The zombies in L4D are fairly intelligent, at least as far as their underpinning AI goes. Yeah, they follow zombie-like rules (such as standing around mindlessly until a disturbance happens), but they express a pretty wide and dynamic range of actions. Setting off a car alarm, for example, will attract attention. The way a horde forms around you is also pretty well done. They also move around intelligently, clamoring over low obstacles and so forth.I can tell you that after 1 hour of PainKiller gameplay my head is about to explode from trillions of dumb pixels.
Basically, Valve isn't using zombies as an excuse for bad AI. These zombies are mindless, but believably so. They're not just simple obstacles whose only routine is to rush towards players but "realistic" zombies that behave "realistically". The game also does a great job of having zombies attack you in unexpected ways. Much of the fun is manipulating the zombies' behavior to your advantage. You know it's safe to pick 'em off one by one, but all it takes is one unexpected event before the rules change and a "safe" situation quickly turns into a massive battle.
This is obviously nothing but pure speculation, but I sort of like the one-beam thing. Human players are a pretty spastic bunch and having 3 other beams flying all over screen might be cause for a seizure or two.You know what I don't like? You can't see the dynamic light casted by your teammate's flashlights. Nor the dynamic shadows that their flashlights cast.
On a side note, I believe there's an engine limit of only one lighting source for dynamic shadows.
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