the work he does in video games. Open-source software Carmack is an advocate of open-source software, and has repeatedly voiced his opposition to software patents, equating them to robbery. He has also contributed to open-source projects, such as starting the initial port of the X Window System to Mac OS X Server and working to improve the OpenGL drivers for Linux through the Utah GLX project. Carmack released the source code for Wolfenstein 3D in 1995 and the Doom source code in 1997, first under a custom license and then under the GNU General Public License (GPL) in 1999. When the source code to Quake was leaked and circulated among the Quake community underground in 1997 after licensee Crack dot Com was hacked, a programmer unaffiliated with id Software named Greg Alexander used it to port Quake to Linux using SVGALib. As this was more feature rich than Dave Taylor's earlier X11 port, he sent the patches to Carmack. Instead of pursuing legal action, id Software used the patches as the foundation for a company-sanctioned Linux port maintained by new hire Zoid Kirsch, who later ported Quakeworld and Quake II to Linux as well. id Software has since publicly released the source code to Quake in 1999, Quake 2 in 2001, Quake 3 in 2005 and lastly Doom 3 in 2011 (and later the BFG Edition in 2012). The source code for Hovertank 3D and Catacomb 3D (as well as Carmack's earlier Catacomb) was released in June 2014 by Flat Rock Software with Carmack's blessing. He has since expressed regret on using the copyleft GPL over the more permissive BSD license. The id Tech 4 engine, more commonly known as the "Doom 3 engine", was also been released as open-source license under the GPL. This occurred despite patent concerns from Creative Labs over Carmack's reverse, while the original Doom source release shipped without music due to complications with the Cygnus Studios developed DMX library (which lead to the Linux version being selected for release). Carmack has since advised developers to be careful when utilizing middleware, noting how it can limit the possibilities of later releasing source code. Tim Sweeney has implied this issue has hindered potential releases of older Unreal Engine source code. On the other hand, despite his technical admiration for the system, Carmack has several times over the years voiced a sceptical opinion about Linux as a gaming