Houdini 20.5 Nodes Dynamics nodes

Gas Combustion dynamics node

A microsolver that applies a combustion model to the simulation.

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The Gas Combustion 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 Combustion DOP applies a combustion model to a set of fields. This is designed to simulate the burning of fuel with the corresponding release of heat, gas, and soot.

Parameters

Enable Soot

Whether to add burnt fuel to the specified soot field. If set, the Soot Rate is used to scale how much soot is output from each unit of burnt fuel.

Normalize Burn Field

The burn field stores the amount of fuel burned in the last timestep. This will vary with the size of the timestep. Normalizing the burn field will divide by the timestep, storing the amount of fuel burned per second rather than instantly.

Time Scale

While all the combustion effects are scaled by the timestep, you can further scale them by this time scale to allow for slow motion behaviors.

Settings

Ignition Temperature

The combustion model will only occur if the temperature field is above this value. If you want all fuel to instantly ignite, use a negative value.

Burn Rate

The amount of fuel to burn per second. This is a ratio: 0.9 means after one second 90% will be burned.

Fuel Inefficiency

Controls how much of the burned fuel is actually not burned, but kept. 0 means all burned fuel is removed from the fuel field. 1 means that no fuel is removed from the fuel field when it is burnt.

Temperature Output

The amount to increase the temperature field by for every unit of fuel consumed.

This is affected by both the heat and burn fields, as per the influence parameters.

Gas Released

A scale factor controlling how much gas is injected into locations where fuel is burnt. This causes burning areas to blow outwards.

The gas release is scaled by both the heat and burn fields according to the influence parameters.

Gas Heat Influence

The degree to which the gas release is controlled by the heat field.

Gas Burn Influence

The degree to which the gas release is controlled by the burn field.

Temp Heat Influence

The degree to which the temperature output is controlled by the heat field.

Temp Burn Influence

The degree to which the temperature output is controlled by the burn field.

Soot Rate

The amount of soot that will be created for every unit of fuel burnt. This is added to the density field that is usually rendered as smoke.

Mappings

Temperature Field

The scalar field to use for temperature calculations. This is used to trigger combustion by comparing to the ignition temperature. It is also updated from the Temperature Output.

Fuel Field

The scalar field representing the fuel in the gas. This is transformed into heat, soot, and gas release.

Soot Field

The scalar field tracking the amount of soot that has been generated.

Divergence Field

A scalar field representing how much gas is released in each voxel by the reaction. This is the burn field multiplied by the gas released value.

Burn Field

A scalar field representing how much fuel was burned in this timestep in each voxel. It could be used to track things such as light output, etc.

Heat Field

A scalar field of the heat of the simulation. This is set to the heat influence whereever the fuel is burning. It is maximized against the previous heat field, and can be used to track the cooling of the air over time without affecting the simulation behaviour.

Stencil Field

A scalar field to use as a stencil for where to perform this node’s computations. Voxels whose stencil value strictly exceeds 0.5 will have the operation applied, while the rest will be left unchanged.

Note

If a stencil field isn’t provided or does not exist, the operation will be performed everywhere.

Advanced

Use OpenCL

Enable GPU acceleration on certain microsolvers. This may not work on all graphics cards or operating systems. Check the System Requirements information in the Support section of the Side Effects Software website.

You should have a fairly recent graphics card and fully updated drivers. Start with a low resolution test simulation (for example, a 643 grid) to verify that it runs with OpenCL, then try increasing the resolution. The additional memory-transfer overhead of using OpenCL will only become worth it at high resolutions, around 2563.

With the plain smoke solver, simulation after the first frame sourcing will use the GPU. If you add a microsolver that isn’t GPU-enabled, Houdini does the GPU required CPU copying instead of raising an error.

For fastest speeds, the system needs to minimize copying to and from the video card. The example file demonstrates several methods for minimizing copying. See the OpenCL smoke example file for an explanation of how to set up a fast simulation using GPU acceleration.

A very high-resolution plain smoke solver simulation should be faster with OpenCL. However, default Pyro effects will not automatically simulate faster.

  • They tend to be very low resolution for fast initial playback, so they don’t have enough voxels for the GPU acceleration to greatly exceed the overhead.

  • They have a lot of non-GPU shaping nodes. While many nodes are GPU-enabled (such as vortex confinement), quite a few Pyro nodes are based on VOPs and are not GPU-enabled.

  • Caching is enabled by default in DOPs.

  • Resizing is enabled by default. Resizing has to go through the CPU to manage the field changes. It can also fragment the GPU memory resulting in out-of-memory errors.

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.

Dynamics nodes