AI Systems of L4D
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This article was based of the presentation "The AI Systems of Left 4 Dead" by Valve's Mike Booth. The original PDF file can be found here: 
Goals of the L4D AI
We all know that Left 4 Dead is a great game to play, and a lot of this is down to the way the AI (artificial intelligence) is programmed. How, for instance, would a survivor bot know to prioritise helping a team mate who is incapacitated instead of healing themselves? Why do you never see zombies spawning out of thin air in front of you? These decisions have been made around the four following goals that the Left 4 Dead AI was designed to achieve:
- Deliver robust behaviour performances - essentially, a survivor bot or infected should always be able to make a decision (ideally a realistic one) in a given situation.
- Provide competent human-player proxies - if less than four players are present in a campaign then the remaining spaces will be filled with bots, so these bots should be both competent enough to complement the human players while not being "overly competent" (eg. consistently deadly accurate).
- Promote replayability - make the game interesting to play and keep it interesting after the second, fifth, tenth and even twentieth re-plays.
- Generate dramatic game pacing - ties into the above; to help keep gameplay interesting and avoid both combat fatigue and lack-of-combat boredom, tune the game pacing relating to how "stressed" the players are.