A space-colonization point network can give a Houdini smoke burst a branching flow pattern: grow connected paths, derive velocity from those paths, rasterize the result into a Pyro source, then vary the network and emission points over time. One important distinction: switching among branch variations adds motion variety, but does not by itself guarantee a seamless loop. For a true loop, the source and simulated result must also be designed to repeat cleanly.
How does the branching network drive a smoke burst?
The network acts as a geometric guide for the smoke, not as the smoke simulation itself. In Karlis Stigis’s 80 Level example, a cylinder is populated with points, a repeated search-and-connect process builds branches through that cloud, and the resulting paths provide the directional velocity field for Pyro. The concept was inspired by Entagma’s VEX space-colonization tutorial and the 2007 paper Modeling Trees with a Space Colonization Algorithm; the implementation adapts the idea to connect existing points rather than create new positions from averaged attraction directions.
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This separation is useful because it lets you art-direct branching before simulating volume motion. The point network determines where the preferred flow goes; Pyro adds smoke behavior, turbulence, and evolving detail.
How do you build the space-colonization paths?
Seed and grow the point network
- Create the point cloud. Place Points from Volume on a cylinder so the candidate points occupy the burst’s intended region.
- Choose a starting group. Mark one or more initial points as active. These seed points begin the repeated search.
- Find candidate neighbors. Each active point searches for unused points within a chosen radius and proposes connections. A candidate point selects its closest proposed parent, which prevents multiple active points from independently claiming the same child.
- Advance the active generation. Connected points become active for the next pass, while the previous generation stops searching. Repeat the search, connect, and advance cycle until the network has the desired reach and density.
- Carry branch identity forward. Track candidate-parent lists and point groups, and propagate root IDs to children so branches can be identified and selected downstream.
In this setup, search distance changes link length, while the candidate limit controls how many neighbors each active point may propose. These are the main structural controls: longer links can make the network more open, while allowing more proposals can create denser branching. The result depends on the point distribution and chosen settings, so inspect the paths before using them as a source.
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Turn connections into smooth, usable paths
The implementation uses an initialization wrangle and three repeated wrangles to manage the growth process. Edge Transport provides normalized distance along the branches. Join the segments into paths, then resample and smooth them so the velocity guide is continuous rather than a chain of abrupt point-to-point turns. Compare neighboring distance values along each path to derive outward direction vectors, and store those vectors as velocity attributes.
Wrap the growth setup as a Houdini Digital Asset (HDA) if you need to reuse it, expose its controls, or generate multiple versions. Stigis nests the solver in an outer loop to create ten packed branch-network variations. Those are separate geometric variations for this shot, not a universal number to use.
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How do you convert the paths into Pyro sources?
- Select emitter points. Choose points around the cluster roots for the main burst, then add some randomly selected points if you want smaller secondary bursts.
- Assign source attributes. Use Pyro Source to add density and temperature, and preserve the path-derived velocity attribute.
- Rasterize the fields. Use Volume Rasterize Attributes to convert density, temperature, and velocity into volumes, then merge those fields into the Pyro Solver’s source input.
- Set source scale deliberately. Stigis reports using a Density Scale of 10 in this particular setup. Treat it as a shot-specific setting, not a generally recommended value; the needed scale depends on the source geometry and the look you want.
A thickened cylinder converted to a signed-distance collision field can contain the burst while leaving a hollow interior. That shape is useful when the smoke should remain inside a cylindrical boundary without filling its center. Adjust collider dimensions and source placement to match the shot rather than assuming this container suits every burst.
Should sourced velocity use Add or Copy?
For this branching effect, the artist found that Copy kept the branch pattern more visually distinct. With Add, the incoming velocity accumulates with the existing flow, and the paths become less apparent. With Copy, the sourced region’s velocity is replaced on each update, repeatedly reinforcing the guide direction. Turbulence can still influence areas outside those regions.
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| Operation | Effect in the reported setup | Best fit |
|---|---|---|
| Add | Combines incoming velocity with existing flow; the branching paths become less distinct. | Use when accumulation is wanted and preserving a sharply defined guide pattern is not the priority. |
| Copy | Replaces velocity in the sourced region on each update, reinforcing the path directions. | Use when the branch scaffold should remain legible in the smoke. |
Stigis also disables flame and buoyancy, then tunes turbulence so the branches remain visible while the volume moves. These are choices for this shot, not universal Pyro rules: retain buoyancy or other forces when they support the intended motion.
How do you make the burst vary and loop?
Use network changes for variation
The example switches among its ten generated branch networks during animation to change the velocity directions, and varies the emitter points as well. That helps avoid a single unchanged source pattern. The number ten is simply the artist’s choice for this setup; it is not a required count or a benchmark against other approaches.
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Changing a source network over time is not the same as making a seamless loop. A loop needs a repeatable source cycle and a simulation whose ending state can transition back to its beginning without a visible jump. SideFX’s Houdini 22.0 Pyro workflow guidance says to use looping sources where possible and discusses source and collider frame ranges for loop-oriented solving. Plan those ranges alongside the animated networks, emitters, and container instead of treating source variation alone as proof of a seamless result.
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Check the loop at the boundary
- Review the source geometry and emitted attributes at the first and last frames of the intended cycle. A sudden change in branch shape, emitter position, density, temperature, or velocity can create a visible pop.
- Check the simulated volume across the wrap point, not just the source animation. A source that repeats can still produce a discontinuity if the smoke’s motion or density does not.
- If you need a strict repeating result, use a loop-oriented Pyro setup and evaluate the resulting cycle at its boundary. The documented example does not specify exact frame ranges or a universal recipe for achieving a seamless simulation.
How should you balance resolution, padding, and simulation cost?
SideFX’s Houdini 22.0 Pyro guidance emphasizes that source geometry strongly affects smoke behavior and that voxel size should be chosen relative to scene scale. Smaller voxels increase resolution and can reveal finer detail, but also increase simulation cost. Padding must leave room for the motion; excessive padding can slow the solve without improving the visible result.
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Start with a resolution that makes the overall branching motion readable, then refine it only if the shot needs smaller features. Check the simulated volume’s bounds through the full animation, especially where branches spread or turbulence carries smoke beyond the source. A container that clips the smoke or padding that wastes computation both undermine the intended effect.
Houdini’s Pyro system can be assembled from DOP building blocks—a smoke object, source volumes, and a Pyro Solver—or through the SOP Pyro Solver, which wraps the simulation building blocks in a SOP workflow. Simulation can be cached for playback, and Pyro Post-Process can help optimize cache file size. SideFX’s product guidance distinguishes COP Pyro, a GPU-oriented workflow with DOP-level permission requirements and edition restrictions, from SOP-level tools; confirm current node and edition availability for the Houdini release you use.
What should you cache for rendering?
Keep density for the volume’s appearance and velocity if you want to drive volume motion blur. In Stigis’s workflow, the output is converted to VDBs, velocity components are recombined into a vector field, velocity is resampled to half resolution, and density is stored at 16-bit precision to reduce cache size. These are disclosed choices for that shot, not mandatory cache settings; inspect the resulting detail and motion blur before adopting them.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsIn Solaris, the artist imports the volume and camera, assigns a Karma Volume shader, reads the density field, and uses density-dependent absorption to shape color and contrast. The cached velocity drives volume motion blur. The reported final-render setup used 128 path-traced samples with OIDN denoising; that is an example configuration, not a quality benchmark or a recommended minimum.
Quick Recap
What is the practical workflow?
- Build and inspect the cylinder point cloud and its branching paths before simulating.
- Derive outward velocity from the smoothed paths, then rasterize it alongside density and temperature.
- Test the source in Pyro and compare Add with Copy; choose based on how strongly the smoke should follow the branches.
- Add the collision container and tune voxel size and padding against scene scale and the full motion range.
- For variation, switch among generated networks and vary emission points; for a seamless loop, also design and validate a repeating source and simulation cycle.
- Cache density for shading and velocity for motion blur, then render the volume in Solaris/Karma if that is your pipeline.
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