Boids (Craig Reynolds, 1986)

Boids is a computer model of a flock of birds, made by the computer graphics researcher Craig Reynolds in 1986 and published at the SIGGRAPH conference in 1987 as "Flocks, Herds, and Schools: A Distributed Behavioral Model". The program contains no flock, only many simulated birds, each steering by a few rules about its nearest neighbours. The flock is what appears when they all do so at once.^1, 2

The three rules

Reynolds "called the generic simulated flocking creatures boids". A boid sees only the flockmates in its neighbourhood, an area set by a distance from its centre and an angle from its direction of flight; all others are ignored.¹ Inside that area it follows three rules, in Reynolds's words:¹

• Separation: "steer to avoid crowding local flockmates".
• Alignment: "steer towards the average heading of local flockmates".
• Cohesion: "steer to move toward the average position of local flockmates".

The 1987 paper gives the same rules as Collision Avoidance, Velocity Matching and Flock Centering, listed "in order of decreasing precedence".²

No leader, no plan

The paper describes the simulated flock as "an elaboration of a particle system, with the simulated birds being the particles". Each bird is an independent actor that moves by what it perceives around it, and "the birds choose their own course". An animator who wanted the flock to go somewhere could add a shared goal, a direction of migration, but no single bird leads.²

New drawing

On screen

The model was used in the short film Stanley and Stella in: Breaking the Ice, first shown in the Electronic Theater at SIGGRAPH '87. Tim Burton's Batman Returns (1992) used modified boids software from Symbolics for its swarms of bats and flocks of penguins.¹

In generative art

Daniel Shiffman's book The Nature of Code builds a flock on the same "three rules: alignment, cohesion, and separation", alongside Reynolds's later steering behaviours, among them following a flow field.³ The model belongs with the other rule systems in these notes where a simple rule, applied by many parts at once, produces a form nobody drew: Conway's Game of Life and diffusion-limited aggregation. See also Generative Art.