Parallax Depth Layers

Scenery is split into two or more planes. Foreground tiles move at the speed of the player, a middle band at some fraction of it, and a far band barely at all. Because the eye reads slower motion as greater distance, a flat image acquires a rough sense of depth. Cost is mostly memory and bandwidth rather than computation, and the effect breaks the moment a background object needs to be touched.

Irem's Moon Patrol in 1982 is the usual credit, with three bands of lunar hills sliding past at different rates behind a buggy. Attribution is contested, and the 1981 arcade game Jump Bug is frequently named earlier. What is not in doubt is why the technique spread: it bought visible production value from spare tile layers, and consoles of the following decade shipped multiple scrolling planes as a hardware feature.

Depth layers gave 2D games atmosphere and scale without changing anything about how they play, which is exactly the appeal and the limitation. Backgrounds became mood rather than space, a place for weather, cities and skylines the player can never reach. Because parallax is purely presentational, it survived the arrival of 3D as a stylistic choice, and side-scrolling games still use it to signal size and speed.