Trooper Dan, on 05 March 2018 - 07:34 PM, said:
The functions that check visibility evaluate whether there is any line of sight between the sprite and the player, they don't take into consideration the angle that the sprite happens to be facing. I don't recall ever being able to sneak up on Doom enemies, though, so I always assumed they worked pretty much the same way.
The enemies in Doom have a FOV of 180 degrees when unalerted, meaning that once you are looking at them from the side, they are able to see you. Map01 of Doom 2 starts you with two zombiemen in front of you that have their back turned to you, for example.
However, note that monsters can be set to ambush, meaning that once they hear a sound through an open sector, they will gain a FOV of 360 degrees, and will activate once there's a clear line of sight between the player and the monster. In this state they function in essentially the same way as the enemies in Build.