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AI and Left 4 Dead 2

First, let’s just get this out of the way: Left 4 Dead 2 is an awesome game. It takes everything good about the first game and makes it better: fun levels, fun gameplay, fun characters and fun zombies. In fact, one of the only things I’d say is a problem in this game is the AI.

Now, the interesting thing about L4D2 is that it actually has quite a good AI for the enemies. These enemies are dynamically spawned based on how the player is doing at that particular point in the game, making for a different experience each time you play it. This is done through a system called the AI Director. It also handles weapon and item placement and other elements of the level like layout, lighting, and weather conditions. Like many games, you are usually rewarded for taking the tougher paths in a level by finding equipment or items that can come in handy then, or later in the campaign. Unlike other games, the rewards change each time you do the level.

In that regard, L4D2 shines. The game does indeed improve its replayability by offering these changeable conditions, making each playthrough slightly different. You’re never a hundred percent sure where enemies will be (particularly the more difficult enemies, known as “Special Infected”), nor which will even show up in a level. On one playthrough of a level, you might be decimated by a “Tank” showing up in the midst of a zombie horde, only to reload and see no traces of him on your second go.

However, not all AI is equal, and while enemies and world building benefit a lot from the Director, the AI controlled allies do not. I usually play this game with my friend, which means two of the four protagonists are controlled by us. The other two are controlled by the computer. Now, again, I want to give a lot of credit to the game programmers: there are good elements to the AIs - they do manage to hit quite a lot of zombies (but not so many that they’re carrying us through the game), they assist in taking down special infected, and they will often split up to follow us (if I go in one direction and my friend in the other, usually one of each of the computers will follow us to make sure we’re not alone).

At the same time… let’s just say that I don’t give them any of my equipment. I hoard the health packs for myself because the computer-controlled players are too stupid to use them correctly. There’s no point in leaving throwable weapons for them (molotov cocktails, pipe bombs and the like) because they won’t throw them. They regularly do strange things like stand in bad places so that they get attacked by special infected, or fail to help you when you are similarly caught by the more powerful creatures in the game. They wander around in pools of acid, taking damage until they die.

To my immense frustration, they regularly walk between me, (presently firing off a gun at as many rounds a minute as I can manage), and the zombie horde. There is friendly fire in this game. Considering that a full shotgun blast can instantly incapacitate a fellow player when on expert difficulty, this is more than a little annoying.

And finally, everyone who’s played this game will likely understand my frustration when I sigh: they startle the witch.

For a game that does so many things exceptionally well, this sticks out like a sore thumb, jarringly unpolished when compared to the control the game has over its enemies. I think, perhaps, this is partially because of the complexities of the remainder of the game. With so much changing, so many things going on in each level, there’s a lot for an NPC to keep up with. Figuring out how to throw molotov cocktails in the heat of the moment is complex and difficult, especially when cooperating with a human player or players.

Games like this have a long way to go before they’re perfect, but I have confidence in the advancing technology and in the people in the industry, and I know that one day we will have computer-controlled NPCs with the same quality that we’ve come to expect from the rest of a game.

Hey, maybe we’ll even see it in Left 4 Dead 3!

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