Flock

A cloud of starlings over a reed bed turns, splits and closes again, and it is nearly impossible to believe that nobody is steering it. In 1986 Craig Reynolds showed that nobody needs to. His boids — a mangling of bird-oids — know only their immediate neighbours and follow three rules: do not collide, pick the same heading, stay together.

That is all of it. There is no leader, no route, no model of the flock as a whole. Every bird decides purely locally, from the handful of neighbours it happens to see. The cloud that results is described nowhere — it is simply there, as soon as enough birds do the same thing at the same time.

What the simulation makes visible is how narrow the band is in which it turns beautiful. Turn cohesion up and the flock clots into a trembling ball. Turn separation up and it scatters into loose gravel. Between the two lies a strip where the flock becomes fluid: elastic, coherent, different every time. That region is not built in. It is a consequence.

The elegance of boids is that they resist the temptation to build in the answer. Anyone who wants to draw a flock could animate a cloud. Reynolds animated the birds and left the cloud to the arithmetic.