So you are looking to see if the player can see the enemy or vica verse?
Yes, or more importantly, if an enemy can fire a shot without it being blocked by a wall.
I apologize for not being too clear on this topic. Let's say that the slope between the enemy and the player is 2/1. The enemy is at (100, 100), and the player is at (103, 106). So the calculator checks (101, 102). Is there a wall? No. Check (102, 104). Is there a wall? YES! So the enemy cannot harm the player.