Houdini 20.0 Nodes Dynamics nodes

Gas Burn Geometry Object dynamics node

Creates an object with appropriate data to use as a fire source.

This node type is deprecated. It is scheduled to be deleted in an upcoming revision of Houdini.

Use the Smoke Object node instead.

(Since version 17.5.)

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The Gas Burn Geometry Object creates a DOP Object which has the appropriate data to run a fire source simulation.

A fire source simulation will consume a fuel point attribute and turn it into a source suitable volume fluid solvers. The Gas Burn Geometry DOP can be used as the solver to process this data.

Parameters

Creation Frame Specifies Simulation Frame

Determines if the creation frame refers to global Houdini frames ($F) or to simulation specific frames ($SF). The latter is affected by the offset time and scale time at the DOP network level.

Creation Frame

The frame number on which the object will be created. The object is created only when the current frame number is equal to this parameter value. This means the DOP Network must evaluate a timestep at the specified frame, or the object will not be created.

For example, if this value is set to 3.5, the Timestep parameter of the DOP Network must be changed to 1/(2*$FPS) to ensure the DOP Network has a timestep at frame 3.5.

Number of Objects

Instead of making a single object, you can create a number of identical objects. You can set each object’s parameters individually by using the $OBJID expression.

Object Name

The name for the created object. This is the name that shows up in the details view, and is used to reference this object externally.

Note

It is possible to have many objects with the same name, but this complicates writing references, so it is recommended to use something like $OBJID in the name.

Solve On Creation Frame

When turned on, newly created objects are solved by the solver on the timestep in which it was created.

This parameter is usually turned on if this node is creating objects in the middle of a simulation rather than creating objects for the initial state of the simulation.

Allow Caching

By preventing a large object from being cached, you can ensure there is enough room in the cache for the previous frames of its collision geometry.

This option should only be set when you are working with a very large sim. It is much better just to use a larger memory cache if possible.

Source

Deforming Object

The DOP Object to use as the both the initial fuel source and the current location of the object. The fuel values are only read on the creation frame as the consumed fuel is stored on the local copy of the object. The deforming object’s location and velocity is copied in every frame, however, allowing the fuel source to be paired with a rbd or cloth simulation. Points should be consistent frame to frame.

Flamefront Reference

The DOP Object, ideally a fire simulation, to use as the reference grid to generate the source mask in. This ensures that the source information is evaluated at the correct resolution and location.

Temperature Source

Optional fluid object to query for temperature values. The temperature scalar field is copied from this object to use as the reference temperature for determining when things should ignite.

The temperature dest should be set to something other than temperature to avoid having the source relationship writing back the previous frames values.

Guides

The source calculated from burning the object is displayed by default as a magenta isosurface. A number of other visualization options are available. The help for the Scalar Field Visualization provides more details about how these work.

Bounce

Specifies the elasticity of the object. If two objects of bounce 1.0 collide, they will rebound without losing energy. If two objects of bounce 0.0 collide, they will come to a standstill.

Bounce Forward

The tangential elasticity of the object. If two objects of bounce forward 1.0 collide, their tangential motion will be affected only by friction. If two objects of bounce forward 0.0 collide, their tangential motion will be matched.

Friction

Specifies the coefficient of friction for the object. A value of 0 means the object is frictionless.

This governs how much the tangential velocity is affected by collisions and resting contacts.

Dynamic Friction Scale

An object sliding may have a lower friction coefficient than an object at rest. This is the scale factor that relates the two. It is not a friction coefficient, but a scale between zero and one.

A value of one means that dynamic friction is equal to static friction. A scale of zero means that as soon as static friction is overcome the object acts without friction.

Temperature

Temperature marks how warm or cool an object is. This is used in gas simulations for ignition points of fuel or for buoyancy computations.

Since this does not relate directly to any real world temperature scale, ambient temperature is usually considered 0.

Outputs

First

The Gas Burn Geometry Object created by this node is sent through the single output.

Locals

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.

See also

Dynamics nodes