Clip texture
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Clip textures are invisible tool textures that limit movement, typically of the player(s). A brush used with a clip texture is generally termed a clip brush. In the Source engine, the general dedicated clip texture is Clip, but alternatives are Player Clip, (Player) Control Clip and NPC Clip. However, any unrendered brush or model can fill this role.
There are good techniques and there are bad techniques when it comes to using clip brushes to limit the player's movement.
Good techniques are the ones that keep the player away from possibly getting entangled and stuck in complicated brushwork such as in between a tree and a hill, or amidst the cracks of several rocks, forcing the player to restart. However, wedge-shaped clip brushes can also be used to prevent the player from getting stuck on small detail brushes like window sills or buttons while trying to run somewhere, and to prevent the player from jumping up on top of an all too narrow fence or window sill.
Bad techniques are known as invisible walls, that block the player from heading into a corridor that he or she is not "supposed" to enter. Apart from frustrating the player, this breaks down the illusion of an immersive world and reminds the player this is just a game.