GDC 09: Creating Replayable Cooperative Experiences in Left 4 Dead
Valve’s 2008 cooperative zombie shooter Left 4 Dead meticulously blends the best of the studios’ critically-acclaimed singleplayer, story-driven titles, like Half-Life and Portal, and its beloved multiplayer shooters, like Counter-Strike and Team Fortress 2. At the Game Developers Conference on Thursday, game designer Mike Booth spoke at length of the team’s approach to cooperative game design during the production of Left 4 Dead.
To begin, the team outlined its project goals for the game. It started with four observations:
- A perceived gap in the market for truly cooperative gaming
- Valve’s own experience creating singeplayer games epic in scale and narrative
- Multiplayer builds community and generates long-term retail sales, as evident by Team Fortress 2
- Valve’s own experience with online multiplayer A.I.
Valve considered the first point to be a major risk for the studio. It questioned at first whether or not “random players in the wild” would cooperate with one another. For any cooperative game to be a success, the team needed structure and mechanics that would motivate players to “do the right thing,” and recognized it would have to require cooperation between players throughout the course of play, and not just at specified moments.
An early driving design principle was to “ensure cooperation was the only winning strategy.” With all these constraints in mind, the team settled on the survival-horror genre fairly quickly. “For one, it’s a well-established mainstream genre for which everyone knows ‘The Rules,’” said Booth. These rules, according to Valve:
- The good guys work together to survive, while the jerks who abandon the group die horribly
- The enemies are savage and seemingly endless
- The good guys are obviously outnumbered and at a clear disadvantage
Valve envisioned a game where players would be faced with wave after wave of fast and vicious zombies, The Horde, and would need to rely on their collective might to overcome insurmountable odds.
To foster cooperation, Booth and his staff decided to treat the entire Survivor team as “the player,” regarding it as one singular unit. Non-cooperative behavior would be penalized harshly and sharply. Abandoning the team would bring about a swift demise. At the same time, Valve wanted to avoid any artificial or arbitrary enforcement, like invisible leases, teleporting laggard Survivors, dealing “out-of-bounds” damage, and so forth.
While the Horde itself was a formidable enemy during early playtesting, Valve saw a need for a more diverse collection of enemies. Each one of the so-called “Special Infected” would not only help to break up the monotony of the main horde enemies, but actually served to reinforce the cooperative aspect of play. During playtesting, Booth noted, “teams that moved like a SWAT team tended to dampen their own drama.” So designers created The Smoker, whose long retractable tongue would pull apart tightly coordinated teams and create unexpected moments of chaos. “We added The Smokers to ensure their perfect plan got messed up.”
The fast and vicious Hunter would teach Survivors that playing as a “lone wolf” wouldn’t work; his debilitating pounce attack could quickly incapacitate solo players. The explosive Boomer was created to break the “shoot everything that moves” rule. He would force players to think before firing. Also, players covered in Boomer bile were subject to Valve’s moments of “dramatic anticipation,” fearfully awaiting the onslaught of the Horde.
Taking the concept of the Special Infected one step further, Valve created two “Boss Infected,” designed to make the Survivors immediately reevaluate their tactics, encourage team communication and foster on-the-fly strategy, and generate dramatic anticipation. The massive and powerful Tank would serve to halt the forward momentum of the Survivors and would demand the full-attention of the whole team. Furthermore, it forced the Survivors to act defensively, a change from their normal tactical approach, and make them reevaluate their surroundings, as a Tank would be able to throw large projectiles at players, topple pillars around them, and launch them off of buildings.
The second Boss Infected, The Witch, was also designed to break the “shoot anything that moves rule” and force Survivors to progress stealithly and cautiously. Her sorrowful cries and incidental music would create powerful dramatic anticipation for the players by notifying them early of her presence.
The team discovered during playtesting that limiting resources, like weapons, ammo, and health, further encouraged cooperation. This lead to the implementation of mechanics that allowed players to heal one another, not just themselves, and give away their own supplies to those most in need. This sharing helped build group solidarity and to break the ice between teammates who hadn’t previously played with each other. “The game is not really about killing zombies,” explained Booth. “It’s about your teammates.”
Survivors who have their health drained to zero don’t die immediately in Left 4 Dead. Instead, they become “incapacitated,” and lay on the ground, slowly bleeding out until a teammate can revive them. This mechanic was implemented to make players feel fearful when separated from the group and reinforces team cohesion. “Putting a player in a clearly helpless and dependent state demands cooperation,” said Booth. It also gives players the chance to help an incapped victim and become “the hero.”
Valve implemented what it calls a contextual “vocalization system” to help improve situational awareness (“Witch!” “Here they come!” “Grenade!” “Pills here!”), communicate short term goals (“Get me out of here!” “The subway is just up the street!” “Get to the chopper!”), and encourage cooperation through basic camaraderie (“Thanks for that!” “Don’t worry, I got you!” “You’re alright!”).
Booth spoke of moments of “dramatic anticipation,” essentially “where event ‘X’ implies interesting event ‘Y’ after a short delay.” “The anticipation of imminent reward or punishment is very powerful,” the designer explained. “If an event is exciting, it will be more so if it broadcasts its impending arrival.” A few of the game’s many examples include the Boomer bile covering a player, “crescendo moments” during which the Survivors are forced to alert the Horde, finale escapes, ledge-hangs, car alarms, incapacitations, and a zombie simply pounding his fists against a door.
“Structured unpredictability” was also an integral part of Left 4 Dead’s design, which Valve defines as a “collection of interesting possibilities selected at runtime using intentionally designed randomized constraints.” One example of structured unpredictability present in the game is “adaptive dramatic pacing,” essentially a method of algorithmically adjusting the game’s pacing on-the-fly to maximize the “drama” players experience. Constant, unchanging combat grows tedious very quickly, but long periods of inactivity are boring. Thus, the much-discussed “A.I. Director” was born.
The A.I. Director algorithmically drives overall pacing and creates peaks and valleys of emotional intensity for the Survivors by modulating the Infected population between four states:
- Build Up: Create full threat population until Survivor Intensity crosses peak threshold
- Sustain Peak: Continue full threat population for 3-5 seconds after Survivor Intensity has peaked
- Peak Fade: Relax period; switch to minimal threat population
- Relax: Maintain minimal threat intensity for 30-45 seconds
This system helps ensure the game is different every time and maximizes the “memorability” factor, which Booth considers to be the coupling of low-probablity and high drama. Manually-placed scripts and triggers, which Valve managed to avoid implementing by-and-large, kills replayability (as players memorize scripting locations), removes any potential drama (the player will always know what will happen next), and diminishes cooperation (creates a “race” mentality).
While creating Left 4 Dead was a decided risk for Valve, its keen focus on creating a stand-out cooperative experience paid off in spades. While Valve does not release Steam sales numbers, Booth concluded with the news that the game has sold 2.5 million retail units worldwide since its release a mere six months ago. With the first downloadable content pack for the game just around the corner, Valve has created a title that will keep players coming back for more zombie brain splattering time and again.
