Wraparound Topology

Coordinates are taken modulo the width and height of the field, so the right edge is the left edge and the top is the bottom. Nothing is lost by travelling; there is no corner to be trapped in and no boundary to bounce off. Objects have to be drawn twice while straddling a seam, and pursuit becomes strange, because any chase can be resolved by running the wrong way for long enough.

Asteroids, released by Atari in 1979, is where the idea became legible to players. Wrapping solved a specific problem for a game about drifting momentum: a ship with no brakes will otherwise pile into a wall, and rocks that leave the screen have to go somewhere. Modular arithmetic was cheaper than either fencing the field or spawning replacements, and it kept every fragment of every asteroid permanently in play.

Consequences run in two directions. As a play space, wrapping removes the safety of corners and forces continuous attention in every direction, which is why it suits games about drift and crowd control. As an idea, it established that a game's space needs no physical analogue and can be defined by whatever rule the program applies to a coordinate, an argument that later games would push considerably further.