From Quake Wiki
|Saved to config.cfg:||no|
When set to a non-zero value, gl_clear will clear the color-buffer. Basically, this means that when the camera can see into the [void], either because of the map extending past the farclip plane or there is some kind of corruption of the map data. This is often used as a debugging tool as the 'gl_clear' color is red, therefore allowing you to see such holes quite clearly against the browns, blue and beiges of the quake world. Other people have also used it when cheating to be better able to make sense of the void whilst noclipping outside of the world.
On older systems (I.E. Pre-Voodoo3) it is advisable to leave this set to 0 (ZERO) as it will have a noticable positive effect on your framerate. Newer systems will be less affected. However, sometimes graphical errors can occur causing seams in the map (especially when gl_keeptjunctions is off), usually causing a few glaring red pixels to show through. For this reason, it is advised to leave gl_clear off, especially on old engines such as GLQuake that use the ugly red colour.
Some custom engines or mods RELY on the behaviour and existence of this cvar, so as a note to engine modders, please leave this cvar intact.
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