Aux System Particle Spawning in After Effects

Aux system particle spawning in Adobe After Effects is a technique used primarily within particle simulation plugins—most notably Maxon's Trapcode Particular—where existing particles generate secondary particles during their lifespan. This article covers what the auxiliary (aux) system is, how particle spawning works, the primary emission triggers, and practical applications for creating advanced visual effects like smoke trails, fireworks, and debris.

What is an Aux System?

In standard particle systems, an emitter creates primary particles that follow a set trajectory, age, and eventually disappear. An auxiliary system—commonly referred to as the Aux System—turns those primary particles into moving emitters themselves. Instead of generating particles from a single static or moving point in 3D space, every primary particle has the capacity to spawn a brand-new generation of secondary particles as it travels.

How Aux Particle Spawning Works

When the Aux System is activated, the secondary particles inherit properties from their parent particles while maintaining their own independent physics, lifespans, and visual appearances.

The spawning behavior is dictated by specific triggers:

Continuously: Secondary particles spawn consistently behind the primary particles as they travel through the scene. This is the most common method for creating continuous trails, ribbons of light, and missile exhaust.
At Bounce Events: When primary particles collide with a designated floor, wall, or 3D layer, the impact triggers a burst of secondary particles. This is ideal for simulating sparks ricocheting off surfaces or dust kicking up upon impact.
At Death: When a primary particle reaches the end of its lifespan, it can trigger an explosion of secondary particles. This function is essential for effects like fireworks, cluster munitions, or bursting bubbles.

Key Controls and Parameters

Controlling aux particle spawning requires balancing parent-child relationships across several settings:

Emit Probability: Controls the percentage of primary particles that actually spawn secondary particles, helping reduce visual clutter and optimize render times.
Inherit Color: Determines whether the spawned particles take on the color of the primary particle at the moment of birth or use a new, dedicated color gradient.
Velocity and Physics: Secondary particles can either inherit the velocity of the parent particle or be ejected in random directions with independent gravity, turbulence, and air resistance settings.
Particle Type and Size: The aux system allows secondary particles to use different forms (such as sprites, glow spheres, or streaks) and size curves independent of the original particles.

Practical Applications

Auxiliary particle spawning elevates simple motion paths into complex, organic visual elements. Common applications include:

Fireworks: Primary particles shoot upward and burst into hundreds of falling secondary sparks upon death.
Atmospheric Smoke and Fire: Flaming embers emit thin trails of dissipating smoke as they float upward.
Rain Splashes: Primary raindrops fall downward and spawn tiny droplet bursts upon hitting a collision surface.
Magic and Sci-Fi Effects: Energy blasts emit trailing particles and sparkle effects that linger in 3D space.

Performance Considerations

Because aux systems generate particles exponentially, particle counts can escalate rapidly. If 1,000 primary particles each emit 50 secondary particles per second, your scene quickly processes tens of thousands of simultaneous calculations. Keeping the lifespan of secondary particles short, lowering emission rates, and utilizing opacity fade-outs are essential practices for maintaining reasonable render times in After Effects.