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r showtris

From Quake Wiki

Start map with r_showtris enabled
The r_showtris cvar is a developer command that shows the outlines of world and object polygons as triangles. It is helpful for optimizing geometry and vis-blocking.

It has three posible states 0 to 2

0: Disable this feature.
1: Enables it like in the picture.
2: Also enables it, but it won't show what's rended behind a wall like 1.

The command is only available in certain engine ports, such as Fitzquake, forks, DarkPlaces and QuakeSpasm.