Deborah R. Fowler

Particles in Dynamics

Posted 2013 Updated Mar 8 2024 Updated Apr 26 2026 - restyled

History of Particles in CG (Click to expand)

William Reeves (particle pioneer 1983 Siggraph paper). Read the wiki entries here and here. Watch The Adventures of Andre and Wally B (1984).

For inspiration: Karl Sims' Particle Dreams (1988-90).

Craig Reynolds created Boids (1986) to simulate flocking. This behavior is integrated into Houdini's POP tools. See the 1987 short Stanley & Stella: Breaking the Ice.

Rendering & Troubleshooting

Rendering is key to getting the effect you want. By default, Houdini Karma/Mantra renders particles as spheres where size is controlled by pscale.

Engine	Key Requirement / Workflow
Karma / Mantra	Renders as spheres by default; size manipulated via `pscale` attribute.
Redshift	Use the specific "Redshift Particles" checkbox and place the material on the object container.
Instancing	To render actual geometry on points, append an Instance SOP after the DOP import.

Common Troubleshooting Tips (Click to expand)

Gaps in Simulation: If you see stepping or "holes" in your trail, turn up your substeps.
Age Ramps: Use the age attribute to drive ramps for color or opacity to create more natural decays.
Missing Color: If POP Color looks white, it is likely the default "headlight" lighting (Mantra). Add a Sky or Sun light to see the actual attributes.
Data Inspection: Right-click a node and select Spreadsheet to verify attributes like age, velocity, or id.
So much data?
Select Hide all and then select only the attributes you cant to view
Reflections: Beware of particle reflections; they can significantly increase render times in dense simulations.

Examples

History in Houdini

Particles have been part of Houdini for a long time. They are now fully integrated in DOPs and were once a separate solver. Why mention this? To make sense of some of the now "older" posts and example files.

Overview

PDF version of my PowerPoint lecture: Intro To Particle Dynamics — here.

Tips from SideFX Particles in Dynamics and the workflow.

If you want to not sim you must disconnect or bypass the pop to disable (or you can use the simulation button at the bottom right corner — the brain — to turn sims off).

To create a particle simulation you can either create one:

from the shelf tool - it will create an AutoDopNetwork — a dopnetwork node — with a default setup having gravity, attributes set to "Add to inherited velocity" and Variance set to 1

dopnet labeled AutoDopNetwork
geo node containing the particles (dopimport or geometry field import)

OR you can create a popnet inside of SOPs by hitting tab and typing popnet (just a dopnet node inside of SOPs).

using a dop import node, or using the convenient object merge tab on the dopnet node to bring out just your particles by typing popobject in place of the * in the Object file of the object merge tab

There are many pop nodes — hit tab and type pop to see the variety.

A few to point out:

Curve Force tool — there is a curve force tool to make particles move along a path (we'll talk about the VOPs/VEX way as well).
Flocking tools
Following a leader (small exercise to try) — here.

In addition, you can manipulate your pops with VEX.

VEXpressions — https://www.sidefx.com/docs/houdini/dopparticles/vexpressions.html
- Note that if the "Use VEXpressions" checkbox is off, the parameters use Houdini expression language, but values cannot vary per-particle. With it on, you can write VEX code snippets to evaluate on each particle at each time step.
Documentation on VEX snippets can be found here.
TIP: Use Ctrl+Enter to commit changes in the VEX snippet window. Pre-made snippets are contained in the multi-line editor drop-down menu.

Other Resources

Steven Knipping has some excellent tutorials on particles
Entagma also has great resources — one of my favorites to dig a bit deeper into POPs is the Production Setup Walkthrough: Color Dust Explosion.
Masterclass on Particles with Jeff Lait (H13)
Particles — Lesson 7 — Ari Danesh (H13)
FX guide on H13 — http://www.fxguide.com/quicktakes/side-effects-houdini-13-we-ask-them-about-their-fea/
FX guide on H14 — http://www.fxguide.com/quicktakes/houdini-14-gives-you-more-crowds-and-sand/