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Post-Play'em - Observations on Game AI


Left 4 Dead: First Look

Ok… half price weekend on Steam finally got me into Left 4 Dead. After watching all my TF2 buddies drifting away to this mysterious “other game”, I needed to jump in and take a look. Also, since it will likely be the buzz of GDC next month, I figured I had better have an idea of what it looked and felt like.

I have to admit that it’s a pretty groovy game. All the stuff we have heard so far is correct. The online, team play is cool. It’s so much better than most team shooters in that it seriously punishes you for separating from your cohorts too much. Also, because there is such a significantly different array of tactics that are necessary for playing as the infected, the versus mode is almost like having a second game. But that’s not what this column is about…

The big AI pull on Left 4 Dead is the “AI Director” that we have all heard so much about. In an interview with Edge Online, Valve’s Gabe Newell talks about all the nifty stuff they tried to take into account. Wikipedia even touches on the AI Director. It’s no secret that this tech was a big deal.

When I finally got to playing it, I was very impressed with it myself. Having played through some of the levels more than once (like No Mercy, for example), I am well aware that I don’t know where anything is going to be next. Compare that to other games where the 2nd time through you already have an idea of what is coming next… by the 3rd or 4th time, you have it down. I don’t like that amount of rigid predictability. With L4D, I have been on my toes at all times.

It does seem like it does a decent job of balancing the flow of the game. There are plenty of times when I’m “on the edge”. Compare this to the rubber banding I suspected in Doom 3 where it just seemed that the lower I got on health, the less damage the enemy did. The result was an (almost) asymptotic approach to 0 health. Exciting? A little. Contrived? Definitely. With L4D, this feeling is much more natural in that it is adjust the pacing of the enemies themselves rather than adjusting the abilities they present. Rather than the confusion of “why is this horrible monster doing very little damage now that I’m almost dead?”, I find myself saying “thank God they aren’t coming after us just now… I need to heal up a little!” It’s a subtle difference, but it is there.

That’s not to say that the game is always winnable in true rubber band fashion. On my first day of play, three friends and I played that last rooftop level of “No Mercy” about 8 times and kept coming up just short. It was exciting each time. Finally, we beat it by only barely dragging our asses into the chopper. Quite the getaway! As we reflected on what had happened (the game was new for all of us), I noted that every single time was different. Different monster placement, different ammo placement, different items, different pacing. We couldn’t do the same thing each time because the computer wasn’t doing the same thing. One time we used gas cans to light fires. The next time, I went looking for the gas cans… only to find that they weren’t there (they were across the level). The next time, there were no gas cans at all… just more propane tanks — which meant a different strategy entirely.

What I have found interesting is, while playing the infected (which I am having trouble getting the hang of), I can watch the other zombies “pop in” as they spawn. It’s kinda fun to see them do that. Oddly, I feel a strange kinship with them at that point. (“Yeah, right here buddy! We’re gonna get ‘em from this closet, right dude? Right? Hey… pay attention to me!”)

The other thing I get to watch as the infected is how their “idle” behaviors work. There are quite a few of them and they blend well. Admittedly, by their nature, zombies don’t have to be terribly engaging, but it is a nice touch to come around a corner or see a group far off actually doing stuff. Sometimes, I feel that the sensory distance is a little short, however. If a little beepy grenade gets every zombie in the same zip code to freak out, why doesn’t my shotgun wake them all out of their staggering stupor as well? Again, people just don’t model the sense of hearing well in games. *sigh*

The AI of your counterparts is pretty decent, for the most part. I’ve seen a couple of odd glitches, however. Therefore, I think I’m going to make that a separate post so I can possibly post some FRAPS-caps of them in action.

(As always, make sure you check the tags below to see what else I may have written about this game.)

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