No collide displacements

[Giloh]

How do I make it so you can walk through a displacement, like with func_illusionary on a normal brush?
I know it's possible, I've seen it on another map.

 

[Freyja]

With the face edit window open, select the displacement and check the three boxes "No hull collision" and the other two I forget the names of.

 

[Giloh]

Thanks, been looking for it for like an hour xD