Id Tech 1

id Tech, also known as id Tech 1 or the Doom engine, is the game engine that powers the id Software games Doom and Doom II: Hell on Earth. It is also used in Heretic, Hexen: Beyond Heretic, Strife: Quest for the Sigil, Hacx: Twitch 'n Kill, Freedoom, and other games produced by licensees. It was created by John Carmack, with auxiliary functions written by Mike Abrash, John Romero, Dave Taylor, and Paul Radek. Originally developed on NeXT computers, it was ported to DOS for Doom's initial release and was later ported to several game consoles and operating systems.

The source code to the Linux version of Doom was released to the public under a license that granted rights to non-commercial use on December 23, 1997. The source code was later re-released under the GNU General Public License on October 3, 1999. The dozens of unofficial Doom source ports that have been created since then allow Doom to run on previously unsupported operating systems and sometimes radically expand the engine's functionality with new features.

Although the engine renders a 3D space, that space is projected from a two-dimensional floor plan. The line of sight is always parallel to the floor, walls must be perpendicular to the floors, and it is not possible to create multi-level structures or sloped areas (floors and ceilings with different angles). Despite these limitations, the engine represented a technological leap from id's previous Wolfenstein 3D engine. The Doom engine was later renamed to "id Tech 1" in order to categorize it in a list of id Software's long line of game engines.