Doom is a First Person Shooter released in December of 1993 by id Software for the PC. The series continued with Doom II, The Ultimate Doom and Final Doom using the same engine. The franchise received a reboot in 2005 with the release of Doom 3.
Doom's engine was revolutionary in its' day, being one of the first games to feature almost true 3D environments. More about the engine's capabilities and shortcomings can be read on The Doom Wiki. Most notably, the Doom engine reads all its' behaviour values from a pseudorandom table in the source code. This allows for demo recording and playback, so players can share their exact play routes and others can play those routes back using the actual game engine via exchange of small demo files.
id Software released the Doom source code late in 1997 under a not-for-profit license then again in late 1999 under the GNU GPL. The availability of this has encouraged ongoing development of "source ports" and continued creation of fan-made mods even today. Ports like ZDoom focus on implementing extra editing features like scripting and extra weapons/monsters although this comes at the expense of compatibility with original demos. Ports like PrBoom focus on extending the features of the game whilst maintaining demo compatibility with the originals. The PrBoom-Plus branch of this port adds several TAS features for demo recorders and watchers such as slowdown and the ability to split demos and continue recording to a new file halfway through.
Official competition for Doom was hosted by Compet-N from 1994 to 2005. Compet-N focused on the original Doom levels as well as a limited range of fan-made modifications which were seen to be of very high quality by the Doom community. Rules for Compet-N were very strict, with demos only played with the DOS executables being allowed entry into the archive.
Compet-N established a format for sorting demos based on both the skill level selected within the game, and the aims of the demo. Some examples are focus on achieving 100% kills on the Ultra Violence difficulty (called UV Max), or a focus on completing the levels as fast as possible without attacking any monsters (called Pacifist). A complete list of categories can be found on the Speedrun page of the Doom Wiki.
Today, Doom speedrunning lives on at Doug "Opulent" Merrill's Doomed Speed Demos Archive. The Doomed SDA features a much more relaxed ruleset, accepting demos recorded with modern sourceports and for any fan-made level.
Despite the fact the game was released in 1993, Doom speedrunning is alive and well with world records still being beaten.
Speed Run Strategies
Doom was the game that first featured straferunning, where players can move both forward and diagonally for a 40% increase in speed. This has been described by one of the creators as an "unintended side effect" and can allow for unforseen "jumps" across gaps in some of the original levels.
Regular speed strafing is called SR40 in the Doom speedrunning community. A faster variant called SR50 is achievable in the engine by moving forward, pressing the strafe sideways button, and turning sideways with the "strafe on" button pressed. This allows players to move 50% faster than just running forward however the extra speed comes at the expense of the ability to turn.
The sourceport PrBoom-Plus includes a TAS option to enable SR50 permanently.
Due to Doom's 3D nature, it is possible to use a rocket explosion to propel the player directions and speeds that wouldn't be attainable by use of normal moving controls (especially as the original Doom has no jumping). Rockets can be fired either by the player or by an enemy. Explosive barrels, a regular hazard in Doom levels, can be used in the same fashion.
The Doom II enemy called the Archvile features an attack which both causes great damage and propels its' target up and back a certain distance. A player can become the target of an Archvile attack in order to be propelled to an upper ledge or across a chasm which would otherwise be unreachable. Of course, the player has make sure they have sufficient health to survive the very powerful attack.
Suicide Exit (also called Deathslide) is a technique where the player intentionally kills the character to exit a level. Dead actors in the Doom engine acquire momentum on death by a weapon (which will have a degree of knockback) or by a nearby explosion. The engine considers a corpse to have 0 height, so a dead player can slide their corpse under locked doors and into otherwise unreachable places which can trigger a level exit. This was discovered in 2001 by speedrunner Vincent Catalaá. More information, including techniques and demos, can be found on the Doom Wiki Suicide Exit page.
When moving along a north/south or east/west wall, the player can move against and along the wall (ie: by straferunning into it) to accelerate to a very high speed known as a wallrun. This occurs because the engine recognises the player runs into a solid object and attempts to "slide" the player along the object, as coming to a sudden stop the instant you touch a wall is not very plausible. However, no check is made to see if the player has already "slid" against a solid object. Given the right angle, a player can "slide" two times per check resulting in achieving double movement speed.
Sprites in Doom (called Things in the engine) all have square "bounding boxes" in the Doom mapping format. As such, it's also possible to run into Things and accelerate in a similar fashion. This is known as "Thingrunning".
An amateur player can easily wallrun on most maps, an ideal setup for thingrunning can be found in the rows of barrels present on Doom II's MAP23: Barrels o Fun. More information and demos can be found on the Doom Wiki Wallrun page.