The Left 4 Dead franchise is based on a series of cooperative first-person shooter, survival horror video games developed by the Valve Corporation. Set in the days after a pandemic outbreak of a viral strain transforming the majority of the population into zombie-like feral creatures, the games follow the adventures of four survivors attempting to reach safe houses and military rescue while fending off the attacking hordes.
The games encourage cooperative play between up to four players, each taking the role of one of the survivor characters and the computer controlling any unassigned character. Players use a combination of melee weapons, firearms, and thrown objects to fend off attacks from the bulk of the infected creatures, while using an assortment of healing items to keep their group alive. Certain unique infected creatures pose a more difficult challenge, typically requiring teamwork to take down effectively. The games are overseen by an AI called the Director, designed to give the players a more dramatic experience based on their performance, penalizing players for stalling while rewarding players with special weapons by taking riskier paths.
The Left 4 Dead franchise take place in the days following a pandemic outbreak of an infection that transforms humans into feral, zombie-like creatures, seeking to kill those yet to be infected. Within the United States, the Civil Emergency and Defense Agency (CEDA) orders the creation of safe zones with the aid of the military, and evacuates as many people to these areas, aiming to transfer them to islands and ocean-going ships, as the infected are unable to cross bodies of water. However, it is revealed in Left 4 Dead 2 that, in its failures to halt and control the spread of the infection, CEDA was stripped of its responsibilities by the U.S government. In its place the military assumes total control of the situation and gives the civilians some rights banned to them by CEDA, such as the encouragement of using firearms and all other weapons.
A number of people are discovered to be immune to the infection, though they can carry and unintentionally spread it to others. In both games, four of these carriers meet and become the game's "Survivor" characters, making their way to safehouses and extraction points. The first game takes place in the Northeastern United States, beginning in Pennsylvania and ending in Florida and features the characters of Bill, an old Vietnam war veteran; Louis, a frazzled manager; Francis, a biker; and Zoey, a college student who loves horror movies. The second game takes place in the Southern United States, beginning in Savannah, Georgia and ending in New Orleans, Louisiana, and features the characters of Coach, so named for his former employment at a high school; Ellis, a mechanic; Rochelle, a reporter; and Nick, a gambler, con artist and ex-prison inmate. Each character has a unique personality and dialogue, but they are functionally identical from a gameplay standpoint.
The Left 4 Dead games are first-person shooters incorporating survival horror elements. A player controls one of the four Survivor characters, and has the ability to move, jump, and use weapons in their possession. Players are limited to two weapons: a main firearm with limited ammunition taken from ammo caches, and either a sidearm with unlimited ammunition or a melee weapon. Players also have three additional inventory slots. The third slot gives the player a thrown weapon, including a Molotov cocktail, a pipe bomb that can be used to lure a horde towards it before it explodes, or a bile jar that can be used to lure a horde to a specific area. The fourth slot provides for either a health kit which they can use on themselves or the other survivors, a defibrillator to revive a dead Survivor, or a special ammo deployment kit providing unique ammo such as explosive bullets. The fifth inventory slot is used for pain pills, giving the player a temporary health boost, or an adrenaline shot, temporarily increasing the player's speed. Some environmental objects like propane tanks or gasoline cans can be carried and thrown at hordes, upon which they can be fired upon as a makeshift explosive, but cannot be stored in the player's inventory. In Left 4 Dead 2, limited-use weapons such as the chainsaw or the machine gun can also be carried in a similar manner. If the player is currently not wielding a melee weapon, they can use whatever object they are holding to temporarily push back any Infected surrounding them.
A health bar is used to track each character's health; players are aware of the state of each other's health and special items, and each character is shown to other players through an outlined silhouette on the game's HUD, regardless of walls that separate the characters, and colored based on their health state. A character's health suffers from attacks from any Infected, environmental effects such as fires, and from friendly-fire incidents. When a character's health falls below a certain level, the character will not be able to move as fast until they can restore their health. If the player's health drops to zero, they become incapacitated, and a new temporary health bar appears, representing a bleed out period. Should this bar drop to zero, the character dies, and can only be restored either through a defibrillator, appearing later in a level in a "rescue closet" from which they must be freed, or when the remaining players reach the safe house. Otherwise, an incapacitated character can be revived with the assistance of any other character. However, if a character becomes incapacitated three times in a row without using a health item, they will immediately die the third time. Similarly, if a character falls over an edge, they will hang precariously for a limited time, falling to their death if they are not assisted in time.
In the main campaign mode, each campaign is divided into three to five sections. For all but the last section, the goal of the players is to reach the safehouse at the end of the level, where fresh supplies of weapons, ammo, and health items are typically found. In the final section, the campaign comes to a climax and requires the players to either make a stand against waves of Infected as they wait for a rescue vehicle, fill up an escape vehicle's gas tank while fending off the horde, or race through a gauntlet of Infected to make their way to the rescue point.
Additional game modes are also available. A 4-on-4 competitive mode is available, where in alternating matches, one side controls the Survivor characters while the other controls unique Infected creatures. When playing as the Survivors, the goal remains the same as the normal campaign mode. The Infected side tries to prevent the Survivors from making their way to the safehouse; should they be killed by the survivors, they will respawn later as a new creature. Scoring is based on how far the Survivors get and other factors, with the team with the most points at the end of a campaign considered the winner. Other modes are based on single-situation standoffs where the Survivors have to hold out as long as possible.
In Left 4 Dead 2 two additional modes have been introduced. In "realism" mode, several of the video game aspects are removed from the game, such as the identification of the location of teammates via their silhouette, the respawning of dead characters later in rescue closets, and more severe damage models. The sequel also features "mutations", game modes based on either the campaign or competitive modes where specific rules may be in place. For example, one mutation may give all the Survivor characters chainsaws from the start, while another may make every unique infected appear as a specific type, such as the Tank.
Both games support user-generated content through new maps and campaigns using tools provided by Valve; Left 4 Dead 2 also provides support for custom mutations. Valve has further supported the user community by highlighting popular third-party maps, and including select ones in software patches for the game.
The Left 4 Dead series uses a collection of artificial intelligence routines, collectively the "AI Director", to monitor and alter the gameplay experience in response to the players. Valve's primary goal with the AI Director were to promote replayability of the games' campaigns, as their previous multiplayer games with this feature, such as Counter-strike and Team Fortress 2 have shown thriving communities of players that continue to the play the games despite the limited number of maps available due to the unpredictable nature of online play. In considering this for Left 4 Dead, Valve identified that many games use static events that always occur at fixed in a level, or limited dynamic events where one of several events could occur at fixed points. Valve themselves had used this idea in some key battles in Half-Life 2: Episode Two where the spawning of Combine forces would be based on the player's location. They recognized that such systems do no promote replayability or cooperation: players could easily memorize where events would occur, and those that had yet to experience the events would slow other players down. With the concept of the AI Director, Valve believed it could capture the same chaos and randomness that would occur in Counter-Strike and Team Fortress 2 in the cooperative gameplay experience, transforming it from simple memorization to a skills challenge. Valve has further terms this approach "procedural narrative", creating a new story each time the game is played.
The AI Director has several facets that combine to form "structured unpredictability" for each playthrough. The Director first procedurally generates a dramatic flow for the level, which identifies the size and location of hordes of common infected and uncommon infected throughout the level. The procedural generation considers each traversable area on the map, using pathfinding algorithms that Valve had incorporated into Counter-strike computer-controlled characters, and the "flow" of the map—the general direction from the start of the level to the safe house. As players progress through the map, the Director will spawn infected in areas near the players and out of sight, while removing infected from earlier portions of the map the players have passed through. The placement also considered the "Escape Route", the shortest path through the map, and will increase encounters along this route to increase the difficulty. Spawning rules are different for each of the infected; hordes are more often spawned behind players to take them by surprise, while special infected like the Hunter and Smoker will spawn ahead of the players, giving their individual AI the opportunity to lay in wait for the players. Procedural generation is also used place weapons and other items throughout the level. Weapons and equipment can be programmed by the map designer to be generated by the Director at fixed points, allowing for some predictability for the players, and to give some creative control to the design for story-telling and visual effects.
Overriding the procedural generation is the aim to create "active dynamic pacing" of the game by continuous monitoring of players, and altering the predetermined pacing to react to this. Each player character is tracked by a metric called the Survivor Intensity, which increases as the character takes damage, becomes incapacitated, or another character dies nearby, among other effects, and slowly decays in time. The Director will alter its previously developed schedule for spawning of infected to build up Survivor Intensity to a certain threshold; when this occurs, the game sustains this peak for a few seconds, then enters a period where it relaxes and reduces the spawning of infected, allowing the players to finish their current encounter and allow their Survivor Intensities to fall away from the threshold. The Director then repeats this cycle until the players have reached the end of the level.
The boss infected encounters are generated through different means, creating a sequence based on cycling without repetition between three situations: Tank, Witch, or "No event". Boss events are then generated based on the number of areas the players have passed through. These events are not modified by the dynamic pacing of the game; Valve discovered when these were controlled by the pacing, players would often be at too high of an intensity to allow these encounters to occur. Keeping the events on a separate track allows the pacing to be unpredictable, further enhancing the replayability goal of the Director. A sample series of encounters (with progress from left to right) during a level playthrough of Left 4 Dead. The top bar shows the original population of common (white icons), special (colored icons) and boss (large icons) procedurally generated by the AI Director prior to starting the level. The second bar is the monitored "Survivor Intensity", with the red "Relax" sections occurring immediately after the Intensity reaches a threshold limit. The third bar is the actual population generated while playing the level; certain common hordes have been removed due to the Intensity being high and in a "relaxed" state, while other hordes and uncommons have been added when the Intensity is very low near the end of the level.The Director also creates mood and tension with emotional cues, such as visual effects, audio cues for specific types of infected, and character communication. Within Left 4 Dead 2, the Director has the ability to alter placement of walls, level layout, lighting, and weather conditions, and reward players for taking more difficult routes with more useful weapons and items. A dedicated Director routine dynamically controls music and other ambient sound effects, monitoring what a player has experienced to create an appropriate sound mix. The process is client-side and done by a multi-track system. Each player hears their own mix, which is being generated as they play through the game, and dead players watching a teammate hear their teammates' mix.
Valve sees the AI Director as a tool to be used for other games. A software patch for the cooperative game Alien Swarm added a game mode, Onslaught, that used a version of the AI Director from Left 4 Dead to dynamically generate enemies for the players to fight.
Valve has investigated the use of biofeedback from the players directly as input for the Director. At the 2011 Game Developers Conference, Valve demonstrated a simple test where a player, fitted with a device to measure skin conductance level, played through a Left 4 Dead 2 level. Certain in-game actions, such as hearing a threatening noise or passing an opening doorway, would cause the skin conductance to rise. Valve postulates that such information fed into the Director could create a much more effective player experience.
The Infected characters within the game are divided into four categories. The most abundant are the "common" infected, the least deformed of the infected creatures. Alone, these infected are generally ineffectual at harming the player, but in large masses, can easily bring down a single player. Often, the Director will engineer a "horde", where a large number of the common infected charge the players. Hordes can also be created by loud noise created by the players, such as activating a car alarm. As such, many individual levels feature a unique, noisy event, such as the raising of a rusty elevator, where a very large horde will be attracted, with the players given forewarning so that they can prepare for the onslaught.
A second type of infected creature, available only in Left 4 Dead 2, are the "uncommon infected". They are unique to specific campaigns and, though not any stronger than the common infected, have a special ability that can hamper the players' progress. For example, the uncommon infected in "Hard Rain" are infected road workers that are still wearing protective ear pieces, making them immune to the lure of pipe bombs.
The third type of infected creature are the "special" infected. Each of these creatures appear less frequently during a campaign, and are designed to keep the players working together as a team. They have very specific vocalizations, as well as leitmotifs which are inserted into the game's soundtrack when the AI Director places one, allowing Survivor players some forewarning. They serve as playable characters for the Infected in the games' competitive modes.
- Hunter - a nimble and agile Infected that is able to leap large distances and tackle a single Survivor, who will take damage until either they or the Hunter is slain. The tackled Survivor is rendered completely incapable of taking action, encapsulating the intent of the special infected: they must be rescued by other Survivors as they cannot help themselves.
- Smoker - an Infected with a long tongue used to snare a Survivor, preventing them from action until the tongue is hit or the smoker is killed. When killed, it leaves a cloud of gas that temporarily obscures the Survivors's vision and makes them cough.
- Boomer - a slow-moving bloated infected that is able to eject bile onto a Survivor; though not harmful itself, the bile lures nearby infected to the character. When a boomer is killed, it sends out more bile, covering any Survivor nearby.
- Spitter (introduced in Left 4 Dead 2) - An Infected that can spit a large amount of acidic bile at the survivors. Survivors standing in the bile zone suffer damage until they move out of the area.
- Charger (introduced in Left 4 Dead 2) - An Infected with a gigantic arm and half the strength of a Tank, the charger races at a single Survivor, pushing everything else out of the way with great force, and pins its victim to the ground, rendering them helpless, and attacking them repeatedly until it or the victim dies.
- Jockey (introduced in Left 4 Dead 2) - An Infected that jumps onto the back of a Survivor and steers them towards danger, such as nearby Spitter acid or Witches. The ridden Survivor has limited control to fight against the Jockey's influence, but cannot use their weapons. Unusually, AI-controlled Jockeys have been known to voluntarily abandon a victim even if that victim has not been incapacitated.
Finally, there are two special "boss" infected characters that the AI Director includes at rare moments. The Witch is an infected woman with glowing eyes. She weeps piteously, but is passive and can be avoided entirely. However, if a Survivor "startles" her, by shining light on her or invading her personal space, she will charge that Survivor, incapacitating them immediately and quickly draining their remaining health. She is not particularly durable, but can run faster than Survivors, making it difficult to slay her before someone takes damage. The other boss infected is the Tank, an enormous infected with superhuman strength. The Tank can charge Survivors and smash them to the floor, or can throw debris, rocks and cars at them, a hit resulting in instant incapacitation. Kiting is a valid tactic, but Survivors must do so for an extended period of time, as a Tank on Normal difficulty has ten times the health of all the Survivors combined. In competitive modes, players may not control the Witch, and may only play as a Tank if the AI director has already chosen to spawn one.
Left 4 Dead was released in November 2008. The first game's developed was started by Turtle Rock Studios, who were bought by Valve during the game's creation, with continued developed occurring at Valve's studios. Left 4 Dead 2 was released a year later in November 2009. Valve attributed the short turnaround between the titles as a result of having many ideas to expand on the first game, but more than could reasonably done through software patching or downloadable content. The fast turnaround of the sequel was initially criticized by many players of the first game, leading to a temporary effort to boycott the second game. Valve helped to placate matters, demonstrating that it was still developing content for the first game, including a crossover campaign, "The Passing", between the characters of both games.
A fan-made remake of Left 4 Dead was developed by independent video game designer Eric Ruth, titled Pixel Force: Left 4 Dead, playing in an 8-bit overhead shooter style instead of a first-person shooter.
Main article: The Sacrifice (Left 4 Dead)Tied to the release of "The Sacrifice" downloadable content, a four-part web comic drawn by Michael Avon Oeming was released to describe the events of the first set of survivors, Bill, Louis, Zoey, and Francis, that led them to meet the second group, Coach, Ellis, Rochelle, and Nick. Each of the parts provides a backstory for each of the first survivor characters at the onset of the pandemic, and also follows directly up on the final campaign from the first game, "Blood Harvest", where the survivors, identified as carriers, are taken to a secure military facility. The use of the comic medium to expand the game's story outside of the game's bounds has been used by Valve before in promoting Team Fortress 2.
Within Left 4 Dead 2, a fictional southern rock band called the Midnight Riders was introduced. Several songs have been produced for the game and tie-in promotions. Two of them, "Midnight Ride" and "One Bad Man", have appeared as downloadable content for the Rock Band series via the Rock Band Network. Valve released a series of holiday cards themed after the survivors, created by Alexandria Neonakis. Three plush toys have been released based on the Left 4 Dead series' infected; first with the Boomer, and later with the Hunter and the Tank. They are to be followed by two more plush toys yet to be released. They were also created by Alexandria Neonakis. Valve sells a number of different apparel on its web site, including t-shirts, caps, and posters, with some of it based on the above mentioned Midnight Riders.
In June 2011, it was confirmed that Valve had signed a deal with NECA, to produce exclusive action figures, which includes Left 4 Dead and other Valve series. The figures are expected to be released in Spring of 2012.