The id Tech 4 engine (also known as the Doom 3 engine) was designed by John Carmack using C++ and OpenGL. It featured several new graphical features such as bump mapping, normal mapping and specular highlighting.
Unlike its widely-known preceding id Tech 3 and id Tech 2 engines, the id Tech 4 has had the least successful licensing to third parties thanks to its competitor, the Unreal Engine, which featured better graphics and level design.
Carmack released the Doom 3 source code on Github on November 22, 2011. The open source does not include the original implementation of 'Carmack's Reverse' code, which was used to efficiently calculate shadows. According to Carmack, that portion of the code had been rewritten to avoid potential infringement with a patent filed by Creative Labs. A settlement was later reached, allowing Carmack to use his code in Doom 3.