DOOM

id Software released DOOM as a program you could take and pass on: the first episode was distributed freely, the rest were sent by mail after payment. The files with the level, texture and sound data lived apart from the engine, in WAD containers, and this turned out to matter more than any single design idea. A few months after release, outsiders were already building their own levels and graphics sets without having the source code.

The engine draws a floor plan with sectors of variable height, split into parts by a binary tree. There is no full 3D here: rooms can't stand one above another, but the scene holds dozens of enemies without a drop in frame rate. Enemies are drawn as sprites captured from several sides, so the silhouette shows which way an enemy is facing. The authors called the four-player network mode deathmatch, and the word stuck to a whole way of playing.