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From HalfLife 2 Knowledge Base

mat_wireframe in action

mat_wireframe is a console variable that shows everything that the graphics engine is trying to draw. It is normal that the engine is trying to draw much more than a human would think it should.

You must have cheats enabled to use mat_wireframe. Then open the console and type

mat_wireframe 1

to turn on the wireframe view.

mat_wireframe 0

will turn it off. If you plan to use it frequently, you can bind a key to it; type the following to make the k key toggle it:

bindtoggle k mat_wireframe

Key:

• Red: brushes, including brush-based entities
• Light blue: props
• Yellow: decals
• Blacks, grays, whites: light halos, particle effects, sprites, and suchlike

Note: brush-based entities are drawn after regular world brushes, so you might encounter the odd behavior that a door will obscure a wall behind it. This can be confusing, but in reality both the door and the brushes behind it are being drawn (except in the case of area portals, described later).