 Commonly known as the ""Doom 3" engine" which was used to power "Doom 3" as it released in 2004, id Tech 4 began as an enhancement to id Tech 3.During development, it was initially just a complete rewrite of the engine's renderer, while still retaining other subsystems, such as file access, and memory management.The decision to switch from C to the C++ programming language necessitated a restructuring and rewrite of the rest of the engine; today, while id Tech 4 contains code from id Tech 3, much of it has been rewritten.