Gl clear

Name: gl_clear
Default value: "0"
Cvar flags: None
FTEQW Builds: OpenGL Renderer Minimal GL Build Merged Build Linux Build

This variable will toggle the clearing of the screen between each frame. This
can be helpful when specing a game and flying out of the map and during map
development when the map maker must fly outside of the map to look has his map
from the outside. Enabling this toggle will clear the areas which are not
rendered outside of the map thus preventing the flickering effect when images
repeat themselves over and over. However it causes problems when playing and
going through the surfaces of a liquid (for example when diving into water), as
you will see a red flash where the liquid texture would normally be repeated.
0 = Off.
1 = On.

To Quote the QuakeSRC wiki:
Whilst 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 wuill be less affected.

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.