How it works
This is here as the counterweight to the flow field. A flow field is a field: it exists before any of the marks do, it does not change, and two strokes crossing the same place are steered identically. There is no field here at all. Each agent is steered only by whichever other agents happen to be near it at the time, so the shape of the drawing is a record of who was next to whom.
Reynolds’ three rules, from 1986, and there are still only three. Separation: steer away from anyone inside your personal space, weighted by one over the distance so a near miss counts for far more than someone at the edge of it. Alignment: steer toward the average heading of everyone you can see. Cohesion: steer toward where they are, on average. Every murmuration, shoal and swarm in computer graphics since is these three with the weights moved around.
The detail that decides whether it works at all is what "steer toward" means. A rule names the velocity it wants; the force is the difference between that and the velocity the agent has, and the sum of the three is capped, so no single rule can simply overwrite the others. Adding the wanted direction straight on instead — the easy mistake, and the first thing this did — gives agents that drift politely past each other and never form a flock. What is drawn is not the flock but its history: one path per agent, the whole way it went.