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Analysis: Interplay in  Left 4 Dead
Analysis: Interplay in Left 4 Dead Exclusive
June 10, 2009 | By Gregory Weir




[In this column, writer and designer Gregory Weir takes a look at enemy interplay in Left 4 Dead to highlight how creating complex interactions enhances the player experience.]

Enemies in video games tend to primarily play the role of obstacle. The player character must get from point A to point B, and an army of goons stands in her way, forcing her to take time to thin the mob before continuing on. In most games, these enemies do not cooperate in any meaningful way.

Yes, they are all pursuing the same goal -- kill the player character -- but they do it in a simple way, without taking advantage of each other's abilities. The power of a group of enemies is equal to the total power of its members.

Sometimes, enemies are more complex. Half-Life is one of the first games to use squad-based artificial intelligence, where human grunts share information and coordinate their attacks to better inconvenience the player.

Occasionally, a particular pair of enemy types are designed as partners to work together. It's especially common in strategy-focused games to find a rock-paper-scissors pattern of enemy strengths, but on a small scale this is more about the units' individual weaknesses than any sort of interaction of abilities.

Valve Software's Left 4 Dead, however, has a very complex example of cooperation and interaction between enemies, compressed into just six types of Infected units.

These units, through the interplay between their unique abilities, have more than additive group strength. The power of a group of these enemies is greater than the sum of its members, because of this interaction.

Blood Harvest

The player characters, or Survivors, are threatened by six types of Infected in L4D's zombie apocalypse: Common, Hunters, Boomers, Smokers, Tanks, and Witches. These remain the same in all gameplay modes, although other players can control Hunters, Boomers, Smokers, and Tanks in Versus mode. These six can be separated into three categories: Common, Special, and Boss Infected.

Common Infected are in their own group. They are fragile zombies who go down in just a few shots, but they are great in number. Common appear in two forms: as "wandering zombies," who begin scattered around the world and provide little more than target practice, and "horde zombies," who rush in groups of 10-30 and mob the Survivor players, causing much more danger. All Common Infected are attracted by beeping pipe bombs and car alarms.

At most difficulty levels, the Common Infected are environmental hazards; they obscure vision, block movement, and require the players to take time to thin their numbers. The other enemy types take advantage of the Common Infected to become more dangerous themselves.

The Special Infected aren't much stronger than Common Infected, but they have special abilities. The Hunter can leap great distances and tackle a Survivor, immobilizing and damaging her until the other Survivors come to the rescue. He is also the hardest to distinguish from Common Infected, making him stealthy despite his loud screams.

The Boomer is large and easily-spotted, but he can attract horde zombies by vomiting on the Survivors, or simply by being close enough to them when he explodes on death. The Smoker is the easiest to spot due to his cloud of smoke and spores, but he is able to pull Survivors with his tongue.

These three combine in powerful ways. The ability of the Hunter to instantly incapacitate makes it a bad idea for the Survivors to split up, while the Boomer's bile attacks are more effective if the Survivors are in a tight group.

The Smoker can pull a Survivor away from the group, which makes the Hunter's pounce more deadly and the horde summoned from the Boomer's bile more dangerous. The Hunter can more easily go unnoticed when the Survivors' vision is obscured by the Boomer's bile. Individually, the Special Infected are fragile enough that they pose only a minor threat, but when more than one attacks at once, they become considerably more dangerous.

The Boss Infected are even more exceptional threats. The Witch can be killed quickly with concentrated, coordinated fire, but if she survives the initial attack she is guaranteed to at least incapacitate a Survivor. The Tank, on the other hand, is simply huge, sturdy, and strong, which makes battles with him last longer than with any other Infected.

These Boss Infected are powerful enough to present significant threats on their own, but when combined with the Special Infected, they become even worse. The distraction of a Special can cause the Survivors to inadvertently set off the Witch, while the Tank is sufficiently distracting in his own right to allow the Specials to be much more effective.

By creating enemies that can combine their abilities to become even more powerful, Valve significantly increased the complexity of L4D gameplay. Instead of simply worrying about six kinds of enemies, the players must worry about 36 combinations. This increases the game's challenge without making any single threat more dangerous. Players enjoy this interplay because it creates memorable situations with interesting solutions.

No Mercy

The technique of enemy interplay can be used in any game which has varied types of enemies. Each enemy should be designed with a specific special ability or set of abilities. The Hunter can leap and pounce, the Boomer can vomit and explode, and the Smoker can pull. The designer should then think about how to make these abilities interact.

Imagine if the Boomer could vomit from a long distance, essentially hurling globs of bile instead of spraying it at close range. This would make it much more dangerous, but would weaken the interplay between it and the Smoker. Because the Boomer has a limited range, it benefits from the Smoker's ability to reposition Survivors.

Paradoxically, this process will often involve weakening individual enemies in order to make them even stronger in combination. Providing an enemy with a weakness lets the designer compensate for that weakness with a different enemy. Instead of two strong enemies working independently, they are two weak enemies working together. This is more interesting and often more challenging than a mob of independent foes.

By creating interplay between enemies, a designer can make a game more interesting and strategic. The extra complexity will be recognized by the player, and will make the gameplay experience more fun and longer-lasting. Games that take "a minute to learn and a lifetime to master" consist of simple elements that combine to provide a hidden complexity, and they have greater longevity and player investment than more straight-forward works. A designer who takes lessons from Left 4 Dead's enemy designs will create a richer, more engaging game.

[Gregory Weir is a writer, game developer (The Majesty Of Colors), and software programmer. He maintains Ludus Novus, a podcast and accompanying blog dedicated to the art of interaction. He can be reached at Gregory.Weir@gmail.com.]


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