Clip texture
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. These all share the same Clips VisGroup category. 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.
Example
The fence on the right is enveloped by a toolsplayerclip tool texture for two reasons: First of all it prevents the player from getting caught on the brushwork when running alongside it, and second of all it prevents the player from jumping over something that would be at least cumbersome to climb over in real life. Be aware that the player could acquire physics objects and pile them up against the fence, and through this jump over the brush. If this is the case, you could consider extending this clip brush all the way to the roof of the map, but as this would create an invisible wall, it would perhaps be better to replace the entire fence with a solid wall instead.
Alternatives
Invisible Displacement
In some cases, clip brushes are not an option, or too difficult to make an accurate clip brush around props.
For example, Left 4 Dead often used displacement brushes using invisible textures to make something like a clip, which covers rubble props.
Env_Player_Blocker
Another useful, but very limited alternative are env player blocker entities. These are rectangular point entities that block players in it's volume. This entity is mostly used in "Cleanup" activities via the Commentary System or LMP file edits after a map has been compiled already.
Clips with Materials
Some games, like Counter-Strike: Global Offensive and Garry's Mod, have multiple clip tool textures that use specific material surface properties, such as concrete, dirt, glass, ect. This allows the clips to sound different when stepping upon and also make use of the materials properties, like the very low friction of "SlipperySlide". Example uses of this is utilizing clips with glass material when working on glass catwalks, which would allow you to still hear glass noises when stepping onto the clip, as it would otherwise play a generic sound which would be out of place.
For games that do not have these clip textures, you should create your own or copy them from a game that has them. Alternatively, you can (with more overhead) use the following workaround to avoid creating new material files:
- Build your brushwork for the clip using any texture that posesses the material properties you require. For example,
glass/glasswindow007a, simply because it is glass. - Turn the brushwork into a func brush that is set to "Don't Render", "Always Solid", and "Solid BSP". This stops your brush from cutting Vis Leaves and becomes invisible, just like a clip brush would.
Warning:Depending on thickness, it will also block bullets. It will also block physical props and projectile weapons, such as grenades, or arrows, because it is merely invisible brushwork.