Houdini 20.0 Nodes Geometry nodes

Pyro Source Spread geometry node

Pyro Source Spread solves for the spreading of flame across a point cloud.

Since 18.0

The Pyro Source Spread operation applies a diffusion and combustion solver on a point cloud. This can simulate the spread of flames and provide dynamic inputs for a pyro solver.


Reset Simulation

Clears the entire simulation cache.

Start Frame

What frame on the Houdini playbar that the simulation should begin at.


The visualize controls provide a quick way to visualize the common attributes used for the spreading of fire.


Visualizes the diffrate attribute. This is a static attribute that controls how fast temperature will diffuse into points. Lower values will heat slower, so impede the spread of auto-ignition due to temperature.


Visualizes the fuel attribute. This is any fuel left after burning that will be carried to the next frame.


Visualizes the burn attribute. This measures how much fuel burned on that point in this frame. It is commonly what will be used as a source into a pyro solve.


Visualizes the temperature attribute. This is how hot the point has become, either from incoming initial conditions, combustion, or the diffusion of temperature.

Total Burn

Visualizes the totalburn attribute. This measures the total amount of fuel ever burned on that point. This is useful to control shading effect such as the charring of a surface over time.


Disables the visualization of attributes.

Temperature Change

Cooling Rate

By what ratio the temperature cools in a twenty-fourth of a second. 0 will not cool the temperature at all. 1 will immediately cool to zero (ambient) temperature. 0.5 will cool to 50% of the temperature in a standard frame.



How quickly to diffuse temperature across the point cloud. A value of 1 will approximately diffuse to the first ring of neighbors, and 2 to the second ring of neighbors. This will be done within a single frame. Note that larger numbers will trigger more iterations so become slower.

Search Radius

The diffusion uses this radius to connect nearby points. The radius must be large enough to connect close points.

Max Neighbors

No more than this number of points will be connected for diffusion.

Diffusion Rate Noise

Add Noise

If no incoming diffrate exists, it is assumed to be 1 and scaled by this noise. If any incoming diffrate exists, it will be multiplied by this noise to get the diffusion rate.

Help for the individual noise parameters can be found in the Attribute Noise SOP.


Initial Fuel

The incoming fuel attribute will have this amount added to it. The amount will also be scaled by the injection noise.

Injection Rate

This amount of fuel, scaled by the injection noise, will be added to each point in one twenty-fourth of a second.

Postburn Rate

After a point has burned, it will then inject fuel at this rate, scaled by the injection noise, in one twenty-fourth of a second.

This is useful for simulating the emission of fuel after an object has caught on fire without saturating the fuel field before the fire reaches it.


The fuel attribute will not be allowed to exceed this amount. This avoids run-away injection in places that are not burning.

Injection Noise

Add Noise

The various injection parameters are all scaled by this noise. More help on the noise parameters can be found on the Attribute Noise SOP.


Ignition Rules

Ignition Temperature

Any point whose temperature exceeds this value will burn its fuel.

Ignition Frame Attribute

If a point has this attribute, and its value for this attribute is less than the current frame, then it will burn. This is useful for statically keyframing when points will ignite.

Ignite Attribute

If a point has this attribute, and it is non-zero, the point will burn. This is useful for dynamically igniting points through custom rules. This can be done in the embedded SOP network or by animating this attribute in the input and adding it to the animated attribute list.

Burn Rate

Points that are burning will consume this much fuel in one twenty-fourth of a second. If there is not enough fuel, they will generate a proportionally smaller amount of burn. If there is too much, fuel will be left behind for the next frame.

Temperature Output

For every unit of fuel burned, the temperature of this point will be increased by this amount.

Burn Output

The burn attribute is the ratio of the burned fuel to how much fuel could have burned if there was infinite fuel at this point. The burn attribute is additionally scaled by this parameter.


Animated Attributes

The set of point attributes to copy every frame from the input onto the solved points.

Attribute to Match

How to correspond the animated input points to the current solved points.

Location Attribute

The rest attribute to use for noise computations. Using this avoids having the injected fuel noise swim as an object moves through space.



How many substeps to perform for each Houdini frame. Matching this to the substep rate of the Pyro Solver allows both to operate in sync.

Cache Simulation

Controls if the simulation is cached to memory.

Cache Memory (MB)

Maximum size of the memory cache.

Allow Caching To Disk

Whether to save simulation data to temporary disk files when the memory cache is filled. This is often slower than recomputing the data.

See also

Geometry nodes