GtkRadiant
About
GtkRadiant is the official open source toolchain for Id Tech 3 (the Quake III: Arena engine). Originally was written for Linux as a port of Q3Radiant using the GTK+ toolkit, it was backported to Windows, OS X and eventually grew to support a number of games including Return to Castle Wolfenstein, and Doom 3.
I owned the level compiler, Q3Map2. I also contributed the DXT image reading library and PicoModel, a BSD-licensed 3D mesh loader.
