The Doom engine powers Doom and Doom II, and is separate from the assets and resources (data) used by those games. In a more technical sense, the engine is the executable elements of the games, based on the corresponding source code, as opposed to WAD and lump files. Various versions of the engine were released by id Software for the PC games, one per release but often shared by both games, as the same executables are used for both, only renamed accordingly.
The engine is composed of a rendering engine which structures game levels and handles movement, effects, and obstructions during play, and an auxiliary Doom networking component to connect computers during multiplayer games. Additionally, the engine includes a sound management system, of which, unlike the rendering and networking functionality, the sources were not fully released to the public, as they included proprietary code written by Paul Radek.
The latest version of the engine released for the games is the one included with version 1.9 of each game, although slightly modified versions were later issued with The Ultimate Doom and then Final Doom, although John Carmack, the main programmer, was not really involved in those two releases, and they are marked as "v1.9" notwithstanding the differences. The released source code, which includes some differences from the DOS incarnations to make the code more portable, is conveniently marked as version 1.10.
As a game engine
In a more abstract sense, the Doom engine is the use of the Doom source code or the executable parts as a starting point for another game. This aspect is similar to the more novel concept of game engine, although the Doom games precede the concept and, while designed with greater moddability than many earlier FPS titles such as Wolfenstein 3D, the Doom engine differs from newer game engines in that it does not offer a definite core separated from the game-specific rules, and does not provide the greater degree of hardware abstraction newer engines may offer. It has provided developers an engine which is to be used as a modifiable example, rather than a multi-platform core to be used as a base.
Use in ports and other games
Aside from the seminal Doom games, the engine has been used for several commercial games:
- Heretic (1994): A fantasy themed shareware game developed by Raven Software and published by id Software, arranged into three episodes, like Doom. The engine is based on Doom version 1.2 and adds flying (for the player), the ability to look up and down, an inventory allowing the user to select when to use certain items and power-ups, and monsters with multiple ranged attacks.
- Heretic: Shadow of the Serpent Riders (1996): An expanded release of Heretic prepared for distribution in retail stores, much like The Ultimate Doom is in regard to Doom (and likewise free for those with a registered copy of the shareware game), adding two extra episodes plus another three additional (bonus) levels.
- Hexen: Beyond Heretic (1995): The second fantasy game title by Raven Software published by id Software. It is the sequel to Heretic and the engine is based on Heretic's. It adds character distinct classes with varying attributes, class-custom weapons with progressive power, a scripting language permitting customized game behavior, and hub system allowing the player to return to previous levels (instead of the more linear episode system).
- Hexen: Deathkings of the Dark Citadel (1996): An expansion for use with Hexen: Beyond Heretic providing additional levels by the same authors and released as a separate product.
- Strife: Quest for the Sigil (1996): A science fiction game developed by Rogue Entertainment and published by Velocity. Its engine is based on version 1.666, or higher, of the Doom engine and the game includes NPCs the player can interact with, a degree of player character progression, and a particularly non-linear hub system.
- Chex Quest (1996): A modification of The Ultimate Doom styled after the Chex Cereal brand, developed by Digital Café and included in cereal boxes as a promotional bonus item. The game uses an engine based on the source code for The Ultimate Doom, modified to accommodate some necessary changes.
- Hacx: Twitch 'n Kill (1997): An add-on modification of Doom II version 1.9, developed by Banjo Software. Unlike the games listed above, its developers relied on reverse engineering instead of the source code, although it includes extensive and comprehensive changes to the behavior of monsters and weapons. In 2000, the game was re-released as freeware, and a standalone version was released in the form of version 1.2 on October 9, 2010.
Fan community variants
With John Carmack's release of the Doom source code on December 23, 1997, the Doom games and community have benefited from ports and modifications of the engine developed by many people around the world. These engines can run the original Doom games and sometimes other games using the engine as a base (see above). These versions produced by programmers among the fan community are commonly known as source ports, as distinguished from the engines included with the games.
As a result of the distinction between the official and fan-made engines, the name vanilla Doom (or Vanilla Heretic, etc.) is sometimes used by fans, especially by regular source port users and developers, to describe the original executables themselves, the act of playing with the original executables, or a feature, phenomenon, or add-on that complies with the standard engine to one degree or another (either being strictly compatible, or not having or using characteristics found only in source ports).