The AI director was created for Left 4 Dead in order to enhance the experience of a zombie apocalypse to the fullest. The director decides where weapons upgrades, ammo, pills, pipebombs, molotovs and medpaks spawn. In addition to these positive traits the director also decides where infected will spawn, how many there will be, where to place an infected rush, and how often special infected spawn. The director decides when and where to place these pros and cons depending on player skill, how long it's been since a previous attack or a health station, how much health the players have, the difficulty set, and how close they are to the end of the level.
The director works on many complex triggers. For instance:
- Players have 100% health
- Players have 100% ammo
- Players are at the end of the level
- Spawn a "Tank" special infected
This of course is a simplified version of the game's actual system, but on the most basic level that's how the director works. The director will work in maps not optimized for its use, since all of its abilities rely on spawning things corresponding to the players' position. Infected can spawn only if they are out of sight, and health and ammo will spawn relatively close in an out of sight location. Because of this system, custom maps are very easy to make for Left 4 Dead since modders do not need to handle spawn locations themselves.