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Pyro refers to Houdini’s volumetric fluid simulation package. The pyro solver is used for capturing the motion of fire, smoke, and explosions, but can also be used to simulate other fluid-like phenomena, such as mist or an avalanche.
Note
In contrast to FLIP, pyro does not use particles to track the fluid. Since particles are very useful for resolving the surface, pyro is not well-suited for simulating liquids.
Getting started with pyro using shelf tools ¶
The tools on the Simple FX shelf generate pyro simulation networks using the 
Pyro Solver SOP, which is a SOP level wrapper around the pyro dynamics building blocks. These tools provide a fast and friendly user experience, and avoids the need to build a dynamics network manually.
You can alternatively use the tools on the Pyro FX shelf to put down example DOP pyro setups. They create pyro networks that include sourcing on the at the geometry level, a DOP network for the simulation, and an output object to import results of the simulation back into a renderable object.
Tip
The tools on the Simple FX shelf are typically a better alternative as they are simpler to get started with, more user friendly, and don’t require a separate dynamics network.
All of these are extremely useful for creating instant simulations that can be further modified to integrate with your scene.
Related pyro nodes ¶
Sourcing ¶
Sourcing is responsible for the injection of new quantities into the simulation. For example, you may want to continuously add soot and heat inside a chimney to animate smoke rising out of it. To do this, you would first generate these source volumes using SOPs, then merge them into the simulation.
The recommended method to create the source volumes is by rasterizing point attributes. This approach allows for easy inspection and manipulation of sourced quantities. It also naturally accommodates more complex sourcing (such as for shrapnel trails), since you can use the results of a particle simulation to emit smoke and fire.
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Converts the source geometry into points with attributes (such as
This node has controls for how to scatter the points on or inside the source geometry, what attributes to create, and what to do with existing attributes. |
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Simulates combustion of fuel and diffusion of heat on the points. This node is especially useful for controlling and directing the spread of flames across a given region. |
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Creates points for sourcing the core of the explosion, and lets you shape how the explosion will look. |
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Creates points for a visible blast from the the muzzle of a firearm, and lets you shape how the muzzle flash will look. |
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Creates points for sourcing a shockwave from the core of the explosion, and lets you shape how the shockwave will expand. |
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Creates points at the core of the explosion that can be used to generate trails, sparks, various particle simulations, and rigid body debris flying out of the explosion. |
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Generates curves to use as trails for the |
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Uses the output of |
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Creates a packed source set from the incoming volumes. You can merge packed source sets into a source library, whose individual elements can be instanced into pyro simulations. |
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Creates spawn points for sourcing pyro simulations with packed sets. You can use spawn points to instance elements of a source library, while precisely controlling their placement and timing. |
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Generates source instance points from spawn points. The
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Uses the output of |
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Adds some randomness to the attribute values on the points to create more interesting and realistic smoke and fire effects. |
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Generates a volume from the points and point attributes, for import into
DOPs or into the |
Pyro Trail Source
RBD Bullet Solver
DOP simulation ¶
The essential building blocks of a pyro dynamics network are the object,
sourcing, and the solver. The simulation object with all the necessary fields is
created by the 
Smoke Object (Sparse) node. The
Volume Source node is responsible for importing sources
and merging them with their respective simulation fields. Finally, the
Pyro Solver (Sparse) takes care of applying forces
and evolving the fluid fields.
See pyro workflow for information on how to use the various parameters of the source, object, and solver to achieve different smoke and fire looks.
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Controls the 3D resolution of the smoke/fire object, as well as the simulation domain. It also has extensive controls for visualizing contents of the object. |
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This node imports the SOP volumes generated by the source network and merges them with the object’s fields. There are several merging operations for combining each source volume with its target simulation field. |
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Similarly to |
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Houdini includes two solvers for grid-based smoke/fire simulations. The
The high-level effects created by the tools on the Pyro FX shelf use this solver. The pyro solver has extensive controls for evolution and output of the
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Solver for simpler effects involving smoke, but not flames. |
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Pulls the simulation results (consisting of the relevant object fields)
back into SOPs for export and rendering. It’s important to note that
imported volumes from this node will not have the visualization
information built in. However, you can use the
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Smoke Solver
SOP simulation ¶
Pyro simulations can also be carried out entirely in a single SOP network, avoiding the need to switch between contexts and directories. You can do this using the Pyro Solver SOP, which is a convenience wrapper around a pyro dynamics network. This node exposes parameters from the simulation object and the DOP solver, and even has a dive target to allow you to apply manual forces. The SOP version of the solver should be adequate for most of the simulation scenarios. Additionally, there is a built-in 
post-process node and 
pyro bake volume node that ensures that the exported geometry is ready for rendering.
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A SOP level wrapper around the pyro dynamics building blocks that provides a fast and friendly user experience, and avoids the need to build a dynamics network manually. |
Simple FX shelf tools ¶
There are several tools on the Simple FX shelf that create simple networks demonstrating this approach. You also can find these tools using the tab menu.
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Creates a visible blast from the the muzzle of a firearm during shooting. |
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Creates thick cloud-like smoke rising from a spherical base. |
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Creates an explosion that can be fully simulated on the GPU. |
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Creates a torch fire that can be fully simulated on the GPU. |
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Creates a large rising and expanding explosion. |
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Creates an aerial explosion. |
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Creates a large ground explosion. |
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Creates a large rising and expanding explosion with a ground shockwave. |
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Creates a bonfire simulation using the sparse pyro solver. |
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Creates a fire simulation spreading across the surface of the selected object. |
Post-Processing ¶
It is important that you process your simulated volumes to save disk space. The simulation can be saved to disk for rapid playback using the 
File Cache, however to save disk space, the Pyro Post-Process node can be used to optimize the file sizes. Alternatively, the Optimize Exports menu option in the Quick Setups dropdown of the Pyro Solver SOP can reduce the disk size by 90%.
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Prepares and processes the volume geometry for saving it to disk, and can greatly reduce the file size of the simulated caches. |
Rendering ¶
The last step is to achieve the best look possible both in the viewport and in the render.
Tip
To achieve the best look possible, you have to change the default color display in Houdini. By default, the viewport uses gamma 2.2, which is not adequate to display and render any pyro simulation. To use a more accurate color display, see how to setup the Academy Color Encoding System (ACES) on the Shading and Rendering page.
To make sure you have to desired color display, right-click the 
Open display options button and turn on the Color Correction checkbox, which will bring up the Color Correction bar at the bottom of the viewport.
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A basic building block node to change the visualization of the volumes. When working with the output of a pyro simulation, the |
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Prepares the volume geometry for rendering by manipulating the visualization of the imported pyro fields or incoming volumes. The resulting viewport visualization is a pretty close approximation of the render results, as this node also creates a matching material for the volumes. When scattering is turned on, it bakes out a |
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The shader used by the render engine to create production-quality smoke, fire, and explosions. |
Pyro Shader