Doom, released by id Software in December 1993, was a watershed moment in real-time graphics on personal computers. Building on the ray-cast approach of Wolfenstein 3D, Doom introduced a far more capable renderer that allowed varying floor and ceiling heights, non-orthogonal walls, textured floors and ceilings, diminished lighting with distance, and a strong sense of three-dimensional space, all running smoothly on the unaccelerated PCs of the era.
The core rendering innovation was the use of binary space partitioning to organize level geometry. Levels were precompiled into BSP trees so that the engine could traverse and draw visible surfaces front to back without overdraw, a technique that made the frame rate predictable and fast. As the released source explains, the renderer worked in terms of horizontal and vertical spans of constant Z with fixed light shading, an approach tuned specifically to the strengths of integer-heavy CPUs without floating-point 3D hardware.
Doom shipped as shareware, with its first episode distributed freely and the full game sold by mail order. This distribution model, combined with the game’s speed and intensity, drove explosive adoption across bulletin boards, university networks, and corporate PCs. Doom also helped popularize the deathmatch, networked multiplayer combat between players, cementing competitive multiplayer as a central pillar of the genre.
Just as important as the game was the modding culture it enabled. Doom stored its content in WAD files separate from the engine, and players quickly built tools to create custom levels, graphics, and sounds. This separation of engine and data turned Doom into a platform, and the community produced tens of thousands of custom levels and total conversions over the following decades.
In December 1997 id released the Doom source code, preserved today at github.com/id-Software/DOOM, under terms that became a GPL release. John Carmack’s accompanying notes, echoed in his archived .plan files, candidly discussed both the renderer’s design and the decisions he would revisit. The source release cemented Doom’s status not only as a commercial and cultural landmark but as one of the most studied codebases in the history of game programming.