Pyro Solver dynamics node

Sets and configures a Pyro solver.

All

Parameters

Inputs

Outputs

Local variables

See how to use the pyro tools for information on the general workflow surrounding this node.

The Pyro Solver DOP sets objects to use the Pyro Dynamics solver.

If an object has this DOP as its “Solver” subdata, it will evolve itself as a Smoke object.

This solver makes use of various field subdata on the object. The object should have a scalar field density to track the density of the smoke in space. The object should have a vector field called vel to track the velocity of each point in space. Optionally, a scalar field called temperature is needed for the internal buoyancy calculations.

To setup an object with the requisite subdata, use the Smoke Configure Object DOP.

Parameters

Sources

Any object with the source relationship will be unioned together and used as a source. Solid objects are converted into a field that is 1 inside the object and 0 outside. The maximum of this source field and the destination field is used for the new destination field.

Note that there are two possible fuel choices. One applies to fuel added to the main smoke object. The second controls how the sources on the second fuel object will behave. The fuel object can only inject fuel, thus the single option.

Emitters

Emit Smoke	Source objects add to the density field.
Smoke Amount	Scalar multiple for the source prior to computing the maximum with the density.
Emit Fuel	Source objects add to the fuel field.
Fuel Amount	Scalar multiple for the source prior to computing the maximum with the fuel.
Apply Temperature	Source objects add to the temperature field.
Temperature Scale	Scales the object’s temperature. Note that unlike fuel and smoke amounts, whether a voxel gets overwritten is unaffected by the temperature amount. A temperature amount of 0 does not turn off the addition of temperature, it just causes the newly set temperature to be 0.

Noise

Add Noise	Noise will multiply the source field with a noise field. This will turn the solid object into a Swiss cheesy object, often resulting in nicer chaotic behavior.
Amount	How much to mix the result of this calculation with the original source. A value of 0 is the same as not adding noise, 0.5 will blend 50% of each, and 1 will use the remapped noise value as the final source value.
Frequency-Flow Roughness	These control an anti-aliased flow noise VOP.
Contrast	How harsh the transition from 0 to 1 should be. A high contrast results in a sharp boundary between the kept and discarded regions, a low contrast results in a smoother drop off.
Map Noise Output to Final Source	Remaps the source value. This allows remapping the fall-off of the transition from sourced to non-sourced voxels. Alternately, by building a hill shaped ramp, one can extract a contour of the source shape rather than a solid version.

Simulation

These controls govern how the simulation advances with time.

Limit Speed, Speed Limit

If set, each axis of the velocity field will be clamped to this maximum speed. This is done after the force computation and before the divergence or advection, so it is possible for the speed to end up slightly larger than this value.

Note

As the clamping is done per-axis, the velocity along the diagonal can be up to sqrt(3) higher.

Viscosity

Viscosity is a force which tries to ensure that neighboring voxels have the same velocity. A zero value allows fluid in adjacent voxels to move any direction without resistance.

Higher values introduce a penalty effect when a voxel’s velocity varies from that of its neighbors. This is currently implemented by applying a diffusive term to the velocity field.

Cooling Rate

Controls how fast the temperature field trends to zero. A value of 0.9 will cause the temperature of hot gas to fall to 90% of its original value after one second.

Buoyancy Lift

An upwards force proportional to the difference between the ambient temperature and this voxel’s temperature will be applied with this scale factor. Thus voxel’s whose temperatures are higher than average will rise and those with values lower will sink.

Buoyancy Direction

The buoyancy force will be applied in this direction. It usually corresponds to the logical “up” direction.

Vortex Confinement

Instead of using explicit vorticles, the vortex locations can be detected from the velocity field directly. This parameter will cause existing vortices to be boosted by this value, increasing turbulence in the simulation that would otherwise be lost by the grid resolution. Too high a value can cause the simulation to become unstable and blow up.

Negative numbers can be used to suppress vortices and smooth out the simulation - but usually a better solution is just to use a lower resolution grid.

The scalar field called “confinement” is also multiplied into the vortex confinement value, allowing you to vary the amount of vortex boosting over space, or using a Gas Ramp, another field.

Velocity Damp

A global drag force on the velocity field.

Fuel

The physically correct thing is for fuel to move through the voxel grid in the same way as the smoke. However, in practice, it is often useful to control the fuel independently. The Original method moves the fuel with the same velocity field as the smoke, but has the option to speed it up or slow it down. The Alternate method allows an entirely independent fluid sim to be used to compute the fuel trajectory.

Advect Fuel

If not advected, fuel will never move from where it is created.

Fuel Speed

A scalar multiple for how fast the fuel should move relative to the other fields, such as temperature and smoke. Making the fuel move slower can keep a fire more contained.

Use Fuel Object

If set the fuel field will be acquired from the second input to the Pyro solver rather than from source relationships in the first input. This allows finer control of the evolution of the fuel as well as the ability to use its own independent forces and velocity field.

Fuel Mixing

How the fuel object will mix its fuel back into the pyro object.

Copy	Directly copy the fuel field every frame from the fuel object. Combustion of fuel within the pyro object will thus be ignored.
Accumulate	Add a time step proportional amount of fuel to the pyro object wherever it is present in the fuel object. Note that this can have the fuel field exceed 1 in value if no combustion occurs. The Fuel Amount will be added to the system after one second.
Average	Drag the pyro objects fuel field toward the fuel object’s field according to Fuel Convergence. This ensures the pyro objects fuel field will remain between its value and that of the fuel object.

Fuel Convergence

How quickly the pyro object’s fuel field will be set to the fuel object’s value. A value of 1 is equivalent to a copy, a value of 0.5 means after 1 second the pyro object will have 50% of the fuel object, a value of 0 means it will never copy.

Solve Independent Fuel Velocity

Run a second fluid sim for the fuel object. This will allow you to independently control the forces and behavior of the fuel’s motion separate from the normal simulation. The various simulation parameters match the correspondingly named ones in the main simulation.

Combustion

The combustion model takes the fuel field and turns it into a burn, temperature, and density fields. If combustion is not enabled, the fuel field is ignored in the simulation.

Most of these options are documented in Gas Combustion DOP

Heat Source

Controls what initializes the heat field. If set to None, the heat field will not be changed by the smoke solver, which means it also will not cool down. Source means objects with a source relationship will set the heat field to 1 inside of the objects. Burn means to apply a maximum operation with the burn field, this is most sensible if a Use Fuel Model is enabled. The heat field tracks the velocity of the fluid so is useful to create a history of the burn field.

Heat Cool Time

How many seconds it takes the heat field to drop from a value of 1 to a value of 0. Note that the heat field will keep dropping below 0, becoming a sort of time-since-sourced field.

Diffusion

Turbulence operating at a finer scale than the sim will spread the the fields out. This can be modeled by blurring the fields over time. A diffusion of 2 will cause the field, after one second, to be blurred by a guassian with radius 2.

Turbulence

Creates a global turbulence field. This turbulent velocity field is modulated by the forcescale field and the lookup ramps provided. This controls where the turbulence shows up, so you can ensure it occurs only in the regions of the sim you want.

The noise parameters come from the Curl Noise VOP.

Scale	Strength of the force to add to the velocity field.
Mappings	These ramps allow one to control the specifics of the turbulence noise according to the `forcescale` value. The result of using the ramp is multiplied with the corresponding noise parameter.

Forces

Force Scale	A global scale factor on all forces applied to the simulation. 2 will make forces twice as strong, 0.5 half as strong.
Force Scale Blur	Diffuses the force scale. This occurs instantaneously, so is the radius of blur to apply on this specific frame. Since the `forcescale` is regenerated every frame, this does not accumulate. Ensures the resulting turbulent velocity field has smooth edges avoiding boundary artifacts.
Force Scale Source	Controls the source of the initial `forcescale` field. Density and temperature mean it is copied verbatim from those fields. Density and Burn will take the maximum of the two fields, useful for ensuring frame one has as much force as later frames when the density fills in more.
Force Scale Field	When the force scale source is other this is the field used for the initial forcescale field.
Force Scale Range	The range of the initial force scale field that will be remapped to the 0..1 interval.
Map Force Scale Source to Force Scale	Remaps the force scale ramp with respect to itself.
Scaled Forces	Determines which DOP Forces will be scaled by the `forcescale` value before being applied. These forces will be proportional to the `forcescale` field on each voxel.
Absolute Forces	Determines which DOP Forces will be applied uniformly to all of the voxels of the simulation. The `forcescale` field will not be taken into account when evaluating these

Advanced Options

Rest Field

Speed	Controls how fast the rest field moves in response to the velocity field. To get the rest field to stick perfectly to the smoke a value of 1 would be used. This, however, quickly results in the rest field smearing out in streaks which is often not desired. By moving it slower than the actual smoke velocity the streaking can be diminished while still letting the rest field move with the smoke.
Initial Reset Frame	Which frame the rest field will be reset on. Delaying the initialization of the rest field until after any pre-roll is done can give a better result.
Reset Every Frames	The rest field will be reset every time this number of frames goes by.
Dual Rest Fields	Creates a rest2 field that is reset in a leapfrog fashion with the main rest field, allowing one to run long sims without popping.

Substepping

Note that if you are using the alternate fuel model, the sub-stepping occurs independently for each object. In this case you are probably better off using dop simulation level sub-stepping.

Minimum Substeps	While the Pyro Solver tries to estimate the correct sub-step size for a stable simulation, if unusual forces are present it may take too large of a step. By setting the minimum sub-step you can enforce stability.
Maximum Substeps	The Pyro Solver will not break the simulation down into more sub-steps than this. It is a very good idea to always have a maximum to ensure frames will be finished regardless of their complexity. Lowering this ceiling can ensure a minimum computation time at the expense of accuracy.
CFL Condition	The CFL Condition is a factor used for automatically determining what size sub-step a scene requires. The idea is that any sub-step should not allow any objects to interpenetrate by more than one voxel cell. This condition is met when this parameter is at 1. A value of 10 would allow a sub-step to move the smoke by as much as 10 voxel cells, possibly tunneling through objects rather than properly deflecting.
Advection Method	Controls how the particle tracing is performed. Single step takes the velocity at each voxel and makes a single step in that direction for the timestep. This is fastest and is independent of the speed of the velocity field, but will start to break up if large timesteps have to be performed. Trace will ensure the backtracking does not move more than a single voxel before it the velocity is updated, allowing for larger timesteps. Midpoint is like Trace but uses a higher order advection for more accuracy but at greater speed cost.
Projection Iterations	When, and only when, the velocity field is center sampled, this specifies the number of iterations used to project it. See the Smoke Object DOP for how to set the sampling mode of the velocity field. The number of iterations also controls the maximum distance any change in the velocity field can have, so should be about the width of the volume in voxels. Lower iterations will be faster but the field will not be as nice.

Forces

Vorticle Strength

An overall scale adjustment for the vorticle forces. Note vorticle forces are only present if data named vorticles is attached to the object.

Feedback Scale

A scale factor used in applying feedback forces to other objects. A value of zero prevents any feedback from occurring.

The value can be thought of as a density, so to have a default RBD object to balance a value of 1000 should be used.

Align Turbulence Calculation

The velocity field is often an offset field. Thus, to compute a turbulence, three passes are needed for the x, y, and z components. This option will instead compute the turbulence onto an aligned field and then average the result into the offset velocity field, greatly reducing the amount of noise calculations required.

Object Merge Options

These apply to both the Pyro Object and the Optional Fuel Object.

Collisions

Restrict Mask to Bandwidth	The collision mask is an SDF built storing the distance to the collision geometry for each node. By default, it is only calculated up to a certain distance from the collision geometry. However, if you want to use this field for special effects (such as having things react before they reach the object), one might want to compute the full range of the mask.
Use Point Velocity for Collisions	When building the collision field for the smoke use the collision objects per-point motion to detect deformation. This allows one to properly react to deforming objects. This only will work if the objects have a consistent point count, however.
Use Volume Velocity for Collisions	When building the collision field for the smoke use the collision objects change in volume representation to detect deformation. This allows objects that are deforming over time and changing point count to still properly affect the smoke.
Temperature Merge	Controls how the source object’s temperature physical parameter will affect the smoke’s temperature field. The options match those for Velocity Merge.
Collide with Non-SDF	Allows the fluid to collide with objects that don’t have Geometry/SDF, such as other fluids.

Sources

Velocity Merge

Controls how the source object’s velocity will affect the smoke’s velocity field.

None

The source velocity will be ignored.

Weighted Average

A blend of the gas’s velocity field and the source object’s velocity will be done with the weights for each dependent on the relative densities.

Net New Source

The object’s velocity will affect the field to an extent that the source’s density is greater than the original density.

If you add smoke with a maximum operator (the default) this has the effect of only having newly added smoke affect the velocity field.

Velocity Type

Rigid Velocity	The velocity of the object treating it as a rigid body. Only the angular velocity and linear velocity will be used, no local deformation will be taken into account.
Point Velocity	The velocity of the object attempting to take into account deformation by using point history. This only works if the topology doesn’t change.
Volume Velocity	Uses the SDF representation of the object to detect deformation. Does not require a fixed topology over time, but cannot detect tangential velocities.

Temperature Merge

Controls how the source object’s temperature physical parameter will affect the smoke’s temperature field. The options match those for Velocity Merge.

Pumps

Velocity Type

Controls how the pump object’s per point velocity is computed.

Rigid Velocity	The velocity of the object treating it as a rigid body. Only the angular velocity and linear velocity will be used, no local deformation will be taken into account.
Point Velocity	The velocity of the object attempting to take into account deformation by using point history. This only works if the topology doesn’t change.
Volume Velocity	Uses the SDF representation of the object to detect deformation. Does not require a fixed topology over time, but cannot detect tangential velocities.

Temperature Merge

Controls how the pump object’s temperature physical parameter will affect the smoke’s temperature field.

The choices are to leave it unaffected or to directly set the interior of the pump to the pump temperature.

Clear

Fields to Clear

Zeros out the specified types of fields after the solve step. This ensures the .sim files, which store the complete state of the simulation, do not have any information not needed, reducing their size and save time.

None	No special clearing of fields is done.
Hidden	Fields not needed for the next time step that do not have guide parameters are cleared.
Static	Fields not needed for next time step are cleared. Note that some guides will thus start showing up as zero as the underlying field got cleared.

Additional

A space separated list of extra fields that should be cleared post-solve.

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.

Usages in other examples

Example name	Example for