How to make a fading wall

[Agent_Wesker]

I know how to make a wall fade as you move away but how do you make a wall fade as you approach it? I want to separate my map into pieces using black textured walls that fade when you get close to them.

(the walls will be non-solid of course)

[kommandojoe]

Use a func_areaportal_window in conjunction with a func_brush.