Particle background

A particle background is a field of many small agents animating behind the content of a page. The varieties are named after what the field looks like rather than after how it is built: drifting dots that wander and wrap at the edges, the plexus or constellation look where a line is drawn between any two dots close enough to each other, a starfield streaming outward from a vanishing point, a flock that moves as a group. Under all of them sits the same loop: a few dozen positions and velocities, stepped once a frame and redrawn, usually into a single canvas rather than into a few hundred elements. Nothing in the layout depends on the field, which is what separates it from a dot grid background, where the pattern is a scaffold the surface actually snaps to. Here the field is decoration, and should be marked as such so assistive technology never has to describe it.

The flocking variety has a real pedigree. Craig Reynolds published boids in 1986 as three local rules applied by every agent to the neighbours it can see: separation, steer away from crowding; alignment, steer toward the average heading of the group; cohesion, steer toward the average position of the group. No agent knows the shape of the flock and no central authority draws it, yet the group reads as a living thing, which is why the model outlived its own field and turned up in film crowds and screen savers alike. Web particle backgrounds are almost always simpler than boids. The constellation look needs no steering at all, only proximity: each pair of dots within some radius gets a line whose opacity falls off with distance, and the illusion of a network doing something comes free.

Three costs decide whether a particle background is a good idea, and all three get worse the larger the field is. The first is vestibular: a full viewport of drifting motion behind text is the canonical trigger case, so prefers-reduced-motion is load-bearing rather than polite here, and the honest response is to stop the loop and rest on one static arrangement, not to slow it down. The second is power: an animation frame loop redrawing a full-page canvas keeps a machine busy for as long as the tab is open, on a page that is otherwise sitting still, and a laptop on battery pays for every idle frame. Pause when the field scrolls out of view, cap the count, and keep the speed low enough that a single frame is not doing much work. The third is contrast: text over a moving field has no stable ground underneath it, because the ratio at any given pixel changes as the dots pass behind the letters. The fix is not a darker particle colour, it is a plate: give the reading surface its own opaque or near-opaque background and let the field stay outside it.

Among its neighbours, aurora UI is the closest in spirit and the furthest in mechanism, a slow wash of blurred colour with no discrete agents in it at all, which makes it much cheaper and much less likely to disturb anyone. A cursor follower is the same kind of ornament reduced to one agent and tied to the pointer instead of to a clock. A gooey effect also puts blobs on a page, but they merge and separate through a filter rather than wander. Libraries in this corner are numerous and mostly interchangeable; particles.js is the one whose defaults you have already seen, on more startup landing pages than anyone would care to count, which is itself a reason to think twice before reaching for the effect.