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The Gas Calculate 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 Calculate DOP performs many different operations on a pair of fields. It can be used to scale or blend two fields. Another use is to find the intersection or union of signed distance fields by using the Maximum or Minimum function.
Parameters
Dest Field
The result of the operations is stored into this field. This field is also used as the first source operand in the calculation. The type of this field can be scalar, vector, or matrix. The source field should match this type. Index fields can also be used, but only if the operation is constant, ie, consists of Copy calculation type with no source field.
Source Field
This is used as the second operand in the calculation. It can be a list of fields in which case they are matched pairwise with Dest Fields. SDFs can also be used. If there are more dest than source, the extra do not get a source field.
Source Index
Instead of the source field, this index field will be treated as a field of object ids. For each voxel, the corresponding object will be looked up and its property used as the source value. There can only be one index field.
Source Property
The property of the object referenced by the index field to use as the source value.
Dest Blend
This is used as the destination value for blending. it must be a scalar field.
Source Blend
This is used as the source value for blending. it must be a scalar field.
Blend Method
The destination and source blend fields are used to blend the result of the gas calculation onto the destination field. The two values that are blended are the original destination field and the result of the calculation, including the post multiplication and addition. If blend fields are not present, they are considered to have a value of zero.
The original value of the destination field is A, the value of the source field is B. The destination blend value is Wa and the source blend value is Wb. The value D is written to the destination field.
No Blending
No blending is performed and the blending fields are ignored.
D = B
Weighted Average
The blend fields are used as weights and are normalized.
D = (Wa * A + Wb * B) / (Wa + Wb)
Weighted Sum
The blend fields are used as weights and are not normalized.
D = Wa * A + Wb * B
Net New Source
Does a weighted average but with the weight for the source field being how much "extra" additional material it has. This matches the expected blending for density where the densities were mixed using Maximum.
D = (Wa * A + max(Wb - Wa, 0) * B) / (Wa + max(Wb - Wa,
0))
Net New Volume
Selects from the source field only if the source blend field represents being inside a volume (less than zero) and the destination blend field represents being outside a volume (greater than zero). This corresponds to copying only for the net new volume that would result from doing a Minimum of the two SDFs.
if (Wb < 0 and Wa >= 0) D = B else D = A
Calculation
The value of the destination field is A, the value of the source
field is B. The value D is written to the destination field.
Dest, Source, Post Time Scales
The relevant add and/or multiply operand is scaled according
to the current timestep. If a is the addition operand and m
the multiplication operand and t the timestep, the operands
are changed as follows.
None
a = a
m = m
Add
a = a * t
m = m
Multiply
a = a
m = e ^ (ln(m) * t)
Both
a = a * t
m = e ^ (ln(m) * t)
Divide
a = a
m = m / t
Linearly Scale Multiply
a = a
m = m * t
Dest Pre-Add, Dest Pre-Mul
The A in the calculation is set to A' = A * m + a, where m is the
Dest Pre-Mul and a the Dest Pre-Add.
Source Pre-Add, Source Pre-Mul
The B in the calculation is set to B' = B * m + a, where m is the
Source Pre-Mul and a the Source Pre-Add.
Post-Add, Post-Mul
The D that the destination field is set to is modified by D' = D * m + a, where m is the Post-Mul and a is the Post-Add.
Dest, Source, Blend Pre-Add, Pre-Mul, Convert, Scale
The destination and source blending fields, W, are set to
W' = W * m + a.
The conversion is then applied to the result. SDF to Fog will map negative values to 1 and positive values to 0 with values lying within the one cell boundary getting linearly interpolated values. SDF to Surface will map cells on the zero boundary to 1 and those more than one cell away from the boundary to 0. Absolute will take the absolute value of the field.
The scaling factor is then applied to the result. A length scale will multiply by the voxel size, area by the size squared, and volume by the size cubed.
Use Timestep
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.
Time Scale
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.
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 $OBJID to vary as expected.
Inputs
First Input
This optional input can be used to control which simulation objects are modified by this node. Any objects connected through this input and which match the Group parameter field will be modified.
If this input is not connected, this node can be used in conjunction with an Apply Data node, or can be used as an input to another data node.
All Other Inputs
If this node has more input connectors, other data nodes can be attached to act as modifiers for the data created by this node.
The specific types of subdata that are meaningful vary from node to
node. Click 
an input connector to see a list of available
data nodes that can be meaningfully attached.
Outputs
First Output
The operation of this output depends on what inputs are connected to this node. If an object stream is input to this node, the output is also an object stream containing the same objects as the input (but with the data from this node attached).
If no object stream is
connected to this node, the output is a data output. This data
output can be connected to an 
Apply Data DOP,
or connected directly to a data input of another data node, to
attach the data from this node to an object or another piece of
data.
Locals
channelname
This DOP node defines a local variable for each channel and parameter on the Data Options page, with the same name as the channel. So for example, the node may have channels for Position (positionx, positiony, positionz) and a parameter for an object name (objectname).
Then there will also be local variables with the names positionx, positiony, positionz, and objectname. These variables will evaluate to the previous value for that parameter.
This previous value is always stored as part of the data attached to the object being processed. This is essentially a shortcut for a dopfield expression like:
dopfield($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 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:
$tx + 0.1
…to make the object move 0.1 units along the X axis at each timestep.
Examples
HotBox Example for Gas Calculate dynamics node
This example shows how to take any object with it’s volume representation and add it to the temperature field. You can change the temperature of the object in two ways: by adjusting the volume density value or by adjusting the Gas Calculate microsolver DOP’s source’s Pre-Multiply field.
The following examples include this node.
FlipColorMix Example for FLIP Solver dynamics node
VariableDrag Example for Fluid Object dynamics node
HotBox Example for Gas Calculate dynamics node
dopexample_gasnetfetchdata Example for Gas Net Fetch Data dynamics node
RBDtoSmokeHandoff Example for Smoke Object dynamics node
SourceVorticlesAndCollision Example for Smoke Object dynamics node
