Creating a predictable range of vision for enemies

So I'm wondering how to show a player what the enemies can and can't see.

At first I drew a semi-circle and used the line of sight to do this. Problem is, the semi-circle sprite goes through platforms where the enemies clearly can't see.

Currently, I'm looking at placing boxes all throughout the game and saying if they're within a certain range of an enemy they will be visible with say 30% opacity. Having troubles with getting them to do this. It also seems quite demanding to do this.

Does anyone have a more elegant solution?

Maybe there's a different way of conveying such information to the player?

How have others gone about this?

use the distance expression in the math part of system's expressions to do this.

toralord

I know how to detect this mathematically. I'm looking for a graphical way to show the ranges to the player.