The Pyro solver handles effects related to hot expanding gases such as fire, smoke, and explosions. The Pyro FX shelf tab contains tools for creating different effects.

Effects are generated from source geometry. For example, the skin of a character on fire. This can be as simple as a sphere if you just want to generate smoke and the source shape doesn’t matter.

See Pyro look development for information on how to use the various parameters of the source, object, and solver to achieve different smoke and fire looks.

In Houdini 17, pyro sourcing changed from being based on volumes representing different properties (temperature, fuel) to being based on points with attributes representing the physical properties.

• This conforms better to how most users source pyro sims, for example running a particle sim and then using the result as the source for smoke.

• This makes it easier to manipulate the properties using the large number of SOPs dedicated to editing points and point attributes.

The points are still converted into a volume before export to the simulation, however the new workflow is to edit the points and point attributes rather than the volume itself.

The attributes have the same names as the volume fields: density, temperature, fuel, divergence, burn, heat, velocity, collision, source, and sink.

When you use the Pyro shelf tools, they add geometry (SOP) nodes inside source object, set up a DOP network for the simulation, and create an Output object to import the result of the simulation back into a renderable Object.

• The source adds temperature and fuel to the simulation.

• Where fuel exists and the temperature is above a certain point, burn is created.

• burn creates heat (visible flames).

• burn inefficiency leaves a certain percentage of fuel behind after burning.

• divergence represents the expansion caused by heating gas.

• Dissipation and cooling reduce the smoke density and temperature over time, respectively.

• velocity = Starting velocity plus temperature times Buoyancy

• Advection affects temperature, heat, density, and fuel (if Advect fuel is on in the solver).

• In Houdini, temperature represent buoyancy, velocity represents momentum.

Notes:

• The “Multifield” visualizer shows both heat (flame) and density (smoke).

• Temperature is one of the most important fields for running a simulation (together with Velocity) and controls to great extend how the sim behaves and looks. Although it isn’t often used for rendering and visualization, internally it drives many forces and shape operators (such as Buoyancy, Shredding and Dissipation).

• When creating a fire sim, that control is extended to the combustion model. Temperature Diffusion and Cooling Rate act directly on the temperature field and offer great control over the simulation’s decay in speed and simulated detail.

The Smoke/Pyro solvers allow you to add collision fields (areas the smoke/fire will avoid) and expansion fields (areas that add outward velocity to the simulation).

Tools in the Populate Containers shelf tab let you create collision and expansion fields. These create the same sourcing setup as the one Pyro tools use for fuel sources, but instead of creating a fuel attribute, they create a collision or divergence attribute.

An expansion field adds outward velocity to the simulation within the field. That velocity is then advected, creating continuous outward force. This can quickly blow up your simulation.

If you're making an explosion, that can be desirable, but if want something more subtle, you can try the following:

• Add a Drag Force node after the solver to keep the velocity from continuing forever.

• Scale down the expansion source (Volume Source DOP > Scale parameter in the divergence source), or animate it on and off.

You can also use DOP forces to push smoke/fire around. However, the effect of the force will be dampened by the solver’s project non-divergence step. Because you set up the expansion field as a direct input to the solver, the solver knows to exclude the expansion field from the project non-divergence calculations.

You can use the fields of a pyro simulation to advect the points of DOP geometry.

• Attach editable Geometry data to the smoke container.

  To create editable geometry, import geometry using a SOP Geometry DOP and copy it using a Copy Geometry DOP. Then use an Apply Data DOP to attach the geometry to the smoke object DOP. Set the data name of the copied geometry to Geometry.

• Connect a Gas Advect to the Advection input of the pyro solver (the third green input). Set the Gas Advect’s Geometry parameter to the data name of the editable geometry (Geometry) Set the Velocity field to vel and the Collision field to collision.

Temperature diffusion is a blur on the temperature field. Increasing it will blur the boundaries between high and low temperature areas, and increase the diffusion of temperature from a source out into the system.

Viscosity is a blur on the velocity field. Increasing it will blur velocity variations, such as shredding and vorticles (small pinwheel-type eddies). If you have a look you like but it has a little too much velocity noise, you can try adding a bit of viscosity to blur it out.

Because these controls blur the fields, high diffusion/viscosity values will decrease the temperature/velocity in the system, because you are averaging together the areas of high temperature/velocity (for example, near the source) with the surrounding (usually much larger) areas of inactive space.

• You may want to decrease the division size (that is, increase the resolution) of your containers before rendering.

• Render Pyro effects using the micropolygon engine (Mantra render node > Properties tab > Render tab > Rendering Engine).

• Set up your lights' shadow parameters (Light object > Shadow tab), set Shadow type to Depth Map Shadows, Pixel samples to 2×2, and Shadow Quality to 2.

• To get more detail in the render, you can try decreasing the renderer’s Volume step size (Mantra render node > Properties tab > Sampling tab). This will not make a difference if the reason the render lacks detail is the pyro volume’s container is low resolution.