From Quake Wiki
|Based on:||Quake or WinQuake?|
id Software released this hacked version of Quake to work with OpenGL while it was constructing Quake 2. There are quite a few problems and differences that should be noted about GLQuake. First off GLQuake does not support fullbrights - this is a minor visual fault, but remember when working with new textures and graphics to always check to see how it looks in classic software quake as, due to poor error diffusion, your new textures may be bespeckled with fullbrights. GLQuake also doesn't support most sprite modes. Although sprites are frowned upon in Quake, as it's considered old tech, modders find sprites incredibly useful for special effects. Thirdly, and not really a problem, GLQuake supports translucent water, but you need to use a special version of VIS to have it in your maps.
GLQuake is the base for most modern ports of the Quake engine. Check DarkPlaces and Tenebrae sources for heavily improved code. Check FitzQuake and the GQ engine for bug-free enhanced classic GLQuake play. Check Telejano and CheapHack for eyecandy enhanced GLQuake/TomazQuake code.