Gameplay
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 one (or two pistols) 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 movement, interaction, healing, and teammate revival 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, grenade launcher or the M60 machine gun can also be carried in a similar manner. The player 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, whereby they fall to the floor and need to be helped up by a teammate, 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 first-aid kit, 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 two to five chapters. For all but the last chapter, 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 finale, 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 named Versus is available, where in alternating matches, one side controls the Survivor characters while the other controls Special Infected. 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 type of Infected. 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, or where two teams compete to fill a generator with as many gas cans 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.
AI Director
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 was 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 play the games despite the limited number of maps available due to the unpredictable nature of online play. [21] In considering this for Left 4 Dead, Valve identified that many games use static events that always occur at fixed points 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. [22] They recognized that such systems do not 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. [21] 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. [21] Valve has further termed this approach "procedural narrative", creating a new story each time the game is played. [23]
The AI Director has several facets that combine to form "structured unpredictability" for each playthrough. [21] 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. [21] The placement also considered the "Escape Route", the shortest path through the map, and will increase encounters along this route to increase the difficulty. [21] 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. [21] Procedural generation is also used to place weapons and other items throughout the level. [21] 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. [21]
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. [21] 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. [21]
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". [21] 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. [21]
The Director also creates mood and tension with emotional cues, such as visual effects, audio cues for specific types of infected, and character communication. [24] 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. [25] [26] 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. [27]
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. [28]
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. [29]
Infected characters
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 weak, but in large numbers, they pose a significant threat to the Survivors. Often, the Director will engineer a "horde", where a large number of Common Infected charge the players. Hordes can also be created by setting off in-game "crescendo events" that usually involve some loud noise created by the players, or by 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 introduced in Left 4 Dead 2, are the "Uncommon Infected". Uncommon infected are unique to certain campaigns and 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 sound of pipe bombs.
The third type of Infected 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. A Survivor that is attacked by these Special Infected is rendered completely incapable of taking action, encapsulating their intent: the victim must be rescued by other Survivors as they cannot help themselves. Special Infected 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' Versus and Scavenge modes.
- Hunter - a nimble and agile Infected that wears a hoodie and is able to leap large distances and pounce on a single Survivor, who will take damage until either they are killed or the Hunter is either killed or shoved off.
- Smoker - an Infected with a long tongue that he uses to ensnare and strangle Survivors, rendering them helpless until the tongue is hit or the Smoker is killed or stunned with explosives or shoving. When killed, he leaves a cloud of smoke that temporarily obscures the Survivors' vision and makes them cough.
- Boomer - a slow-moving bloated Infected that vomits bile onto Survivors; though not harmful itself, the bile attracts Common Infected to the character and obscures their vision for a time. When killed, a Boomer explodes, covering any nearby Survivors in more bile.
- Spitter - an Infected that spews a large amount of acidic "spit" at the Survivors. Survivors standing in the "goo" suffer quickly-increasing damage until they move out of the puddle. The Spitter herself is quite weak and upon her death, she leaves a small puddle of acid that also damages Survivors.
- Charger - an Infected with a large mutated arm that attacks by charging Survivors at high speed. The charge attack is capable of knocking down and effectively stunning every Survivor at once, and the Survivor at the forefront of the attack is carried as far as the Charger runs or until the Charger hits a wall, at which point the Charger begins to pummel the Survivor. Survivors being pummeled are completely helpless and will be killed unless the Charger is killed. The Charger is also unique among the Special Infected in that, due to his hulking size, cannot be shoved; only frag rounds and explosives can stumble a Charger.
- Jockey - a small Infected with a hunchback that can "ride" a Survivor, hence the name. A Jockey can control a Survivor's movement while riding them, steering the Survivor into danger. A Survivor suffers damage as long as a Jockey rides them and is completely helpless unless they are incapacitated or the Jockey is shoved or killed.
Finally, there are two special "boss" Infected characters that the AI Director includes at rare moments:
- The Witch - a female Infected with glowing eyes and long clawed fingers. Witches are unique in that they are passive, preferring to be left alone to cry. However, if a Survivor "startles" her, either with their flashlight, by invading her space, or by damaging her, she will charge that Survivor and automatically incapacitate them. Once incapacitated, the Witch will viciously attack the downed Survivor until either they are killed or she is, or if she changes targets by being set on fire. If she kills the Survivor who initially disturbed her, then she will run away. On higher difficulty levels, a Witch can automatically kill a Survivor. She is particularly durable, and can run faster than Survivors, making it difficult to slay her before someone takes damage. She is the strongest and fastest Infected on foot. However, she has the second most hit points, behind the Tank. She can be stunned by explosives and frag rounds.
- The Tank - an enormous Infected with superhuman strength. The Tank can punch Survivors and smash them to the floor, or can throw debris and rocks, or punch cars, dumpsters, and other large objects at them (the latter resulting in instant incapacitation). Kiting is a valid tactic, but Survivors must do so for an extended period of time: a Tank on Normal difficulty has ten times the health of all the Survivors combined. In Versus, players may not control the Witch, and may only play as a Tank if the AI Director has already chosen to spawn one. He is the second strongest infected and has the highest number of hit points by far over all of the other infected. Due to his gargantuan size, the Tank can only be stumbled by exploding Boomers, oxygen canisters or propane tanks.
