The Fountain Burst Mode: An Arc That Is Culled, Never Clamped

The catalogue line for Fountain Burst is one sentence: Bounded beat-emitted arcs that settle under gravity. The word doing the work in it is bounded, and it does not mean what it usually means in this catalogue. Everywhere else, bounded means a soft limit that compresses a value so it cannot leave the frame. Here it means the opposite: a particle that falls past the bottom edge is simply not drawn.

The age clock, and why it is not a timer

Every particle in the Particles engine is seeded once with its own position, depth, phase and size. This mode reads the seeded depth as a phase offset rather than as depth: it adds the motion clock to it and takes the fractional part, so each particle runs a 0 to 1 arc and then starts again from the beginning. The whole population is therefore permanently spread across the arc, with no spawn queue and no birth event to schedule.

The rate that clock advances is divided by the Lifetime control, so a longer lifetime does not keep a particle alive longer in any literal sense. It slows the arc down. That is worth knowing before you reach for it expecting more particles on screen: you will get the same ones, moving more slowly.

One launch angle, one parabola

The launch angle is drawn from the particle's own horizontal seed and spans a fan of about 137 degrees, always upward. Position along the arc is then the age times a fixed horizontal reach and a fixed vertical reach, with the beat pulse widening both slightly, plus one more term: the age SQUARED, scaled by the frame height and by gravity. That squared term is the fall. It is small early in the arc and dominant late in it, which is exactly what a parabola is.

Gravity is scoped to this mode and three others in the engine, and this one ships it at 0.38. Only its positive part counts, so a negative value reads the same as zero rather than launching the arc back upward.

The cull line

Once the arc carries a particle below 98.5% of the frame height, the draw loop skips it entirely for the rest of that trip. It is not moved, not clamped, not faded. This is the detail the mode lives or dies on, because the alternative is what a clamp would give you: a line of particles pressed flat along the bottom edge, growing denser every second, which reads as a rendering fault rather than as a fountain.

What the beat actually changes

Two things. The beat pulse widens the arc's reach in both axes by a bounded fraction, so the fountain throws further on a hit without changing where it launches from. And the onset burst feeds the engine's population swing: the Spawn rate control divides into that swing, so a higher value makes an onset pull more of the pool into the drawn count. The swing is capped at 1.3x the target and floored at 0.55x on purpose, because a field whose population jumps by more than half on every onset reads as flicker rather than as emission.

Sizing it

The number you set is a target, not a promise. It is scaled by the preview scale, by the canvas area relative to 1080p, and by a further reduction on machines reporting four or fewer cores, then capped. Export with capture quality on and the last two of those are switched off, so the exported frame is the count you asked for. The engine publishes 520 elements by default and 220 on mobile, with a hard ceiling of 4000.

When to reach for it

Drops, builds, percussion-led electronic music, and anything where you want the picture to throw something on the hit and then let it fall. It composes from the bottom edge, so it sits under lower-third text and titles without competing with them. If you want emission without gravity, the same engine's drifting modes answer that brief instead; The Flow Field Mode: Steering Is the Whole Difference covers the closest thing to it in Trails.

Open it at Fountain Burst in the template gallery to watch the arcs run, or go straight to the editor with a track of your own.