Editing Doom (1993 video game) (section)

===Engine===
{{see also|Doom engine|l1=''Doom'' engine}}
''Doom'' was written largely in the [[C (programming language)|C]] programming language, with a few elements in [[assembly language]]. The developers used [[NeXT]] computers running the [[NeXTSTEP]] operating system.<ref name="CGWLight"/> The level and graphical data was stored in [[Doom modding|WAD]] files, short for "Where's All the Data?", separately from the engine. This allowed for any part of the design to be changed without needing to adjust the engine code. Carmack designed this system so that fans could easily modify the game; he had been impressed by the [[video game modding|modifications]] made by fans of ''Wolfenstein 3D'' and wanted to support that by releasing a map editor with an easily swappable file structure.<ref name="MOD166"/>

Unlike ''Wolfenstein'', which has flat levels with walls at right angles, the ''Doom'' engine allows for walls and floors at any angle or height but does not allow areas to be stacked vertically. The lighting system is based on adjusting the color palette of surfaces directly. Rather than calculating how light traveled from light sources to surfaces using [[Ray tracing (graphics)|ray tracing]], the game calculates the "light level" of a small area based on the predetermined brightness of said area. It then modifies the color palette of that section's surface textures to mimic how dark it would look.<ref name="CGWLight"/> This same system is used to cause far away surfaces to look darker than close ones.<ref name="MOD124131"/>

Romero came up with new ways to use Carmack's lighting engine, such as strobe lights.<ref name="MOD124131"/> He programmed engine features such as switches and movable stairs and platforms.<ref name="GDC2011"/><ref name="IGNvideo"/> After Romero's complex level designs started to cause problems with the engine, Carmack began to use [[binary space partitioning]] to quickly select the reduced portion of a level that the player could see at a given time.<ref name="GDC2011"/><ref name="MOD132147"/> Taylor, along with programming other features, added cheat codes to aid in development and left them in for players.<ref name="IGNvideo"/><ref name="Guardian30"/>