Game maker Id Software has published the Doom 3 game engine source code under the GPL3 license, allowing developers to incorporate its mechanics into their own projects. The release does not contain any game data, according to the release notes, as those elements are still covered under the Doom 3 commercial end-user license agreement. In addition, the shadow-rendering technique known as “Carmack’s Reverse” has been substituted for patent reasons.
The company has traditionally promised and eventually published the source code to its game engines, and promised to do so with the Doom 3 engine as well. A roadblock that could have prevented the code’s release for Doom 3 was that both lead programmer John Carmack and another company independently developed a shadow-rendering technique, but the other game company patented their version.
The company didn’t want to expose itself to possible litigation by open-sourcing a very similar technique. Carmack later found a solution to the problem by further re-coding the technique to be different enough from the original that it could be handed off to Id co-founder Timothee Besset, who has traditionally handled all the Linux and open-source porting and other cleanup of the various game engines and did most of the work on the Doom 3 release.
