid source. Any outside surfel that is inside this field will be deleted.
Source Velocity
A velocity field representing the velocity of a fluid source. Surfels inside of the source volume will have the velocities overwritten by this field.
Apply Only To New Surfels
Apply the Source Velocity field only to the newly created surfels inside of the source volume.
Sink Field
A signed distance field representing a fluid sink. Any inside surfel inside this sink will be deleted.
Boundary Layer Field
A signed distance field representing the surface in the boundary layer. Surfels above the surface in the boundary layer will be deleted and voxels with too few surfels will have additional ones added.
Boundary Velocity Field
A vector field representing the velocity field in the boundary layer. Surfels in the boundary layer will have their velocity overwritten by this velocity field.
Particle Velocity Attribute
The boundary velocity will be applied to the indicated attribute for all the surfels.
Slice Index Field
An index field that indicates the slice number that a region of a simulation corresponds to. If an index field voxel does not match the slice number, a surfel will not be created.
Note
This index field is only used in the boundary layer.
Surfels Per Voxel
The number of surfels per voxel cell. Surfels will be added to cells within the bandwidth until this number is reached.
Birth Threshold
Surfels will be added to a voxel once the current number drops below the product of this parameter and Surfels Per Voxel.
Death Threshold
Surfels will be deleted from a voxel once the current number rises above the product of this parameter and Surfels Per Voxel.
Surface Oversampling
Oversample surfels by this multiple when within Oversampling Bandwidth of the surface.
Oversampling Bandwidth
Oversample within this number of voxels from the surface or any Surface volume boundaries, if Oversample at Boundaries is enabled.
Oversample At Boundaries
Oversample within Oversampling Bandwidth voxels of the boundaries of the Surface volume.
Move To Isosurface
Move surfels that are slightly inside or outside the Surface field back to the zero isosurface.
Build Inside Surfels
Construct marker surfels on the inside of the surface. Inside surfels increase volume as they correct the surface by pushing out the boundary.
Build Outside Surfels
Construct marker surfels on the outside of the surface. Outside surfels decrease volume as they correct the surface by pushing in the boundary.
Add Particle System
Create a particle system in the Surfels geometry if one does not exist.
Kill Outside Bounding Box
Kill any surfels outside the bounding box of the field specified in the Surface parameter.
Copy Nearest Surfel
Instead of explicitly setting the Cd and pscale attributes when
creating a new surfel, create a copy of the nearest existing surfel,
including all other attributes.
Use Boundary
Activate the boundary layer option to seed particles inside the layer even if there is no surface. Additionally, particles inside the boundary layer but above the boundary volume will be deleted.
Lower Border
The thickness of the boundary layer into the simulation from the bottom corner.
Upper Border
The thickness of the boundary layer into the simulation from the upper corner.
Volume Size
The size of the current simulation volume. This is used to help determine where the boundary layer is positioned in the simulation.
Volume Center
The center position of the current simulation volume. This is used to help determine where the boundary layer is positioned in the simulation.
Interpolate Velocity In Boundary
By default, when new surfels are created in the boundary layer, they will be assigned the default velocity (or sampled from the Boundary Velocity Field). With this toggle enable, the new boundary surfels will instead interpolate the velocity vector from nearby surfels. This option is useful for maintaining water level of a flat tank by freely allow surfels to flow out of the boundary layer and into the simulation.
Use Waterline
The waterline option replaces the Boundary Layer Field with a plane, where particles are instead seeded into volume below the waterline plane and inside the boundary layer.
Waterline
The Waterline level and Waterline Direction define the waterline plane. The volume inside the boundary layer and below the Waterline plane will be seeded with particles. Alternatively particles inside the boundary layer and above the Waterline plane will be deleted.
Waterline Direction
The Waterline Direction specifies the “up” direction of the Waterline plane.
Use pscale Attribute
This toggle determines whether the surfel radius used to reseed at the
boundary surface is derived from the pscale attribute or the user-provided
value below.
Boundary Surface Offset
When reseeding the boundary layer, the surfels will be offset from the surface based on this radius. The default is 0, meaning the surfels will be placed exactly on the surface.
Interpolation Attributes
All attributes matching this pattern will be interpolated to newly seeded particles from the surrounding existing particles. This interpolation is more expensive but yields a smoother particle sampling for attributes like velocity.
Minimum Radius
Surfel’s radius will not be smaller than this. They will created this distance from the surface. This is measured in terms of voxel cells.
Maximum Radius
Surfel’s radius will not be larger than this. Even if they are farther from the surface than this distance, their radius will be clamped to this size. This is measured in terms of voxel cells.
Use Bandwidth
Use the bandwidth settings to limit the size of the surface boundary within which surfels can be created.
Birth Bandwidth
Surfels will be created up to this number of cells from the surface if the current number of surfels in the cell is less than surfels per voxel.
Death Bandwidth
Surfels will be deleted when they stray this number of cells from the surface boundary.
Seed
The seed to the random number generator that determines the position of the newly create surfels or which surfel to delete in an overfilled voxel.
Slice Number
If non-negative, only particles from this slice will be eligible as seeds. This avoids wastefully seeding particles in external slices that will be eliminated in the next border update. This only takes effect if the slice attribute is present and Copy Nearest Surfel mode is turned on (and an index field is provided for the boundary layer).
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.
