id Software has once again offered up one of their older game engines to anyone who wants to work with it. The company's co-founder and lead programmer John Carmack released the files for the Doom 3 engine today, much like he has released the engine source code for their Doom and Quake engine. The source code files do not contain any of the data from Doom 3 itself which are still covered by the game's original end user license agreement.
Carmack recently revealed that he had to make a few tweaks to the engine source code in order to avoid some patent infringement issues. Specifically, the code does not contain any data concerning the rendering of stencil shadows via the "depth fail" method, which is also known by programmers as "Carmack's Reverse." Lawyers for id Software's parent company ZeniMax Studios feared that the code would infringe on a similar rendering and shading code that's been patented by Creative Labs.
The Doom 3 engine has been used not only in id Software 2004 first person shooter but also in the 2005 game Quake IV and the 2006 shooter Prey. id has now moved on to what it called id Tech 5 for its latest game Rage which was released in October 2011.