One way wall

Is it possible to make a wall that you can shoot through 1 way but not the other?

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Red lights are optional.

No. Anyway, why would you want to make a wall that you can only shoot through one way (apart from to encourage camping, which everybody hates)?

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Actually it can be done, but it will only allow visible projectiles through (rpg, grenades, crossbow) not weapons with invisible projectiles (shotgun, 357, assualt rifle). Visible projectiles have models, and can be used to trigger entites.

Make a wall three brushes thick, like a sandwich. Each brush must be thicker than the size of the projectile. I tested this with 16" thick brushes and it worked fine, but 4" made it unreliable unless I was shooting from an angle.

Make the middle brush a standard func_brush entity named, say, "middlewall". Make the left brush a trigger_multiple, with an Output to OnStartTouch middlewall disable, and tick the Everything flag. Make the right brush a trigger_multiple, with an Output to OnStartTouch middlewall enable, and tick the Everything flag.

When you fire a projectile at it, the wall will vanish for a fraction of a second allowing the projectile to pass through. It only works in one direction. I made the middle wall glass and it looks extremely cool. (have you ever seen Goku Midnight Eye 2? Reminds me of the phasing rocket launcher from that)

Of course, you can make the wall appear to remain solid by covering the middle brush with the nodraw texture, and making a func_illusionary brush occupying the middle brush space and with the texture of your choice. (I think func_illusionary has been depreciated ... maybe use another func_brush and make it non-solid.)

Also note that if a player tries to walk though it, they'll get stuck inside the wall. There's a few ways to get creative there. Blocking the lower portion is probably the easiest.

You could turn the brush into a displacement. I know rockets pass right trough the back side, but i don't know about things like the smg or shotgun.

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Reality has become a commodity.