Deborah R. Fowler

Fluids - Pyro

Posted 2013 Updated: Aug 27 2024 Updated July 24 2026 - restyled
Keywords: pyro, sparse, karma

H20.5 made Houdini Pyro even easier to use including a Karma Pyro Shader in the material library in lops.

At this point, sparse is recommended, with legacy being deprecated.

Axiom is a third-party gpu solver compatible with Houdini that is very popular (not covered here).

Other more "recent" changes:

H18.5 introduced the Minimal Solver (released Oct 20, 2020) timestamp 1:39:37

gpu explosion on the new shelf tool. New workflow with pyro burst tool, pyro trail on sop level.

Materials related to the new tools:

Excellent recent tutorials on pyro to get started:

Check out SideFX new pyro workflows in Houdini 18.5 here

See also

Sparse pyro is now recommended with the classic pyro (legacy) hidden from the shelf tool

  • What is a fluid? It is anything that takes the shape of its container (liquids and gasses).
  • voxels (think of 3D pixels) is where we store data. Pyro is a voxel fluid (grid based fluids with information in "containers". I the pyro object you can control the number of voxels or resolution of your sim as show below highlighted in blue.
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  • Fluid simulation - wiki "increasingly popular tool in computer graphics for generating realistic animations of water, smoke, explosions and related phenomena".
  • An excellent introduction by Mike Seymour in fxguide on The Science of Fluid Sims is a must read.
In order to understand pyro, it is helpful to have an understanding of what happens during combustion.

Fuel, a source of heat and oxygen are required. When combustion occurs "exhausts" are created and heat is released (as well as light). To control or stop the combustion you manipulate the fuel available, oxygen available or source of heat. Fuel can be wood, fossil fuels (coal, oil, natural gas), hydrogen and so on.

Fuel + oxygen + heat ---> energy (heat,light,sound) + waste

combustion that is very fast explodes (rockets, car engines). Fire releases energy slowly, explosion is nearly instantaneous.

Okay, so how does this interpret into CG speak. General terms here, for specifics see the sections on specific types above.
Fuel --> flames (heat) smoke (density) temperature (temperature) expansion (divergence)

Going one step further, interpreting it into houdini:
  • geometry is prep'd by converting to points, and attributes are added (source fuel)
  • convert the information into a volume (rasterize points) suitable for the solver
  • solver simulates over time, like all dynamic systems - we can adjust to emulate real references

REFERENCE, REFERENCE, REFERENCE - ensure you find a reference for the type of fire/explosion you are trying to emulate

Study - what is the source? Is there wind? gravity? what is burning (materials)? What is the behavior - much like particles it is behavior and rendering

  • If you google Kelvin Color Temperature Scale - that will give you a better idea of the kelvin system - note that what was called heat in Houdini is now called flame
  • temperature will map to the color, flame will map to the fire's intensity
  • color in reality determined by the chemical components (ie. what you are burning)
  • fire releases energy slowly, explosions release energy extremely quickly (milliseconds)

DO NOT use CG work as a reference! That is someone else's interpretation. This is true for all your references, not just pyro.

Houdini Specific
Documentation on Pyro

What is new:

  • H18 - SPARSE VOLUMES - there now exists a shelf tool for Sparse Pyro Effects
  • Sops pyro is the sparse version
  • SPARSE does not work with OpenCL - but sparse is more efficient
  • H17: Sourcing for pyro changed. https://www.sidefx.com/forum/topic/58467/
  • Now Pyro is sourcing volume from points
  • so if you are dealing with geometry brought in from an alembic be sure to unpack it.
Other Resources
Some cool tutorials
More in-depth reading

Rob Bridson, co-founder of Exotic Matter (creators of Naiad) from UBC, has some excellent references on the underlying equations controlling fluids for computer graphics. Many of the references are to Siggraph 2006 and 2007 courses as well as his co-authored book on Fluid Simulation for Computer Graphics. I would suggest to start by looking at the course notes.