This shelf tool sets up a sparse pyro simulation of a medium-size explosion with a ground shockwave. Rather than building a DOP network, it uses the SOP-level tools to build a simple network to work with sparse pyro.

The explosion consists of an initial blast, followed by a rising fireball, similar to the Simple Fireball shelf tool. As long as there are sufficient reactants (corresponding to the emissive flame field), the fireball will continuously emit soot and heat and continue to expand. These various outputs, as well as the time it takes the fireball to burn up, are controlled through the parameters on the Flames tab of the Pyro Solver.

The shockwave starts slightly after the initial explosion, spreads out radially from the center of the explosion, and has no reactant sources (corresponding to the emissive flame field). It’s considered to be smoke/dust, so only density and temperature are sourced.

1. Click the Simple Shockwave tool on the Simple FX tab.

2. Select the location of the shockwave in the viewport.

• If you have a Houdini FX License, you can dive inside the solver and edit the subnet with custom DOP forces. Otherwise, you can stay at the SOP level and use the exposed parameters on the Pyro Solver node.

• pyroburstsource_fireball sets up the initial shape as well as the density, temperature, and burn sources for the fireball. Burn gets merged with the pyro flame field (to refresh the available reactants) and the divergence field (causing the initial outward explosion).

• pyroburstsource_ground_shockwave sets up the initial shape, the density, and temperature sources for the shockwave. It does not source flame, since the shockwave is made of dust and smoke.

  To...	Do this
  Change the expanding size of the shockwave	Modify Interior Expansion and Exterior Expansion parameters on the Burst Animation tab of the Pyro Burst Source node.
  Change the shape of the shockwave	Turn on the Add Shape Noise checkbox on the Burst Shape tab of the Pyro Burst Source node. This will change the expanding shockwave from mathematically circular to a more imperfect and natural shape.

• adjust_shockwave_velocities is used to further modify the velocity source of the shockwave. Velocity sourcing is an important part of defining the desired look of the shockwave.

• merge_all_sources combines the fireball and the shockwave sources together before simulation.

• The parameters on the Pyro Solver SOP have the greatest influence on the motion and emergent shape of the fireball and its smoke.

• If you dive inside the Pyro Bake Volume node, you can use the following controls on the Pyro Shader to do the following.

  To...	Do this
  Change the color of the smoke based on density	Change the menu next to Smoke Color from Constant to Use Ramp. Set the Density Range parameter to control the color of the smoke at given density values. Set the Smoke Color Ramp to change the color of the smoke based on Density Range.
  Change the color of the smoke based on how light scatters through the volume	Change the Absorption Color. The value of 0.425 (red),0.36(green), 0.3(blue) could be a good starting point for more realistic looking smoke.
  Scale smoke density using an other volume	Turn on the Use Control Volume checkbox. Set Density Control Volume on the Bindings tabs to the field that you want to use to affect the density. Usually temperature or flame are good choices for this purpose.