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11 Secrets about LEFT 4 DEAD’s AI Director and its Procedural Zombie Population

Alex J. Champandard on October 7, 2009

As more games strive for replayability and unique experiences, AI becomes increasingly important to help synthesize the gameplay and adapt to what the players are doing. This can help significantly increase the lifespan of single player games or story-based games, even if they are mostly linear like LEFT 4 DEAD. With large open sandbox worlds, technologies like the AI Director become even more important to help make the experience a little less mediocre during the worst moments, since emergence is rarely good enough on its own!

This in-depth feature article looks into LEFT 4 DEAD's procedural gameplay systems and the famous AI Director that helped Valve solve such problems, and secure the "Best Game AI of the Year" in the 2008 Awards. You'll learn how the zombie population is generated using a layered approach that's similar to Perlin noise, and how the results are adapted to what the players are doing. Dig into the a simple four-state system monitors and controls the whole experience, and discover what tricks Valve used to balance and tune the results.