Developer Commentary (Left 4 Dead 2)

The Developer Commentary is an extra mode in Left 4 Dead 2. In it, a player is able to listen to audio comments made by the developers, while they play through a campaign in the game with a significant lack of difficulty, as all Infected will ignore the player and only attack the AI Bot Survivors. The mode takes place in The Parish with icons called Commentary Nodes throughout a campaign, which trigger a specific audio comment when the player presses the "use" key while looking at the icon with your crosshair. Some Commentary Nodes, when activated, will spawn objects, characters, and even scripted events. Though there are enemies in the game, there are no Special Infected unless the player has spawned them by a node, and the Common Infected generally ignore the player unless the AI Survivors are all dead, in which case Common Infected will attack the player. No common infected will spawn in Crescendo Events or the finale. If all of the AI Bots are dead, spawned Special Infected will go idle. Due to this, no Achievements may be earned, but it is a way of practicing a number of skills.

Freezing problems while loading forced Valve to remove the Developer Commentary option from the Xbox 360 version of Left 4 Dead 2, but it has been fixed and restored back into the game.

The Waterfront
Number of Nodes: 11

Welcome
[Node next to the starting point ]

[Gabe Newell] Hi, my name is Gabe Newell, and welcome to Left 4 Dead 2. We love this style of zombie-driven cooperative gameplay. Tom Leonard and the rest of the Left 4 Dead 2 team had a great time building on the design and game mechanics of the original and we hope you have as much fun playing the game as we did making it. To listen to a commentary node, put your crosshair over the floating commentary symbol and press your use key. To stop a commentary node, put your crosshair over the rotating node and press the use key again. Some commentary nodes may take control of the game in order to show something to you. In these cases, simply press your key again to stop the commentary. Please let me know what you think after you have had a chance to play Left 4 Dead 2. I can be reached at gaben@valvesoftware.com. I get about 10,000 emails each time we release a game, and while I can't respond to all of them, I do read all of them. Thanks, and have fun!

Campaigns
[Node in front of the fence]

[Scott Dalton] We did a lot of work in Left 4 Dead 2 to make every campaign unique in terms of flavor and game play. This drove our decisions in regards to locale, lighting, population, music, and events. We wanted to create a continuous set of locations proceeding through the South, with the key goal of making each location iconic and evocative. We drew up an overall world map of our version of the South, and charted out a journey through a variety of locations. While each location has its own particular common zombie types, we also further differentiated each campaign by creating "Uncommon" zombies that were thematically tied to that location. The uncommon zombies also had specific game play mechanics that made them unique. In each campaign, the music blended specific elements of the location with the central recurring Left 4 Dead theme. We also designed new elements in each campaign where the Director could take control in interesting ways - be it weather, unique paths, or types of Crescendo Events and Finales.

Wound System
[Node in front of the market entrance]

Note: This node repeatedly spawns Common Infected.

[Gray Horsfield] Since players spend the vast majority of their time shooting the common infected, we wanted to improve the feedback and visceral nature of this experience. In Left 4 Dead 1, we provided only the ability to shoot off limbs with blood decals for bullet hits appearing only on the PC. Now, in Left 4 Dead 2, there are 43 unique ways to damage an infected - from gunfire through melee weapons, all the way up to explosive damage. Because many of these wounds are non-fatal, players are able to wound an infected more than once, resulting in about 780 possible damage combinations. To create the appearance of a wound, we project a texture modified by an ellipsoid that culls the pixels of the wounded area from the infected, creating a cavity for the wound to fit into. To avoid memory overhead, instead of creating wound variants for each of the infected, the wounds are spawned as separate objects that work for the entire horde. Our scripting system allows us to spawn specific wounds from specific weapon hits. For example, the sniper rifle headshot explodes the head, and the axe creates a slash across meaty areas of the body.

Body Piles
[Node& over a body pile]

[Marc Nagel] The body piles were generated by tossing self-colliding ragdolls into a level in-game, then exporting out the result. We were able to use a debugging tool to drag them into artistic piles - content creation at its most enjoyable. Unfortunately, the process was so efficient that it took only about two hours.

Sound Design
[Node on a balcony overlooking the street ]

[Mike Morasky]The guiding sound design principles in Left 4 Dead, as well as Left 4 Dead 2, were to stay as organic as possible and to reflect the world within which the game takes place. For example, the monsters are all former human beings, grotesquely transformed; and as such, all of their vocalisations are human performances with very little effects processing, if any at all. The challenge with the new character sounds in Left 4 Dead 2 was to keep them clearly audible for game play, reasonably believable at unrealistic distances, and clearly identifiable as unique. The existing bosses already used the pitch spectrum for differentiation, so we expanded into more characterizations. The Charger mutters unintelligibly to himself; the Spitter is trying to screech out her bile hairball; and the Jockey...well, who knows what he's on about?

Enemy Variation
[Node next to the broken-down bus ]

Note: This node repeatedly spawns a group of Common Infected.

[Ricardo Ariza] We wanted to improve the variation on the infected while keeping their memory footprint identical. The entire horde is never comprised of more than five head textures and five body textures. We use luminosity lookup into a gradient texture for tinting variation, allowing us to not only get a hue variation but also luminosity variation; for example, we can make a black or white T-shirt out of the same texture map. The gradient is broken up into zones, so that we can tint areas of clothing and skin differently. The infected texture also includes four distinct masks for blood and detail such as dirt or pond scum. We randomize both the gradients and the masks each time an infected is spawned. We also randomize the body and the head meshes, resulting in nearly 20,000 available variations in a typical map - up more than ten-fold from the original Left 4 Dead.

Music
[Node next to the jukebox in the bar]

[Erik Wolpaw] When deciding what to do with the music for Left 4 Dead 2 we faced some interesting challenges. Some of the music in Left 4 Dead 1 plays an iconic and important role in game play, and we felt that it shouldn't really be changed. On the other hand, the game is set in the Southern United States, which is rich with musical identity, so we also felt that adding some local flavor to each campaign would really help set the tone for that campaign. The solution to bridging the gap between the new "local" campaign music and the more traditional horror music from the first game was solved in several ways. First, we kept all the original themes from Left 4 Dead 1 but arranged them in a style consistent with the local campaign's theme. Second, we wrote an overarching set of cinematic "southern goth" pieces for the entire game. Finally, we wrote new pieces for the new characters in the style of Left 4 Dead 1. By doing all this, we establish that these are new characters in new places but they are sharing the experience of everyone else in the Left 4 Dead universe.

Infected Textures
[Node in a back alley behind the bar ]

Note: This node spawns a Common Infected.

[Bronwen Grimes] The infected textures are part hand-painted, part photographic reference. One of our team members had a nightmare folder full of photographs of people suffering from bizarre diseases and injuries. They were so hard to look at that the infected actually contain none of these. Instead, the secret ingredients for infecting normal-looking human textures are photos of housing insulation and potato skins.

Glowing Details
[Node in the back of the kitchen]

Note: This node spawns a Common Infected, a Hazmat-suited Infected, and a Worker Infected.

[Thorsten Scheuermann] We wanted to invoke the feeling that the infected have lost their humanity and behave like feral animals. Seeing their eyes glow in the dark like those of a deer in headlights helps illustrate this. To create that effect we authored the shader for the infected so that an artist could mark the regions that should reflect light towards the viewer in a texture. The artists quickly realized that they could also use this feature to make retro-reflective safety materials on clothing, which they put to use on several of the uncommon characters. You now see this effect on riot gear, CEDA hazmat suits and construction vests. As a side-benefit, we found that the retro-reflective effect also helped players identify targets in very dark areas.

Explosion Debris
[Node in the restaurant]

Note: This node spawns a Boomer.

[Sergiy Migdalsky] This restaurant was one of the first areas we populated with tables and chairs. In early playtests, a Survivor would pop a Boomer, and the resulting explosion sent tables and chairs flying everywhere. It was a great effect. We decided to furnish a few more areas like this, so that more players would be likely to experience it at some point in the campaign.

Pipe Bomb
[Node outside of the safe room]

[Torsten Zabka] The pipe bomb has always been a crowd pleaser, but in Left 4 Dead 1, we were forced to disable ragdolls and gibs and replace them with a blood mist due to performance issues. This time, with a new wound system, a new ragdoll solver, and new particle effects, we've finally returned to our original vision. The pipe bomb causes visually catastrophic damage to the infected horde, to the delight of gamers and designers alike.

The Park
Number of Nodes: 13

Narrative
[Node in the safe room ]

[Chet Faliszek] In Left 4 Dead 2 we wanted to give players who were looking for a narrative a little more. To do this, we introduce the four Survivors to each other and the infection as we begin the game. This lets us see the world changing through their eyes. And the world is changing, each new Campaign shows a different stage and reaction to the infection. We start with CEDA's naive underwhelming response and end with the Military's cold but needed resolve to save only those they can. We also connect each Campaign. So while you know you escape Whispering Oaks in Dark Carnival, you also know that is just one escape of many on your journey to safety in New Orleans. Time does pass between campaigns, but each campaign starts with the previous rescue vehicle. We also continue to spread hints and clues to the infection. There is more "Story" in the dialogue this time around, but players should still search for and read graffiti and notes in the world. Observant players in Left 4 Dead will notice some tie-ins to the story as both games are set in the same world.

New Special Infected
[Node in the street outside of the safe room ]

Note: This node spawns a Charger, Spitter, and Jockey consecutively. The Charger and Jockey will attack AI Survivors within range, but the Spitter will usually run away, eventually disappearing. The Survivor AI seems to almost completely shut down during this node, seeing Survivors chase after a Jockey riding a Survivor but doing nothing to shoot it off, and letting themselves be repeatedly meleed by a Special Infected without reacting.

[Tom Leonard] The new Special Infected were designed with a variety of goals in mind. They needed to fill in gaps left by the other Specials; they needed to provide interesting combos with other Infected; and they needed to offer a new, fun set of skills for players to master, whether they were playing as or against the Infected. When we began Left 4 Dead 2, we had a list of several dozen Infected types that had been discussed over the past several years. We added a fair number of new designs to those, and eventually whittled our list down to the final three: The Charger, the Spitter, and the Jockey. The best survivor teams stick together and buckle down under attack, quickly fending off infected attacks. To provide an opening for the infected to capitalize upon, we created the Charger. His charge attack not only separates members from the group, but will bowl down tightly-clumped survivors, giving a few crucial moments for other infected to attack. The only non-boss infected who is immune to being bashed, his design encourages players to keep a quick trigger finger and a watchful eye on their companions, while also keeping their distance. The Spitter's Area of Denial attack serves a variety of purposes. While the initial pool of acid permits a few moments for escape, its damage increases as the acid begins to do its work. Lingering in or passing through the pool of acid is a dangerous proposition, and it can quickly force a group of survivors out of a tight spot, or drive a wedge between members of a group. The Jockey is the one Special that retains control after he's begun his incapacitating attack. He's also the only special that allows a survivor enough control to fight back. A well-played Jockey has many ways of driving a survivor to a deadly end, either at the hands of other infected or through the environment itself. Just getting a survivor far enough out of the group for another special to prey upon is often the Jockey's greatest strength. All of these special types provide unique opportunities for combined attacks alongside the classic Hunter, Smoker, and Boomer.

The Park
[Node in front of the entrance to the park ]

Note: This node takes control of the camera to get an overhead view of the park.

[Dario Casali] The park started out having several paths that were dynamically changed for added replayability. However, playtesters found the paths confusing. When we removed the dynamic objects that forced survivors to go in certain directions, we found that having all paths open led to a better experience. Players still spent a lot of time in the park, but they could choose to go different ways every time they played the level.

Ammo
[Node next to the ammo pile near the horse statue ]

[Matt Boone] With the added variety of weapons in Left 4 Dead 2, we wanted to encourage players to scavenge weapons and switch between them frequently. We found that placing ammo piles scattered through the level fought against this goal as players tended to find one gun they liked and then never switch, as long as they had practically endless ammo. We experimented with removing ammo piles from most of the environments so that the only way to get more ammo was to find a new gun, and we immediately saw players switching weapon types more frequently. We also experimented with giving players deployable ammo packs, similar to the upgrade packs such as the incendiary and explosive ammo that are in the game today. Carrying an ammo pack was perceived as being less valuable than carrying a healthkit though, and many players bypassed the item, counting on finding another gun along the way. We found it was important, however, to continue to place ammo piles in the finale areas, to give players a home base to fight from in the longer set-piece battles.

Animation
[Node at the exit of the park]

Note: This node spawns a Spitter and a Jockey

[Karen Prell] The animators experimented quite a bit with the movement styles of the new special infected in order to help them contrast with the existing specials. Some early passes at the Spitter featured awkward, bouncy, bird-like movements with knock-knees and pigeon toes. She certainly looked different, but it was a little too humorous and didn't project enough of a feeling of strength and danger, so she ended up with much more aggressive posing and actions. We tried a few animal-inspired runs for the Jockey, including sideways, leaping hops like a lemur and a four-legged, hyena-like gallop. These fit with his maniacal laughter, but again, needed to be more aggressive. Such movements also would have caused problems maneuvering the character and reading its intentions in game, especially in versus mode. They also seemed too much of a mutation from the character's original human state when considering the amount of time from the start of his infection. The Jockey's final movement pass has him on two legs with just the right touch of animal inspiration - a menacing, predatory hunch and hands bent rather like a praying mantis, ready to give survivors a big, deadly hug.

Hallways
[Node in the narrow alleyway]

[Chris Carollo] In Left 4 Dead 1, long narrow hallways usually meant relative safety for the survivors. One player could cover the front and another could cover the rear, while players in the middle healed. With the addition of the Charger, however, the long narrow hallways in Left 4 Dead 2 turn out to be extremely dangerous.

The Overpasses
[Node under the destroyed overpass]

Note: This node takes control of the camera to view the destroyed overpass.

[Randy Lundeen] The two overpasses in this area continue throughout the rest of the campaign, and eventually meet up with the bridge that the survivors must cross to escape. When we initially designed the campaign, we planned for these overpasses to guide players to their eventual goal.

Gauntlet Mode
[Node in the army trailer]

Note: This node takes controls of the camera to get an overhead view of the gauntlet.

[Aaron Barber] The alarm scenario at the bus station was the first Left 4 Dead 2 Crescendo Event to feature the gauntlet mode, where players must navigate along a path to deactivate the zombie rush. This proved to be a successful approach to countering the camping strategy of finding an optimal corner and holding out.

Weapons
[Node next to the army tent]

Note: This node spawns a variety of weapons.

[Tristan Reidford] Left 4 Dead 1 had a good, fictionally justifiable cast of guns--the M16 being a standard police and military weapon, for example. In 2 we had the opportunity to do something a bit less generic, and we use some more interesting guns, whilst still being aware that we had to justify their place in the game. The Desert Rifle, for example, is used to suggest that the military rerouted guns meant for the Middle East back to deal with the domestic crisis; hence the desert camouflage scheme on the gun. The ever popular AK47 (which is actually part AK74) is so plentiful that it's easy to justify its appearance almost anywhere, whereas the silenced SMG is more of an underground criminal-looking weapon, so it's interesting to think how this has found its way into the survivors' hands. The hunting rifle from Left 4 Dead 1 is back, and we've added a military sniper rifle which has double the clip size and is more of a scoped semi-automatic assault rifle, rather than the classic bolt-action sniper rifle, which would be ineffective against the horde. Left 4 Dead 2 also features a variety of pistols including a custom civilian handgun, a standard issue police sidearm, and a large caliber magnum.

Player Guidance
[Node further in the gauntlet]

[Brandon Idol] Because Left 4 Dead 1 took place entirely during the night, it was fairly straightforward to light the path we wanted players to take. For campaigns that take place during the day, we had to find other tools to help the player along. The most valuable tools were visible landmarks, such as gas station signs, overpasses, the bridge, and smoke from wrecked aircraft.

Adrenaline
[Node next to the alarm button over two adrenaline shots]

[John Guthrie] As with our efforts to replace the first aid kit, replacing the pain pill item was a challenge. Initial versions of Adrenaline allowed players to run faster, and perform certain actions faster, such as reviving other players. But the pills already give 50 temporary health points, which is enough health for injured players to run to full speed again, so the adrenaline needed something extra. We expanded the actions that could be affected by adrenaline, shortening the time it takes to use healthkits, defibrillators, and upgrade packs; we sped up the player beyond their normal maximum speed and made them resistant to being slowed down by hits from the infected. Although it doesn't give as much health as the pain pills, we found many interesting strategies began to emerge from the well-timed use of adrenaline.

Special Infected Sounds
[Node outside of the bus station]

[Bill Van Buren] As we added more Special Infected characters to Left 4 Dead 2, we faced a challenge in designing unique sounds for them that players could easily recognize and distinguish from the other Specials and from the Common Infected. In Left 4 Dead, the four male Specials each had a distinct tonal range and a character attribute to keep them unique - for instance the Boomer's vocalisations were kind of a slow, deep, bassy gastric horror. As we added the new Specials to the mix, it became increasingly difficult to avoid infringing on the sonic space of the previous specials. We found that we couldn't rely so much on using the tonal range for distinction and instead had to push harder on characterization to help them read clearly to players. For example, the Jockey is in the high, bright tonal range of the Smoker, and even the upper end of the Hunter, but since the infection has left his brain in a constant state of hysterical mania, his vocalizations read clearly apart from these other specials. Sometimes it can be challenging to find a characterization that will lend itself to both a Special's active and idle or lurking modes. For instance with the Charger, we started out with a non-verbal angry muttering, kind of a griping bark that we all liked. Though his personality in his idle states was interesting and distinct, we found that we needed to ramp his vocalizations up to a more sustained yell for his attacks to get the right sonic intensity for these events. In subsequent playtests, however, we found that players were often surprised when attacked by the Charger, as they would associate him only with the aggressive attack vocalizations that he make from the point that he sees them through his charge, and since he would generally see players before they saw him, they didn't have a chance to connect him with the angry non-verbal mutterings of his idle states. So we ended up recording new vocalizations for these states, mixing in elements of his more strident calls and yells amid the gripes and barks, so that players could more readily make the leap to recognizing a lurking Charger in the vicinity.

Jockey Animation
[Node outside of the safe room] This node spawns a Jockey that will attack AI Survivors when they are within range.

[John Morello] The source engine's animation system is a powerful tool that lets us create animation procedurally instead of authoring each of them individually by hand. There are various reasons for doing this, which become clear when we consider the challenge of animating the Jockey riding a Survivor. From the start of the Jockey's design, we wanted the Survivor to be able to fight back to some degree. With that in mind, we decided it would be vital for the Survivor to know the Jockey's intent, regardless of how successful he was at acting on it. So even though you may be stuck against a table going nowhere with a Jockey on your back, we want you to understand that the Jockey is really trying to get you out that door at the end of the room, and around a corner where your friends can't help you. To accomplish this, we implemented a system of animation layers for the survivor that would work in conjunction with the Jockey. These layers are attached to different controls that the code interacts with, called "pose parameters". The first layer consists of the survivor's upper body from the waist up, which allows the survivor's reactions to work in sync with the Jockey's motion. The second layer is the Survivor's locomotion, constrained to the lower half of his body from the waist down. So, in the scenario we just described, when the Jockey is in full control and has a clear path to the door, the code tells the upper body of the survivor and the body of the jockey to lean in that direction, and sets a pose parameter for the survivor's legs to move in that direction as well. However, things get more interesting when the Survivor regains some control and steers himself to get stuck on the table. Even though you have stopped moving towards the door, we don't want the Jockey's animation to suddenly sit straight up and give the impression he isn't still trying to steer you there. By splitting the survivor's animation into 2 different animation layers that we composite together, your upper body and the Jockey can remain leaning towards the door, while the code tells the legs to stop walking because you're stuck on the table and can't advance. This robust system of procedurally compositing upper and lower body animation layers and position in realtime, as we showed in this case, saved us a lot of authoring time as well as memory inside the game - both of which are finite and very valuable on any project.

The Cemetery
Number of Nodes: 10

Playtesting
[Node in front of the safe room].

[Charlie Burgin] Up until Left 4 Dead 2, all playtests conducted at Valve required an observer from the team to sit behind each tester and observe them directly. For Left 4 Dead 2, we developed a system where a single observer could view each playtester's screen in parallel with webcams that captured the playtester's reaction. We recorded every playtest so we could go back and reference specific moments at will. This meant that most of the team could continue working during the playtest - which often takes several hours out of every day.

Returns
[Node on top of a bus]

[Alireza Razmpoosh] This drop onto the bus shows a good example of what we call a "return". If a survivor walks on top of the bus, he has a height advantage against the infected. However, a Smoker can pull him down off the bus so that he has to go around and up the stairs to get back with the rest of the team.

Incendiary Ammo
[Node in a tool shed]

[Eric Strand] Setting fire to zombies was always a high point in Left 4 Dead and we wanted even more ways to light zombies on fire in ''Left 4 Dead 2. ''This dream gave rise to incendiary ammo! Initially, we dispensed incendiary ammo, and later explosive ammo, from fixed boxes placed through the levels. These boxes could be used many times by the player. But while this was fun for playtesters, we found they were either hesitant to progress away from the item, or hoarded the special ammo until they really needed it. We tried replacing one of our previous experiments, the ammo pack, with a version that deployed a one-use-per-player ammo upgrade box. This allowed players to carry the upgrade pack until they sensed they would need a little more firepower in an upcoming arena, and the player carrying it gets the satisfaction of providing a substantial buff to his teammates.

The Spitter
[Node on top of a trailer]

Note: This node spawns a Spitter.

[Peter Conig] As with the Smoker, the Spitter has a gameplay style which necessitates making her more visible to the player at a distance. Since the Spitter's strategy is to hit and run, striking at the survivors from far away, we needed distinct visual markers to differentiate her in the dark spaces during the chaos of combat. To this end, we concocted a glowing drool that would illuminate her and create a sharp contrast to the other game colors, while playing off the "Green Death" motif associated with the infection. The trails she leaves, which is visually similar to her projectile attack, allows players a brief chance to track her movements when she retreats to hide.

Lobby Browser
[Node outside the burned-out house]

[Zoid Kirsch] After we shipped support for custom add-on campaigns via the VPK format, we realized that while many people can install and play these campaigns, it was difficult to discover what add-on campaigns other players were playing. We also noticed that few players were joining games of different difficulties since it wasn't clear how many other games were available on each setting. To solve this, we implemented a browser that shows nearby lobbies. The lobby browser displays the campaign, the difficulty level, and how many lobbies have open slots are available. We also display lobbies for campaigns you have not yet installed, offering the chance to download them directly. Once the lobby browser was released, we saw a sharp increase in the number of custom campaigns being played. Custom survival maps are particularly popular, with themes ranging from supermarkets to medieval castles.

Laser Sights
[Node inside a house]

[Jay Pinkerton] The laser-sight weapon upgrade came from an experiment during the development of the first ''Left 4 Dead. ''Though we didn't end up shipping them at the time. While developing Left 4 Dead 2, we were talking about interesting items that players could scavenge out of the levels, we remembered the visual impact of those red laser beams. The original intention was to increase the accuracy of the guns, but the more valuable benefit was accidental: you could now see where the other members of your team were aiming. We also liked the choices that confronted players when they had to decide whether or not to swap out their upgraded weapon for one of a different style.

Impound Lot
[Node at the beginning of the impound lot]

[David Sawyer] After Left 4 Dead 1 shipped, we wanted to experiment with an "optional Crescendo Event". The impound lot is the final version of that experiment. It's difficult for a survivor team to navigate without setting off a single car alarm, and if they manage it, the experience of sneaking through an obstacle course full of traps is itself quite rewarding.

Defibrilators
[Node beside an ambulance]

[Lars Jensvold] Since managing health with the first aid kit is such a large part of the player's decision process, finding a new item for the backpack slot proved to be difficult. We wanted a new item to be as valuable as a health kit in certain situations and have a big benefit for the team - as opposed to being merely another offensive weapon. It needed to have a user interface similar to the health kit, requiring a target and an action that take some time to perform. We liked the idea of an item that would act like a "get out of jail free card" - an insurance policy in case of a tank or witch attack. Our exaggerated version of a hospital defibrilator immediately conveyed the purpose of the item, plus it gives the survivors a chance to yell "Clear!"

The Cemetery
[Node at the beginning of the cemetery]

[Kim Swift] Something that was always sad for us to watch in Left 4 Dead was when players would always choose an optimized path through a level once they played through a map a couple times. That meant the hard work we put into other areas of the map would never be seen. So one thing we wanted to add in Left 4 Dead 2 was the ability for the AI Director to change the path of the survivors through a level, so they have to take a different path each time. The cemetery contains one example of this new approach. There are four different paths that the AI Director can create for the survivors. It does this by spawning in and out particular crypts and gates.

New Survivors
[Node further in the cemetery]

[Jeremy Bennet] When we started development on Left 4 Dead 2, we decided early on to cast a totally new band of survivors in the lead roles. This involved extensive concept sketches, then a period of evaluating actors with the help of several talent agencies. Once the actors were cast, we brought them for a full costume photo shoot, and conducted 3D scanning of their facial features. This reference was then used by our artists to create high detail digital sculpts, which formed the basis of our in-game characters. The goal from the beginning was to provide the player with a lineup of believable survivors - real people juxtaposed against extraordinary situations.

The Quarter
Number of Nodes: 13

Nick
[Node outside the safe room]

Note: This node will spawn a non-interactive version of Nick.

[Andrea Wicklund] The writers originally devised a biography for a convict character. There was an idea that here in the lawless world of the infected, this escaped convict, sick of wearing prison clothes, had taken the effort to loot a very expensive white suit from an abandoned clothing store. He might as well survive the apocalypse in style! This concept gradually evolved into the riverboat gambler.

Bourbon Street Balcony
[Node on a balcony over the street]

Note: This node takes control of the camera to show off the street

[Jess Cliffe] This balcony was designed as a "fish in a barrel" moment where the survivors can come out to the street and pick off a bunch of the wandering infected without being in much danger. We felt it was a good introduction to our interpretation of Boubon Street, giving a nice atmospheric vista without too much immediate threat.

Item Spawning
[Node next to the garage]

[Ryan Thorlakson] Level designers in Left 4 Dead 1 placed individual items, such as pipe bombs, molotovs and pills, all over the levels. The AI Director had the ability to pick which instance of each item to spawn. This meant that pills would never spawn in the same place as a pipe bomb or molotov. In Left 4 Dead 2, we decided that we wanted more variety in item spawning, so we now simply place an entity called a "weapon_item_spawn." The AI Director not only chooses whether or not an item spawns in a given location, but also chooses what type of item will appear.

Jazz Club
[Node inside the jazz club]

[Paul Graham] We deliberately sought to design iconic French Quarter locales that would be fun to fight in. This jazz club gave us a nice large interior space that offered a break from exterior streets and alleys.

Melee Weapons
[Node near the pool table]

Note: This node spawns an axe

[Gary McTaggart] The challenges with melee weapons is getting them to look right and feel decent. In order to get the fire-axe to look right when playing the game, it had to cross the screen within 3 or 4 frames. To enhance visibility, we swap between a normal axe and a smeared version, along with a particle effect to cheat a motion blur. The smeared axe hits at the correct time, but allows your eye to follow the action.

Temp Voices
[Node in the alley]

[Kelly Thornton] Because sound is such a crucial element in Left 4 Dead, the designers needed temporary sounds to test out ideas for the new boss infected. Sometime we were able to pull temp sounds from unused performances in our library; but in Left 4 Dead 2 the sound designers did their own temp performances. The goal with these is to be just functional enough to test the new characters' viability. As such, some of the temp voices for the characters were quite funny. While inappropriate for the actual game, some of the voices were highly popular within the team, and it was common to hear animators up and down the halls mimicking the early Charger test calls.

Splitting Up
[Node on the balcony]

[Phil Co] Left 4 Dead 2 's level designers started out by experimenting with variants on the proven designs of Left 4 Dead 1. For example, we tried out several situations where there was a benefit to splitting up the group. We soon found that Left 4 Dead 1 had done such a good job of training people to stick together, it was incredibly difficult to get them to separate. We abandoned that experiment, but retained some of the elements to apply to crescendo events. For example, in the mall gun-shop sequence, it can sometimes be advantageous to send one or two survivors to retrieve the cola while the remaining survivors stay on high ground to provide cover.

Traditional Crescendo Events
[Node on top of the roof]

[Kerry Davis] This is the only "traditional" crescendo event in The Parish campaign. In Left 4 Dead 1, every crescendo event was essentially a button press where players held out in one spot until the infected stopped coming. While we added the onslaught and optional crescendo variations, we felt that the old school Left 4 Dead style event still had some life in it.

The Float
[Node on top of the parade float]

[Danica Wright] Looking at reference from New Orleans, we saw a lot of images of Mardi Gras. We decided we couldn't have a campaign set in that region without a parade float. We designed our float to look a bit sinister. Whereas most jester heads are typically bright and goofy, we wanted ours to be a bit darker and more scary - so we made the model appear more angular. We also chose colors that were less cheerful and more in keeping with a Fall festival.

Add-On System
[Node in the alley]

[Mike Durand] Valve has a history of supporting community developers and helping to promote their creations and with the Left 4 Dead games we've taken the opportunity to make the great content our community produces even more accessible with our new add-on system. Content creators can package campaigns into a single add-on file which end users can easily install. Users can now view titles, descriptions, authors and other information for all of their installed add-ons via an in-game interface. Add-ons work seamlessly with the matchmaking system so that users can quickly get into games featuring community campaigns which they've already installed and can easily download new ones to try. The response to Left 4 Dead add-ons has been very positive and we were happy to see huge, all-new campaigns being played within weeks of the system going live.

Motion Capture
[Node in the street]

[Nick Maggiore] Given the realistic nature of Left 4 Dead, players expect the zombies and survivors to move in natural ways. However animating that kind of motion by hand is very difficult and time consuming, and anyone can tell when it's not quite right. Since we don't want every zombie to move the same way, a large number of motions are required to provide enough variety. All of these constraints drove us to use Motion Capture. The process of motion capture really begins with the character designs. Once we've decided how we want the survivors and zombies to look and behave, we audition actors just as we would for any other performance piece. For instance, a 265 pound high school coach moves very differently from a 115-pound TV producer, using one actor for both roles would be a mistake. So we look for people who fit the body type and who can bring out the character's personality with their motion. On the other hand, in the case of the zombies, we consider them more like feral animals, so we only use one actor for all of them, regardless of gender. We then provide the studio with a digital character, a list of the moves we would like to do, and the props we'll need. Once actors are chosen, we head to the studio and spend a day with them recording the motions we need for the game. It's a very physically demanding day, and in order not to exhaust the actor early in the session, we oscillate the captures between high and low intensity actions. Just try crouch-walking 40 feet, like our survivors do, and you will feel the burn - but that's just one of hundreds of motions we record for each character. During the session we can watch the actor's motion applied in real time to our character model and work with them to get the look we want. In some cases we do multiple takes to let the actor provide us with different perspectives or to add some variety. We also recorded the actor on video during the motion capture to provide extra reference. With the motions recorded, we let the studio process the data, then it is up to us to extract the motions we want to keep. For some motions we need to create loops. For example, to create a run cycle, we may only use half a second from the three seconds of running we recorded at the session. We augment these recorded motions with adjustments or additions by hand, and then compile them into the character models you see in-game. Realistic human motion is very subtle and noisy, and we can all tell subconsciously when it isn't quite right, even if we can't articulate why. Without motion capture, the characters of Left 4 Dead wouldn't be nearly as convincing or interesting as they are.

Uncommon Common Infected
[Node in the alley]

Note: This node spawns a riot cop infected.

[Ted Backman] After we shipped Left 4 Dead, we heard a lot of positive responses about the infected that were unique to their locales. There were patients in the hospital, cops in the streets, and TSA agents in the airport. We wanted to add further variety to Left 4 Dead 2 's campaigns, so we decided to add an uncommon common infected for each campaign. The first to be added was the CEDA guy. Once we got him in the game, we realized that his uniqueness should not merely be visual, but should extend to gameplay as well. In the case of the CEDA guy, his hazmat suit suggested that he should be impervious to fire. We extended this notion when we added the Mud Man, the Clown, the Construction Worker, and the Riot Cop.

Dynamic Car Alarms
[Node next to the overturned truck]

[Jason Mitchell] In Left 4 Dead, our designers could place alarmed cars in levels. When shot, these cars would flash and make noise, attracting the infected horde. Unfortunately, the cars would be located in the same position on every playthrough, taking away from the desired unpredictability. In Left 4 Dead 2, designers can now place groups of alarmed cars and let the AI Director mix and match which cars have active alarms, if any. For example, in the section of the game that you have just played, there are three such cars whose alarm state the director can control dynamically, making this area less predictable from game to game.

The Bridge
Number of Nodes: 10

Bridge Finale
[Node next to military radio]

[Ido Magal] This finale felt risky at first, since it involved the players moving in a straight line almost the entire time. We wondered if the experience would feel rich and varied enough. However, given the fiction and fantasy of the space, we found players were constantly making small movement decisions that kept them interested and immersed, and our concerns about monotony fell away. In addition, while we originally designed it to require constantly moving forward, we found that different players have different comfort levels for moving and shooting. Some players just can't do both at the same time. So we tuned the director to adapt to different play styles adjusting the pacing so that different players would still have a good sense of progress and be able to reach the end.

Multi-Purpose Assets
[Node next to bridge ramp]

[Matt Wright] Game assets are more likely to make it through production if they serve the game on multiple levels. Take this bridge ramp for example: Movement-wise, it acts as the starting gate. Audio-wise: it acts as the starting gun. Visually, it blocks and breaks up a potentially uninteresting view. Fictionally, the seizing malfunction and resulting loud bang make it clear that the players have attracted every infected on the bridge.

Left 4 Dictionary
[Node next to the falling truck]

[Steve Bond] One of our most important tools for creating and evaluating environments was what we called the "Left 4 Dictionary." This collectively developed set of terms, ideas and gameplay grammar describes spaces via their physical properties, functions to gameplay, and the relative advantage or disadvantage to the Survivor or Infected sides. This provided the team with a shared language for types of gameplay spaces, with examples such as capillary, close quarters, open space, funnel, king of the hill, etc. It also allowed the team to talk about and explore new permutations of spaces in a very clear way. The Left 4 Dictionary proved instrumental in allowing us to efficiently refine our environments, and tune difficulty based on an understanding of how each space would influence the gameplay. It was especially useful in this bridge map, where we start with a very uniform bridge, and instead of rooms, hallways, etc., we create desirable game spaces using vehicles and damaged portions of the structure.

Elevation
[Node over hole in bridge]

Note: This node takes control of the camera to get a wide shot of the bridge.

[Chris Chin] Most of the region where Left 4 Dead 2 is set is flat. There are few hills or changes in elevation. But maps without any elevation change can be monotonous, both visually and in terms of gameplay. That's why this double-decker bridge was a visually strong set-piece; it added the much-needed element of verticality to the campaign.

Weather Effects
[Node on upper deck of bridge]

[Tim Larkin] Stormy weather effects give the Director yet another way to influence gameplay by altering the players' sensory capacity. One goal of weather effects is to draw the team together and put them in a defensive stance, such as taking refuge within a building. In this respect, weather is much like a Crescendo Event, but because it's entirely under director control, it can happen any time, any place. The storm itself is created using a combination of particle effects, fog, tone mapping, local contrast, post-processing, DSP alterations, and sound effects. While technically a storm can strike in any environment, we chose to limit it to the Milltown campaign in order to provide a fictional background for the flooding of the maps. Thus the storm is thematically tied to a new navigational challenge, and creates a new experience for that campaign.

Grenade Launcher
[Node above weapons cache]

[Noel McGinn] The grenade launcher came about purely out of our desire to see more explosions and more zombies killed by explosions. We found that something interesting happened when we gave one of the four survivors a grenade launcher: They became more vulnerable to close-up attacks, and their teammates adjusted their strategy accordingly. A pattern emerged, where one player would lob grenades from afar at approaching mobs, and the other players would clear off the stragglers when they got too close.

Water Rendering
[Node above damaged bridge portion]

Note: This node takes control of the camera to provide a wide shot of the water under the bridge.

[Alex Vlachos] The new campaigns in Left 4 Dead 2 featured a lot more water than in the original game. Therefore, it was a priority to improve our water rendering. One of the most important new pieces of technology enabled artists to paint water flow uniquely on each water surface. This allows us to make the water look like it's flowing in any direction we desire, without any constraints. This not only looks better, but it aids gameplay by subtly guiding the player where they need to go. As an example, in the swamp levels, the water usually appears to flow towards the end of the level. And water is far more convincing when you can actually see it flow around obstacles like trees and rocks.

Matchmaking System
[Node on top of a bus]

[Vitaliy Genkin] Traditionally, players were connecting to our multiplayer games by selecting a server with available player slots from a very big list. Players might select a server by an appealing server name or network latency. This worked well for 24 or 32 player games where many players could have a great experience even without tight cooperation with their teammates. But for the Left 4 Dead franchise a different approach was required. We wanted to build a group of 4 players who could start a campaign together, possibly as complete strangers, but quickly acquire the feeling of being a bonded team. Valve matchmaking operates on a vast set of player statistics, from headshot percentages or friendly fire incidents, to the likelihood of sharing a first aid kit with a wounded teammate. In the year since Left 4 Dead's release, our matchmaking system has undergone many changes, resulting in better player experience and longer average team playtime together. With the introduction of new game-modes, we now even measure performance of the entire team and do our best to provide the players with reliable teammates and worthy opponents. We continue to gather and analyze an abundance of statistical information from our real-world players, and use that to improve our matchmaking system.

Improved Source Technology
[Node next to the military vehicle]

[Brian Jacobson] For Left 4 Dead 2, The Source engine received many new features and optimizations that enable it to handle denser, higher resolution environments, effects, and characters. This gave us leeway to fit additional weapons, creatures and larger environments into the same footprint as Left 4 Dead, while increasing the quality of each element across the board. Many of our particle effects use several new techniques which at least double their visible resolution while requiring no additional memory. This has enabled us to create complex weather effects such as heavy thunderstorms in the Hard Rain campaign. The new zombie horde is considerably more variable than that of the first game, with higher resolution meshes and skinning. Not only do the infected look better, but using a combinatorial meshing and shading system, there's over ten-fold the number of variations possible in Left 4 Dead 2 while consuming no more memory. The environments also feature considerably more polygonal density than the first game. Many elements that would have been painted into a texture are now fully modeled in the environment. Foliage looks cleaner while also responding naturally to weather conditions. Additionally, water has gotten a major visual upgrade over Left 4 Dead 1, with far superior lighting, reflections, and flow mapping. All of these pieces come together to improve the visuals of the game while keeping it smooth to play.

Modeling World Environments
[Node next to the helicopter]

[Yasser Malaika] The source engine uses two different methods for modeling world environments: a volume-based system for general forms and a polygon-based system for details. Each has its own advantages and disadvantages and they must be integrated together to create a game map. One of the challenges associated with this setup is that the two systems have mostly separate authoring pipelines, and it's easy to make a change in one and break a tightly integrated set. As part of making the modular "kit of parts" that was used to create this bridge, we developed a new hybrid workflow that allows content authors to create and iteratively edit both types of world geometry simultaneously in a single tool. The resulting modules are pre-optimized "Lego blocks" that can be fitted together, allowing the level designer to focus more on gameplay and less on modeling.