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Why Left 4 Dead Works
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Why Left 4 Dead Works

December 21, 2010 Article Start Previous Page 2 of 4 Next

AI Director

Controlling the common Infected as well as other types of game mechanics such as the placement of weapons or health kits is the AI Director, a module that works behind the scenes of the game and may be considered the "intelligence" directing what the player experiences during gameplay. The AI director monitors the actions and conditions of players and uses that information to make changes to the game environment in real time.

According to L4D team member Gautam Babbar, game levels don't have pre-scripted placement of enemies: instead, the AI Director is comprised of a "system that tracks each Survivor's 'stress level' by watching for events like 'how much damage you are taking', 'how many zombies have you killed near you', and so on.

If a Survivor's stress gets too high, the system will step in and forcibly throttle back the zombie population system to make sure the team gets a break every now and then." In this way, the role of L4D's AI Director could be seen as determining if players are engaging in effective rule/skill/knowledge based behavior and raise or lower the difficulty or number of enemies accordingly.

Special Infected

However, through play-testing early versions of the game, the development team found that in order to expand and improve the gameplay experience they would need to include additional game mechanics to complement the Common Infected enemy type that populated the game world.

This lead to the Boss/Special Infected, which were developed, according to L4D team member Sean Keegan "directly from weaknesses observed during... daily playtesting." As there was only one enemy type present in the game, players exhibited a more-or-less skill based behavior of shooting everything that moved, which quickly became repetitive and tedious.

The five types of special Infected in L4D are horribly mutated creatures that are randomly spawned in the game environment by the AI Director to further impede a player's progress through the game environment. Each come with their own unique actions in game designed to fill a specific niche with abilities that the common Infected lack, such as disrupting a team's ability to protect one another.

For example, the Smoker, which uses a ranged attack to drag players towards its claws, was created to pose a threat to experienced players who stayed in tight groups when moving through the level. Team member Khanh Nguyen summarized the role of this dynamic as follows:

"Highly cooperative and experienced Survivor teams can become very good at staying together and watching all around them, to the point that it became difficult for the zombies to cause the kind of chaos and panic that makes the game exciting. The Smoker was created to forcibly pull these tight teams apart. Moving just one Survivor far out of position with the Smoker's tongue attack proved to be enough to break things up and bring the chaos back."

The Smoker's presence also encouraged teams to be aware of their proximity to each other in the game environment, as players who either lagged behind or charged ahead of their teammates became more vulnerable to the Smoker's attack and reduced their chances of a timely rescue.

The other special Infected have similar objectives in mind; the sweatshirt- wearing Hunter leaps great distances onto a target player, pinning it to the ground and tearing into them with its claws. Players that are too close to the hunter when it pounces onto its target can be knocked aside and temporarily stunned and only by killing it or using the default melee attack can players save a hunter's victim from disembowelment.

On harder difficulties or when under attack by other enemies, the few precious seconds lost while stunned can mean the difference between surviving the Hunter's assault or losing a team member; like the Smoker, players were encouraged by the Hunter to stay close to each other for protection, but not too close as to take collateral damage from his pounce.

  • The Boomer, a heavily obese zombie, serves as method of aggravating the common Infected to attack as they make progress through a level; its primary attack is to vomit a blinding gunk that both disrupts players vision and attracts hordes of nearby common Infected that will specifically attack players covered by the Boomer's bile. Killing a Boomer at close range however will cause it to explode and cover any nearby players with the same horde-drawing vomit if they're within the blast radius. Players needed to make sure there was distance between the Boomer and themselves or their teammates to avoid anyone getting sprayed with Boomer's bile.
  • The Witch takes on the appearance of an Infected woman with long sharp claws and is often heard crying mournfully. Initially in a passive state and huddled somewhere in the game environment (sometimes directly in the player's path), the Witch ignores players unless disturbed by light, stray gun shots or a player's proximity. When startled, the Witch will target the player responsible for disrupting her and can incapacitate or kill them in the first hit. Players often must use stealth, breaking from the usual run and gun action present in other portions of the game.
  • The Tank, by far the biggest of the special Infected, serves as a kind of boss character. As a hulking monstrosity, players have to combine their efforts in order to bring the Tank down before he can use his highly-damaging melee attacks to incapacitate players - often he is capable of doing so on the first or second hit, and can even knock environmental objects such as cars or dumpsters onto players as well.

Recognizing the cognitive behaviors of players allowed Valve to fine-tune and develop the special Infected to make them efficient, challenging and able to play off of each other's strengths and weaknesses. An example of this can be found in the development of the Boomer special Infected.

Initially, the Boomer first was an enemy that players wanted to avoid shooting at close range, as upon death he would detonate and cause heavy damage to nearby players. Its role of drawing down hordes of Infected onto players was held by another type of special Infected called The Screamer, a prototype enemy type created early in development whose role was discussed by team member Jaime Sue:

"The trick to the Screamer was that if he saw you, you had a moment to kill him before he ran away. Once the Screamer got away from the Survivors to a hiding place, he would emit a loud howling scream that would cause a huge mob of zombies to attack the Survivors. While there were several exciting moments of knowing you had to chase him down to shoot him before he screamed, dodging zombies all the way, ultimately it proved too confusing for the Survivors to discover how he worked, or even to reliably notice him in the crowd."

Article Start Previous Page 2 of 4 Next

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