Developer Commentary (Left 4 Dead)

The Developer Commentary is an extra mode in Left 4 Dead. In it, the player is able to listen to audio comments made by the developers, while they play through a level in the game with a significant lack of difficulty. The mode takes place in No Mercy with icons called commentary nods throughout the level. These icons trigger a specific audio comment when the player presses the "use" key while looking at the icon. Some commentary nods, 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 until the finale, instead going for the other Survivors first (if all of the others are dead, spawned Special Infected will target the player). Sometimes it glitches and some Infected will attack the player. Due to this, no achievements may be earned, but it is a way of practicing a number of skills.

= Left 4 Dead Commentary =

The Apartments
Number of Nodes: 18

Welcome
[ Node over the First aid kits ]

[Gabe Newell] Hi, my name is Gabe Newell, and welcome to Left 4 Dead. Left 4 Dead is our first attempt to combine the strengths of single player and multiplayer games. We wanted to take the character driven story-telling of our single player games, like Half-Life, and combine it with the social replayability of our multiplayer games like Counter-Strike and Team Fortress 2. Plus Left 4 Dead has zombie hordes and co-op, so what's not to like. 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. 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!

No Mercy Introduction
[ Node in front of the weapons table ]

[Doug Wood] We experimented with a variety of different introductions for each campaign. For No Mercy, we tried a 40 second fly-in of the helicopter to give the player a movie-like introduction to the game. Ultimately, we found that such elaborate cutscenes are hard to watch over and over in a game that’s built for replayability. Playtesters wanted to get into the game and start playing right away, so we ended up going with a much more streamlined game intro.

Development
[ Node beside the rooftop door ]

[Mike Booth] While we were developing bots for Counter-Strike: Source, we discovered that a few of us armed to the teeth with automatic weaponry against 30 knife-wielding enemy bots was a lot of fun. After shipping Counter-Strike: Source in late 2004, we started experimenting with new game prototypes. That basic kernel of 'small team of friends against hordes of clawing enemies' was something we kept coming back to and we soon realised that the 'co-op vs the horde game' had a ton of potential. We started work in earnest on what was to become Left 4 Dead in early 2005 and in about a week we had a very rough, but very playable and fun prototype. We began our daily cycle of playtest/discuss/modify that continued for the next three years. Left 4 Dead is the result of this evolutionary design process.

Griefing
[ Node between the rooftop skylights ]

[Steve Bond] Originally in Left 4 Dead there were some features that made it possible for players to cause trouble for others. This behavior, known as “griefing” can really ruin the experience of a game, especially for newer players. For instance, at this starting location while players were still getting used to the controls, other players might push them off the roof and cause them to fall and die before the game even really gets started. This is one of the main reasons we changed the Survivors’ behavior so they could no longer push each other.

Common Infected
[ Node in the first apartment kitchen doorway ]

Note: This node spawns a non-interactive Common Infected.

[Phil Robb] We treat the ‘the Infected horde’ as a major character in Left 4 Dead, and spent a lot of effort in making their movements believable. This includes hundreds of motion captured animations that are algorithmically blended with the physics systems to create characters that realistically interact with their environment and each other. The common horde stagger around, cough, vomit, fight amongst themselves, lean against walls, sit on the ground, and lie down as part of their ‘wandering’ behaviors. If something wakes them from their stupor, they become ‘alert’ and look around for the source of the disturbance. When they see a Survivor they become enraged and ‘acquire’ that Survivor as a target, taking off at a full sprint, leaning into their turns, jumping and climbing over everything in their way, trying to get to their victim. We wanted to express this rage in their faces as well, so we found efficient ways for each member of the horde to make intense facial expressions. We chose to make their eyes a solid milky white, partially because it looks creepy and disturbing, but also because it’s much less expensive than drawing and controlling separate eyeballs for each Infected.

Extra Areas
[ Node in the apartment bathroom ]

[Phil Co] Left 4 Dead requires a variety of areas off the main path where the Infected can spawn. We learned that once we had created these places, players naturally wanted to explore them. So, fairly late in development, we decided to reward the players for going into these areas by allowing the random creation of bonus items such as pain pills, molotovs, pipe bombs and extra pistols.

Music
[ Node in the lower apartment kitchen ]

[Kelly Thornton] The most common attitude toward music in multiplayer games is that it just gets in the way. We wanted the music in Left 4 Dead to heighten the key emotional elements that should be inherent in a horror game – keeping in mind that everything in a game should contribute to gameplay. Therefore, we made a simple set of goals that the music should meet, to keep people from turning it off. Players who leave the music on are treated to a variety of subtle audio cues that not only deepen the horror experience, but make them better players.

Playtesting
[ Node above the apartment “drop hole” ]

[Marc Nagel] Once a game is in a playable state, we bring in friends and family who have never played the game to playtest specific areas, maps or campaigns. These players bring a fresh perspective and objectivity to the game experience that often gives us new insights into what’s working and what’s not in our games. We observe their play and experience their exhilaration and frustrations. While playing, we encourage them to talk and ask questions, to get a better idea of what they’re thinking – though we let them know that we won’t be able to answer any of those questions or offer them any help. Watching these playtests helps us to tune the game so that the first time players understand the game mechanics and the basic premise for how the game is played, and that we’re achieving the right balance of challenges and rewards. After the play session is complete, there is a final Q&A session where we ask them about their experience to gain more specific insight than we were able to obtain through observation. One approach we use here is to ask them what the best and worst moments of the experience were for them and why. This is especially useful feedback to help us identify and prioritize our key issues in improving the game experience.

The Boomer
[ Node directly outside the apartment exit in the alleyway ]

Note: This node spawns an interactive Boomer.

[Sean Keegan] The designs of the Boss Infected were driven directly from weaknesses observed during our daily playtesting. For example, in the very early days of the game there were only Common Infected. As a result, the Survivors had a very simple rule of ‘shoot everything that moves’. Although this was fun, it lacked tactical depth and variety. The Boomer’s goal was to shake this simple rule up a bit and he became the zombie that you do not want to shoot. Originally, the Boomer just exploded when shot, instantly dealing a lot of damage to anyone nearby, or even the whole team if someone made a bad decision in close quarters. The Boomer didn’t yet have his vomit attack – that horde drawing ability belonged to the Screamer. As the design of the game continued to evolve, the Screamer was cut and his horde attracting ability was transferred to the Boomer’s vomit attack. This attack fit the hugely obese and gluttonous visual design of the Boomer well. Once the Boomer could vomit, we realized that this was a much more interesting game mechanic, and changed the explosion from an instantaneous damage-causing one into the current vomit-splattering one. The Boomer’s vomit has a couple of very interesting game mechanics: along with being temporarily blinded with vomit in their eyes, whoever is hit by the vomit becomes ‘it’, and the incoming horde of zombies only attacks that player, essentially leaving him to the mercy of his teammates’ willingness to protect him. The other players, who have not been vomited on, have to make a quick decision about how to deal with this new situation. Another feature of the vomit attack is that it is not an instantaneous blow that causes damage; unlike the previous exploding version of the Boomer. Instead there are often several seconds of dread and anticipation where the player knows that the horde is coming and can prepare for the onslaught, but there is an uncertainty about how things are going to turn out. This kind of anticipatory drama is amazingly powerful and makes the whole Left 4 Dead experience work well, and keeps the players on their toes.

The Screamer
[ Node in the alleyway beside the garbage bags ]

[Jaime Sue] The Screamer was a boss zombie who didn’t have any actual attacks; he was bound in a straitjacket. You knew when he was around because of his crazed, maniacal cackling. The trick to the Screamer was that if he saw you, you had a moment to kill him before he ran away. Once the Screamer got away from the Survivors to a hiding place, he would emit a loud howling scream that would cause a huge mob of zombies to attack the Survivors. While there were several exciting moments of knowing you had to chase him down to shoot him before he screamed, dodging zombies all the way, ultimately it proved too confusing for the Survivors to discover how he worked, or even to reliably notice him in the crowd. He was cut, and his horde drawing-attack eventually evolved into the Boomer attack.

Francis
[ Node further down the alleyway ]

Note: This node will spawn a non-interactive version of Francis. However, his head and eyes will follow your movements.

[Jeremy Bennett] To overcome the horror and darkness of Left 4 Dead our highest priority was to create four believable people for the Survivors which you could connect with. With the design of Francis we were looking for a large imposing character that would fit in well during the zombie apocalypse. He needed to feel huge, fit believably into the world and read well as a silhouette. The biker look was a strong choice as the read of bare arms, a stained white shirt and a leather waistcoat, all on a six foot five frame, certainly made for an imposing pared back shape that read clearly amongst the chaos. When dealing with an environment that is as dark as Left 4 Dead, a strong graphic read would be essential in combating the visual noise of hundreds of Infected frantically flailing about. The Survivors are in effect visually our last hope and read as warm and enduring people in a world gone mad. The zombies on the other hand are desaturated, fairly cool in their color palette a lack a lot of the contrast of our Survivors. The four characters were designed as a unit so that while they stand out together from the rest of the world, their strong silhouettes and color contrast provide clear reads for each individually yet still signal ‘team’. We wanted to conjure four Survivors who would resonate realism, and thereby complete our horrifically rendered version of the zombie apocalypse which you, the player, now find yourself in.

Visuals
[ Node in the alleyway connecting to the street ]

[Randy Lundeen] To find a coherent visual look for Left 4 Dead, we first had to make choices that would enhance gameplay in a very dark setting. The darkness is crucial to Left 4 Dead as it forces players to stick together and encourages cooperative gameplay. We took advantage of lessons learned from our previous work on Team Fortress 2 – particularly in regard to maintaining the readability of character silhouettes in a dark, chaotic environment. Meanwhile we studied film techniques that have been employed successfully in dark settings, to address the unique challenges and opportunities in the highly dynamic game environment. To accentuate the notion that you’re playing through the ultimate zombie movie with a band of your friends, we added various filmic effects to the Source engine. Using these effects including color correction, film grain, contrast and vignetting – we were able to define a stylistic look that balances visual drama with clear readability for players.

AI
[ Node beside the police car ]

[Mike Booth] When we started what was to become Left 4 Dead in late 2004, or Terror-strike as it was known at the time, our team was quite small, however, we knew we had certain strengths - one of them being 'AI'. AI is just shorthand for 'software that results in apparently intelligent behavior', which can take many different forms. In our case, we wanted to explore procedural content generation; creating an AI algorithm that would generate an endless amount of content for players to consume. The advantages of this are many - not only would the game be infinitely replayable, but we could theoretically create huge amounts of game experience with a small team and some clever programming. The reason we decided on a 'human vs the zombie horde' motif was that the actual core game mechanic - shoot lots of zombies - is kind of a known fun quantity. That allowed us to take more risk on the side of procedurally populating our zombie world, and to experiment with a procedural system that manipulates the dramatic pacing of the game. Taking this risk has really paid off so far. Not only do we, the dev team, still enjoy playing our game after doing so for three years, but the procedural population system has really saved a ton of effort. For example, if we tweak some portion of how the game plays, we only have to do it in one piece of code which then automatically populates all 20 of our maps, and all future maps we or our fans create, in this new way!

Contrast
[ Node in front of the truck with active headlights ]

Note: This node takes control of the camera.

[Matt Wright] Local contrast adjustment is another film effect we've implemented in Left 4 Dead. We control contrast dynamically, keyed to peak events in the game. When everything suddenly gets sharper, this is a visual parallel to an adrenaline rush. We use contrast to emulate a heightened sense of awareness. While players may not be consciously aware of these effects, they eventually come to associate subtle changes in visual presentation with shifts in the game's intensity.

Lighting
[ Node between the truck and the flaming tanker ]

[Matt Campbell] Anyone designing a post-apocalyptic city has to deal with the problem of how to light it. Initially our take on it was that the city still had power. You could see lights in many windows and the streetlights were all lit up. It didn't give us the atmosphere we wanted, but we felt we needed those light sources. When we started to unify the art and lighting design, one of the decisions was to eliminate a lot of lights. This gave us an opportunity to come up with other ways of bringing light into scenes. As we removed lights, we found that we were also removing any sense of normality in the city. Without lit windows, or even the illuminated signage in front of stores, we removed some familiarity from the player's experience. Things are lit from lower angles than they would expect - from car headlights and oil barrel fires. These dramatic angles really enhance the sense of a creepy, desolate, almost hopeless environment. Now it's a city that you've seen but not in a way that you've ever seen it before. Something's amiss, something's off.

Car Alarms
[ Node beside the alarmed car ]

[Brenda Kennedy] Car alarms are an example of what we call 'Panic Events' - brief, intense, and maybe avoidable moments. In the classic post-apocalyptic movie, car alarms go off but there's nobody around to respond. It's just another sign of things having gone down the tubes. We initially put the car alarm cars in just as a cool effect. But when we hit on the idea of having the alarms call up a horde of Infected, we knew we'd found the right use for the car alarms in the world of the game. It's really great when this kind of classic genre element can serve a gameplay purpose as well.

Buildings
[ Node at the dead end of the road ]

[Sean Keegan] Buildings, by nature, tend to be pretty square. If there's a well-lit building with a dark sky behind it, you clearly see the squareness of the building no matter how many AC units, chimneys or interesting shapes are stuck on top of it. So what we did here was flip the typical lighting scheme, to put a bright sky behind dark buildings. Suddenly the pipes and chimneys blend into the building's silhouette. Now instead of seeing individual details like the AC units, you're seeing a much more complex, organic looking skyline in the background. And because the mind expects a similar level of detail throughout an object, your imagination fills in the detail in the dark areas of the building, based on the silhouette.

Teamwork
[ Node outside the safe room ]

[Dave Kircher] Early versions of Left 4 Dead allowed more skillful players to run off on their own without much fear of deadly consequences--bobbing and weaving around the Infected before they could react. The addition of player slowdown, when hit by a Common Infected, severely reduced the player's ability to simply run through the horde. More importantly, the Hunter pounce and Smoker hang presented fatal situations for a player on his own. These additions instilled the fear of being alone even in the most seasoned of FPS players, reinforcing the value of tight teamwork.

The Subway
Number of Nodes: 17

Game Instruction System
[ Node above the ammunition ]

[Jeep Barnett] Through playtesting we found that Left 4 Dead ' s unique cooperation mechanics were tricky even for seasoned gamers to grasp immediately. Failing because of rules that are not clear is never fun. Therefore, we designed the game instructor system to educate players as quickly as possible. Because first time players can potentially join an in-progress game at any point, it was critical that the game instructor dynamically interpret game events. It keeps a running list of the lessons that can be taught in the current context and displays the ones that are most important. It also tracks how many times the player has successfully demonstrated that they've learned the lesson. Once the player has proven competence at any specific lesson, the hint is never shown again.

Contextual Dialogue
[ Node before the escalators ]

[Elan Ruskin] Left 4 Dead evolves some of the contextual dialog technology that we used in The Orange Box. Each Survivor has a large database of lines to choose from based on their present activity and a variety of factors such as their health, stress level, kinds of Special Infected seen so far, and many others. Each line can potentially trigger an automatic response from another character, allowing rich conversations to be dynamically generated based on the players' history together in the game so far.

Weapon Spawning
[ Node beyond the escalators ]

Note: This node will spawn tier 2 weapons on the adjacent table if they are not already present.

[Steve Kalning] Another way we try to give players a new experience every time they play Left 4 Dead, is that in addition to randomly placing bonus items like pain pills and pipe bombs, we vary the location of the more powerful weapons: the autoshotgun, the assault rifle, and the hunting rifle. These weapons will always appear somewhere on the main path, but there are four different areas in the subway station where they can spawn. This means that players can't take these positions for granted on repeated play-throughs and must keep their eyes open for the weapon upgrades.

Advanced Infected AI
[ Node on top of the overturned subway car ]

[Matt Campbell] Although it sounds ridiculous to talk about advanced zombie AI, we spent a great deal of time on the AI systems for the common horde. First and foremost is their ability to navigate. The environments in Left 4 Dead are geometrically complex and littered with breakable and movable objects. One of the design goals for the zombie horde was that there can never be a place where a Survivor can stand that a zombie cannot navigate to. To make this happen required not only robust path finding code, but also path-following algorithms as well. These path followers have to continuously evaluate the local geometry around them and decide whether to crouch, stand, jump, climb over, and otherwise navigate nearly arbitrary environmental obstacles.

Covered Corpse
[ Node on the covered corpse beside the ammunition table ]

[Jaime Sue] This covered body is an example of how we tell a story in the levels without using any words or overt storytelling. We wanted to show that there are other people in the world that are Survivors like you. We figured most people wouldn't survive very long, and you'd come across their bodies—but we needed a way to set these apart from all the dead ragdolls of the Common Infected. By simply covering a body with a blanket or sheet, it becomes really obvious that this guy had a buddy, a friend, and when he went down, his friend had compassion and covered him up. This also tells a story about the state of the world that they wouldn't want to go out and bury the body—they couldn't expose themselves to Infected. So they'd have the bodies right there with them, but covered. Sometimes you'll see a covered body right outside a checkpoint. So it's like they were in a safe area and, maybe during the night, their buddy died from his wounds; so the next morning the other Survivors didn't really want the body in there with them, so they dragged it outside. This was the best they could do for the guy.

Smoking The Set
[ Node before entering the subway car ]

[Lars Jensvold] Smoking the set is a common film technique used to help separate background and foreground elements. We use particles and distance-based fog to do our own version of smoking. Our first experiments proved frustrating for players. We used a realistic dark fog, but this obscured character silhouettes and gave no sense of the vast surrounding environment. Once we lightened up the fog, the maps gained a greater sense of scale and distance, and the readability of characters greatly improved so that players could more easily tell friend from foe. Fog also helped players spot important events like zombies climbing over distant fences. So changes in visual design not only improved the look of the maps, but aided players in anticipating and coordinating their team ahead of attacks.

Player Outlines
[ Node after exiting the subway car ]

[Alex Vlachos] Early playtesting revealed that players often didn't know that other Survivors were in need of help, even though the status of the other Survivors at the bottom of the screen changed to red. When being attacked by a horde of zombies, we found players focusing exclusively on the in-game action, and missing any changes to the HUD and the other player's status. We solved this problem by using glows around the silhouettes of the Survivors' character models. We use red glows when other Survivors are in need of assistance, and blue glows to show the Survivor's location when hidden behind walls or far away.

Flashlights
[ Node after ascending the stairs and entering the large columned area ]

[Jason Mitchell] In Left 4 Dead, players carry flashlights to illuminate the environment. These shadow-casting light sources not only add surface richness and provide important visual depth cues, they tie into gameplay. For example, because a player's flashlight is attached to his or her weapon, the light becomes pointed off to the side when reloading or performing a shove attack. In a sufficiently dark area, this can cause the player to be effectively blind until they are done reloading or shoving back an enemy. Like so many of Left 4 Dead ' s game mechanics, this encourages cooperation, as players know they may be left in the dark if they choose to reload at the wrong moment.

Rescue Closets
[ Node in front of the closet in the far left corner of the columned area ]

[Chris Ashton] Before there were 'rescue closets', Survivors who died during the game had to wait until the next checkpoint before they could rejoin. As the game matured, the time between checkpoints reached 10 to 15 minutes – clearly too long for a player to sit in spectator mode. Although it may seem obvious now, it wasn't really clear how to bring dead Survivors back into the game in a simple and believable way. Once we hit on the mechanic that you're rescuing a Survivor who has barricaded himself in the problem was solved. This solution is typical of Left 4 Dead game mechanics: Hearing a trapped friend call for help creates a dramatic situation for the Survivors, and creates a new short term goal for the team to accomplish. Also, finding a lone Survivor that joins your team is a staple of the horror movie genre. Finally, it provides plenty of opportunities throughout the environment to bring dead players back into the game.

Grenades
[ Node at the Generator room entrance ]

[Scott Dalton] We set out to create an interesting trade off between the pipe bomb and Molotov from day one. Given the limit of a single item in your inventory, the choice needed to be meaningful. From early on the Molotov was a clear winner, providing a dynamic area of denial with interesting strategic purposes and pitfalls, especially against the Boss Infected. The pipe bomb was intended to provide a balance against a rampaging horde, but initially it proved far less successful. Given the frenetic pace of enemies and action in Left 4 Dead with typical engagement ranges of five to ten feet, using a traditional pipe bomb in any sort of intentional way was nearly impossible. You'd see a mob of zombies, whip out the pipe bomb and by the time you were winding up for a throw, they'd already be eating your face. No amount of tweaks to the detonation mechanic or timing solved this issue, it simply wasn't fun. Given that the horde is drawn to shrill, high pitched sounds, we decided to take the home-made nature of the pipe bomb one step further and attached the guts of a smoke detector onto it. This provided just the element that the weapon needed. Against mobs, you now have the ability to draw them to a point and destroy them. It has the strategic advantage of being able to deflect an incoming rush from a user or a teammate who's being overwhelmed and rarely feels like a wasted opportunity when used. Now when presented with a pipe bomb and molotov tradeoff, you have to weigh what situation you are more worried about facing next and how the rest of your team is outfitted. The mechanics of the weapon help to reinforce fictional elements within the world while falling in line and fulfilling our initial goals for its purpose. And seeing a bunch of zombies turn into a goopy red mist as the result of a big explosion never hurts either.

Crescendo Events
[ Node beside the minigun ]

[Miles Estes] Crescendo Events were the result of a lot of playtesting. Playtesters really loved the finales but since it can take like an hour or so to get to each one, they were missing these kinds of intense experiences along the way. What playtesters liked most about the finale was that it was the one time they were able to stop, catch a breath, make plans and set up traps for the Infected. During the rest of the campaign, you're always on the run, reacting to the Infected, and trying to move forward, so these Crescendo Events came out of wanting to get more of that planning and strategy into the rest of the campaign. These events also helped a bunch with team-building. Like the minigun is really important and, if no one's manning it, your team probably won't do too well. But the minigunner is really vulnerable; they have a limited angle of view, they can't watch their backs anymore, so they have to depend on their teammates to cover 'em. With Crescendo Events on the way to the finale, we not only vary the pacing, but we help build the skills and coordination that will enable teams to survive the ultimate battle at the finale.

First Crescendo Event
[ Node in front of the Crescendo Event activation switch ]

[Dario Casali] At one point in the development process, playtesters told us that, with the exception of the finale maps, they found the experience of going through a campaign fairly flat, with not enough peak moments. So we decided to mix up the gameplay by creating Crescendo Events. This generator room was one of the first maps where we tried one of these events. To proceed from this room, the Survivors must turn on the power to open this door, which attracts a large horde of the Infected. These Crescendo events create multiple peak moments that vary the pace and intensity of the campaign, ultimately building up to the finale.

Louis
[ Node in the second floor room with weapons and ammunition ]

Note: This node will spawn a non-interactive version of Louis. However, his head and eyes will follow your movements.

[Moby Francke] One distinguishing characteristic about Louis is that he represents the 'every day' man. While Bill and Francis are characters who obviously live on the outskirts of society, Louis is really the only one in the group who looks like he's trying to hold onto aspects of normalcy and civilization in a world of utter chaos. Designing a sleek fitted silhouette, and adding detail elements such as designer shoes, neck tie, and a designer watch, not only distinguishes his character from the rest of the group, but also gives the player insight into who Louis was prior to the apocalypse.

Dynamic Lighting
[ First node on the street ]

[Chris Chin] Lighting was one of our toughest challenges. We had to find a light level that was atmospheric without striking players as frustrating or unfair. Wherever possible, we let lighting tell the story and enhance the gameplay. Playtests proved that in a dark game, players will go wherever there's light. All we had to do was set up a few critical lights, and players are drawn to them like a moth to a bug zapper. This simplified lighting also made it easier to read silhouettes, so that players could more easily tell Survivors from Infected. And car headlights proved to be one of our most flexible props, casting long dramatic shadows that tell a story of abandonment, while being easily positioned to lead the Survivors where we want them to go.

World Models
[ Node above the car ]

[Tristan Reidford] Usually each model in the game has its own unique texture maps painted specifically for that model, which give the object its surface colors and detail. To have a convincing variety of cars using this method would have required as many textures as varieties of car, plus multiple duplicates of the textures in different colors, which would have been far out of our allotted texture memory budget. So we had to find a more efficient way to bring about that same result. For example the texture on this car is shared with 3 different car models distributed throughout the environment. In addition to this one color texture, there is also a 'mask' texture that allows each instance of the car's painted surfaces to be tinted a different color, without having to author a separate texture. So for the cost of two textures you can get four different car models in an unlimited variety of colors.

The Tank
[ Node to the left of the truck-cab ]

Note: This node will spawn an interactive Tank.

[Kerry Davis] Along with the Boomer, the Tank was one of our earliest bosses. Where the Boomer broke the rule of 'shoot everything that moves', the Tank changed the team's tactics to require immediate and direct cooperation against a single major threat. If everyone stays calm and engages the Tank, they should be okay. If they panic and scatter, or if someone gets selfish and runs off, the team will likely die. One difficulty with the Tank was that since we offset his incredible toughness by slowing him down a bit, when Tank battles occurred in large outdoor spaces, he was far less effective. Coordinated Survivor teams could easily take down the Tank before he would close in on them and cause chaos. To offset this vulnerability, we added his 'rock throw' and car-slugging abilities. Now, if the Tank finds himself far away from the Survivors, he can still be effective by ripping up the ground and lobbing huge rocks at them.

Color Correction
[ Node to the left of the alarmed car ]

Note: This node will take control of the camera.

[Thorsten Scheuermann] One of the film techniques we relied on was color correction. Color correction simplifies and unifies the visual palette. Our color correction allows us to set a specific saturation threshold below which everything falls off toward gray, while critical items like health packs and blood stay bright and eye-catching. We also vary color correction by area. For instance, safe rooms have a warm color quality, evoking safety, in distinct contrast to the cooler palettes of the outdoors, which feel cold and forbidding.

The Sewer
Number of Nodes: 12

Map Population
[ Node outside the safe room in the adjacent doorway ]

[Charlie Brown] Initially, designers placed the spawn locations for common Infected mobs in each map. However, it became obvious after a few play-throughs, that skilled Survivors knew exactly where each of these were, killing the atmosphere of anticipation and constant danger. To address this, we tried creating 'sets' of these mob-generators and randomizing which sets were active for a given session. This helped somewhat, but ultimately it meant that experienced Survivor teams would just have to check a few more known locations. Since a primary design goal of Left 4 Dead was to create an experience that could be replayed indefinitely, we needed a new approach. We decided to go for the ideal, but riskier, solution: A completely procedural approach that would determine where to create mobs 'on the fly' as the team was playing. Although it took longer to build this system, in the end it worked better than we even hoped. Not only were experienced teams no longer able to expect where mobs would hit, but the process of populating maps with enemies was hugely simplified. What used to take weeks of painful iteration and constant updating as maps were changed, became a five minute process that was much more tolerant of map changes and actually played better. It was so successful, in fact, that we can't imagine building Left 4 Dead without a system for this kind of procedural content. We later expanded this system to add wandering zombies to the environments to increase the Survivors tactical choices and add additional variety to the game experience. Once this design decision was made, these wanderers were added to every map in the game with just an afternoon's worth of code from a single programmer. We playtested the idea the same day.

Server Bandwidth
[ Node outside the lit doorway ]

[Zoid Kirsch] Network bandwidth usage when playing Left 4 Dead is a huge concern. We're constantly looking for ways to reduce how much data the server needs to send to each of the players. In most of our multiplayer games, we're only concerned with other players and their projectiles and weapons; but in Left 4 Dead we must update not only the behavior of other players, but those of a horde of Infected. In the hospital campaign, a lot of the surfaces are flat--such as halls and roadways. By making sure that the Infected only send their height when it changes, instead of giving complete position updates, we shaved off almost ten percent of the bandwidth from the server.

Zoey
[ Node inside the restaurant ]

Note: This node will spawn a non-interactive version of Zoey. However, her head and eyes will follow your movements.

[Andrea Wicklund] The process behind designing Zoey's character wasn't as straightforward as the other three. Developing a young female in a zombie apocalypse isn't easy with a backstory like hers. She had to be attractive and worn down at the same time. She needed to be tough and able to hold her own even though she is occasionally terrified. Her readability was also something we had to take into consideration. A bright red jacket solved the problem. It pops out amongst the gritty, desaturated zombies and the dark, subdued environment. As far as the rest of her appearance, there was a little bit of back and forth as her character came into focus through iteration. Finally, we were certain that the character model met our goals, and Zoey unfolded into an everyday young woman who everyone can relate to.

Level Design
[ Node at the end of the overturned truck ]

[Matt T. Wood] Our first maps created specifically for the game were giant, nonlinear city sections. The idea was that the Survivor team would have to find their way through the zombie infested city. Playtesting quickly revealed that keeping track of your teammates and fending off the hordes of zombies completely consumed player's attention. Adding on-the-fly team discussions of which way to go were slow and frustrating. More importantly, however, was that teams quickly found their favorite route, and tended to always run the same way every time, minimizing their on-the-fly decisions. By paring down the city to a representative and fairly linear route, we could better spend our limited resources making that area highly detailed.

The Gas Station
[ Node in front of the gas pumps ]

[Gray Horsfield] Originally, the Survivors' path forced them through a gas station convenience store and out by these pumps. Due to this path many players accidentally shot the pumps from close proximity, leading to a premature and seemingly unfair end to their Survivor experience. Through a cycle of playtesting and iteration we ultimately arrived at this layout, which preserves both the life of the player and the spectacle of the gas station detonating.

Scissor Lift Crescendo Event
[ Node beside the scissor lift ]

[Phil Co] This loading dock was another arena that we designated for a mini-finale. This time, instead of locking the Survivors in a defensive position, we decided to have them ride the lift up to a rooftop encounter with the horde.

Stress Levels
[ Node in the second floor windowed-room ]

[Gautam Babbar] Based on the success of procedurally populating the world with wanderers and mobs, we decided to try to guarantee a consistent dramatic experience for the Survivors. Through playtesting, we observed sessions where some Survivor teams would occasionally get into a downward spiral of being overwhelmed to the point that they were never able to regroup and 'get ahead'. This was partially due to some poor choices of the Survivor players and varying skill levels, but also due to the 'luck of the draw' with the procedural population system. A random streak of 'bad dice rolls' could either be incredibly exciting for some teams, or a death sentence for others. To address this, we created a system that tracks each Survivor's 'stress level' by watching for events like 'how much damage you are taking', 'how many zombies have you killed near you', and so on. If a Survivor's stress gets too high, the system will step in and forcibly throttle back the zombie population system to make sure the team gets a break every now and then. These breaks are critical for a couple of reasons. The primary reason is battle fatigue – constant fighting and gunfire is tiring. Secondly, the team needs an end to these skirmishes along the way to give them a chance to regroup, heal, and reassess. Lastly, these big, exciting battles are only exciting if there are also periods of quiet, creepy, tension and anticipation to contrast them against. This dramatic pacing is really critical to creating a fun experience. Once this system was in place, we realized we had all this useful information on how the Survivor team is actually doing. From a dramatic standpoint, ideally we want to see the Survivor team just make it to the next checkpoint, limping in and being chased by the horde, so they can celebrate when the door closes and they yell 'we made it!'

Film Grain
[ Node in the alleyway outside of the warehouse ']

Note: This node will take control of the camera.

[Alex Vlachos] One film technique we brought to Left 4 Dead was the idea of grain. Film grain is most apparent in dark scenes, which made it an attractive visual effect for our game. Grain does a good job of implying detail in darkness, adding grittiness and texture. However, we found in playtesting that if we applied grain uniformly, people would quickly tire of the effect. We hit upon a method for dynamically varying the grain effect based on light level. Scenes range from grainy texture in darkness and shadows, to no grain at all in the brightest spots.

Speech Randomizing
[ Node in the sewer's T-shaped junction ]

[Chet Faliszek] Left 4 Dead was created with the goal of being replayable. This goal causes a problem for writing where characters will have the same speech opportunity multiple times over a single session. So we have to avoid having the speech become repetitive; we do that in multiple ways. One is to have almost all speech have a random element to when it plays. This stops the player from being able to tune out the speech because it is expected for a certain action. We successfully tested this on Team Fortress 2 and have expanded on that system. In Left 4 Dead not only does this initial speech have a random element, but we also have follow-up lines that may or may not play after the initial line, randomizing the speech opportunity further. We also limit how often some lines play. No matter how witty or funny a line is, you are sick of it the 100th time you hear it in an hour long play session. To address this, our vocalization system allows us to control the rarity of individual lines or even complete speech opportunities. For instance, a speech opportunity may only fire if you are playing the character Bill and Zoey is healing you and she has just healed you a few minutes earlier. This type of randomization based on a combination of criteria help make the speech seem natural and avoid having the player hear it too often.

Versus
[ Node in the sewer room on the way out to Mercy Hospital ]

[David Sawyer] For much of the development of Left 4 Dead, Versus mode was the default and only mode. Balance problems eventually forced us to make it a separate game type. The problem was that if we balanced it to provide the Survivors a reasonable chance of escape, the experience was a long and frustrating one for the player-zombies. Changes that were necessary to make the player-zombie experience satisfying also served to make the experience dramatically harder for the Survivors. Separating Versus mode allowed us to break out of this dilemma, rather than aiming for a 50/50 shot at the Survivors escaping, now we're aiming for a shorter more intense experience where the Survivors are unlikely to make it to the end; their goal is simply to get further than the other team did. It's more fun for the zombies, and your reward for having a harder time as the Survivors is that you get to switch teams after you're slaughtered and do the same to them.

Ladder
[ Node beside the sewer's ladder to Mercy Hospital ]

[Jerry Bennett] This ladder, from day one, has always been a really exciting place in the campaign because it forces the team to separate a bit, and players can't shoot while they're on the ladder. If a team is in great shape, they just fly through this area. But if the team is really hurting, everyone will stop and look at each other, to see who has the most health. That guy will generally go up first because, chances are, there's going to be Infected up there. It's not always the case, but often enough that whoever goes up first is going to take a bit of a beating.

The Witch
[ Node beside the ambulance ]

Note: This node will spawn an interactive Witch. The Witch will attack you if you startle her while you are the only Survivor alive or by shooting her with a Hunting Rifle.

[Jerry Bennett] The Witch is much rarer than the Boomer, and her penalty is much more severe. Basically, if you shoot the Witch you will likely die. The Witch has two functions: one is to provide an exception to the rule of 'shoot everything that moves'. The primary function of the Witch, however, is to force a change of pace on the Survivor team. Ideally, discovering a Witch near your path will result in the team going into 'stealth mode' – shutting off their flashlights and walking carefully around her so as to not disturb her. This is a large departure from the intense and loud pace of normal gameplay. In earlier versions of the Witch's behavior, she was far more dangerous. Once she was set off – she went berserk and attacked all of the Survivors until she was killed. This proved to be too harsh, particularly for teams of new players. One stray bullet could wake the Witch and she would slay the entire team, ending the game. Now she limits her wrath to the Survivor who harassed her. If she successfully kills that player, she retreats from the scene entirely.

The Hospital
Number of Nodes: 15

Statistics System
[ Node in the safe room ]

[Charlie Burgin] Valve has a long history of gathering real-world gameplay data and using that to improve our products. In Left 4 Dead, we automatically gather data from internal playtests and use that to tune the game. We look at every angle, including things like how often Survivors help each other, how effective different types of Infected are, how effective weapons are and which weapons players favor. We learn a lot from analyzing gameplay data and it lets us make decisions based on real information rather than guesswork. We will gather that same gameplay data from our real-world Left 4 Dead players and use that information to continue to improve the game. We'll also share the gameplay data we gather with the community, as we do with our other products.

Death Animations
[ Node at the foot of the staircase ]

Note: This node will spawn three non-interactive and continuously dying Common Infected.

[Miles Estes] Since killing zombies is such a big part of this game, we invested a lot of time into making their death animations more dramatic than simple ragdolls. We had a professional stuntman on a motion capture stage perform about a hundred different dying animations from different kinds of weapons and hit from different directions, like from the front or behind. We then combined these mocap animations with the physics-driven ragdolls. The result is a really cinematic experience, with zombies that stumble for a few steps into a wall, then slide down the wall and collapse, for example. It's the best of both worlds.

The Music Director
[ Node in the hallway after the staircase ]

[Tim Larkin] We took several steps to keep the music interesting enough that players would be inclined to keep it on as they play. We keep it changing, so it won't become tedious. To this end, we created a music director that runs alongside the AI director, tracking the player's experience rather than their emotional state. We keep the music appropriate to each player's situation, and highly personalized. The 'music engine' in Left 4 Dead has a complete client side multi-track system per player that is completely unique to that player and can even be monitored by spectators. Since some of the fun of Left 4 Dead is watching your friends when you're dead, we thought it was important to hear their personal soundtrack as well. This feature is unique to Left 4 Dead.

Infected Variation
[ Node in the open room beside the kitchen room ]

Note: This node will spawn six non-interactive Common Infected.

[Vitaliy Genkin] In Left 4 Dead players are constantly getting attacked by mobs of Common Infected and have lots of opportunities to view individual Infecteds up close. Playtesters tend to notice and remember the variety of faces and clothing of the different types of Common Infected, and it helped to reinforce the sense of a vast epidemic. It was a challenge to create this diversity of looks for Common Infected, and take advantage of unique clothing or uniforms in specific areas, without blowing our model and texture memory budgets. We came up with a system of marking navigation areas to spawn specific populations of Infected. Each population type defines probabilities for which Common Infected versions can be spawned: for instance, lots of TSA agents are spawned in the airport, hospital patients around the hospital, cops in the streets, office employees in the offices, etc. We also paid special attention to Infected heads and upper bodies because playtesters were noticing those parts of Common Infected the most. By randomly selecting from a lot of small head textures packed in a single texture sheet, using random tint colors for clothing materials and using random body groups for Infected bodies we could generate over 1500 different looks for Common Infected.

Bad Mistakes
[ Node up the staircase and through the double-doored office ]

[Mike Booth] The idea that a bad decision by one player directly impacts the whole team is central to the design of Left 4 Dead. It is emotionally very powerful to know that you need your teammates to succeed, and to have clear and visceral feedback that you just made a mistake that is going to cost the team dearly. Examples include: - Shooting a Boomer that explodes on a friend, causing a horde of zombies to attack him

- Causing the Witch to attack, which usually means someone is going to die

- Throwing a bad Molotov and burning your friends to a crisp

- Shooting a car alarm, causing all hell to break loose

- Running away and leaving your team incapacitated and dying

- And of course, shooting your friend in the back as he was limping into the safehouse, incapacitating him while the horde is hot on your heels.

These kinds of actions may not actually end your game, but your friends will certainly let you know just how they feel about them.

Music
[ Node in the open office waiting room on the elevator floor ]

[Mike Morasky] A lot of overreaching dynamic music systems go to great lengths to organize and control a very expressive art form, often to the point of making the results perfectly 'controlled' but also perfectly boring. Our Music Director aims for 'planned serendipity.' By designing the music and rule sets to increase the probability of beautiful happenstance, and to minimize the probability of inappropriate mistakes, we end up with the highest percentage of musical events working as planned, a nice mid percentage of acceptably artful mistakes, and very few actual poor moments. If you over-design the music and rule sets, there are no surprises; and without surprises, listeners are quickly bored. Ironically, by keeping things simple, the music seems planned; greater complexity just leads to greater randomness and many more poor moments.

Bill
[ Node outside the glass quarantine room ]

Note: This node will spawn a non-interactive version of Bill. However, his head and eyes will follow your movements.

[Ariel Diaz] One of our goals when designing the four Survivors was to have a very diverse group of real-world characters who fit in the post-apocalyptic story line, but that also have heroic and aspirational qualities. Bill was a challenging design problem when it came to finding the correct balance between realism and idealism. In our first attempts, we made him look a bit too old and frail, which made him seem physically incapable of fighting against the horde. On later iterations, we erred on the side of making him look too young and handsome, which broke the fiction that we had for the character. In our final design we balanced the character backstory and the aspirational qualities, portraying Bill as a once heroic army veteran, who though older and wiser, can still excel at physical combat.

Box Art
[ Node in the rooms at the opposite end of the elevator hallway ]

[Greg Coomer] Designing the box art for Left 4 Dead was lengthy process, beginning well over a year before the game shipped. All in all, more than 30 versions of the box art were produced internally, each one exploring different ways to visually communicate about the game. When introducing a new franchise like Left 4 Dead, its box design needs to engage people who are less familiar with the game. At first, we believed that in Left 4 Dead's case we needed to be very literal, communicating all of the important things about the game right there on the box-front. We went through many versions of the art featuring four human characters working co-operatively as a team, firing their guns into a horde of zombies, all the while spattered with blood and looking pretty for the camera. As you can imagine, when you add the game's logotype and the other stuff that has to go on the front of the box, things were looking pretty complicated. We kept struggling with those compositions and the feeling that, overall, they felt forced. Finally, with time running out, we realized that the game deserved more than this conservative approach. We began looking for a piece of key art that was more confident, sophisticated, and simple. The image of a hand with four fingers was an immediate hit, both internally and when we brought it to local stores to test it out with gamers -- it presents a grisly visual pun that can't be ignored. We think it's a great iconic representation of the game.

The Elevator
[ Node in front of the elevator ]

[Matt Scott] The Crescendo Event centered on this elevator has a very simple mechanic: survive until the elevator arrives. In order to distinguish it from other Crescendo Events, we added breakable walls that allowed the horde to smash through, surprising the Survivors and providing access for them to attack from all directions.

Dynamic Music
[ Node in the elevator ]

[Mike Morasky] We base the music on what the player is actually experiencing and not what on what we want them to experience. Working from artificial life work our audio designers had done on Lord of the Rings and the Matrix sequels, we implemented a simple system to examine what's going on in the player's immediate environment, then added the appropriate reactive, scalar rule-sets to control the music and its volume level. Most of the more prominent musical cues are thus 'reactive' results from rule sets processing this input--making the musical experience specific to each player. This system also controls the dynamic mix system—another feature unique to Left 4 Dead.

Dynamic Dialogue
[ Node outside the elevator on the 28th floor ]

[Elan Ruskin] Another dialog feature added for Left 4 Dead is the ability for characters to dynamically notice and comment upon objects in their world. In other Source games we could write explicit scenes for supporting characters to act out at certain points in the world, but in Left 4 Dead we can never be sure which Survivors will actually be alive at any given moment. So, the game considers which characters are present and their condition before automatically choosing the best available conversation that includes whoever is around.

Open Construction Area
[ Node when first heading towards the roof edge ]

[Chris Ashton] The first version of this area didn't have any openings on the walls at all. Everything was closed off. We decided that since you were so high up in the hospital and there wasn't any real geometry below, we could add windows and use 3D skybox that would provide this great vista. Once we put windows in, sometimes the Hunter attack would push you out the window. It became scary to get by those windows and once we realized how much fun it was to get near the risky area, we decided to open up this level as much as possible. So, all of the walls around the extents of the level were just removed entirely. We've got a bunch of areas where some of the props and debris in the environment force you to walk even closer to the edge than you might want to.

Ledge Hanging
[ Node on the building edge, past the forklift ]

[Khanh Nguyen] Given all the rooftops in the game, we saw Survivors falling off a lot, and it wasn't much fun. Players would make it all the way up here, heading for the checkpoint, and then a zombie would come around the corner, and they'd back right off the edge of the building and fall. We wanted to give them a second chance, and in the process encourage the cooperative type of gameplay that we were aiming for. So we came up with the idea of having them grab the ledge and hang there until teammates helped them. The nice thing about this feature is that it also builds tension. As soon as you grab the ledge, the clock starts ticking. You watch your character go from hanging with two hands to dropping off to one hand till, eventually, he slips and falls. In the older version, you'd simply fall and that was it. It was over. So solving a problem consistent with our game design principles led to another opportunity to build teamplay and tension.

Audio Silhouettes
[ Node through the doorway at the small edge corner ]

[Tobin Buttram] In playtests, people were often confused by certain events and elements in the game. The addition of musical cues helped distinguish these events and diminish player confusion. This was similar to the way that we experimented with distinct visual silhouettes in order to make our characters stand out in a chaotic environment. Audio silhouettes work on a preconscious level, and are rarely perceived as intentional.

Three Strikes
[ Node further into the construction and towards the safe room ]

[Sean Keegan] During our internal playtesting, we ran into situations where a player who had a habit of making bad decisions kept becoming incapacitated to the point where the other Survivors became frustrated; continually reviving and escorting this player five or six times in the space of a couple of minutes. By adding the 'third strike and you're dead' rule solved this issue in two ways. First, it temporarily took these players out of the game and allowed the rest of the team to proceed and enjoy their experience. Second, the players who died found themselves as a spectator watching the more successful players. This not only gave them the opportunity to learn how the remaining players were surviving, but how they might improve their own performance as well. It also let them cool off and regain their composure in preparation for rejoining the game a minute or so later.

Rooftop Finale
Number of Nodes: 16

Round Restarts
[ Node in the safe room ]

[Phil Robb] Players need to learn from their mistakes as quickly as possible. In early versions of Left 4 Dead, if Survivors failed a campaign at any point, we just put them right back at the beginning and made them play the whole thing over again. That meant that if the team dies in the last level it'll be close to 45 minutes before they ever get back to that map to try something new. This made it hard for players to remember exactly what they might have done wrong and so it was hard for them to decide what they might want to do differently. So now when a team goes down, we just reset to the beginning of the level that they died on. This gives them a lot more freedom and flexibility on how they approach the most challenging sections.

Music Design
[ Node at the top of the staircase ]

[Bill Van Buren] All the music in Left 4 Dead--themes, motifs, hits and effects--are based on a hybrid scale derived from the main opening menu theme. This theme, representing the near death of humanity as it fades to a distance only to be accessed through a radio, is an incremental melodic modulation collecting up all the notes used along the way. This scale is not absolute, and some notes outside the scale are used occasionally to achieve even greater dissonance; but by using this singular and very chromatic scale, almost any piece written in it will generally dovetail pretty decently with any other. At the very least, the pieces seem to spring from the same musical universe. The resulting tunes are deceptively complex--easy to remember but difficult to sing accurately.

Auto-crouch
[ Node in front of the ventilation hatch ]

[Brandon Idol] When players are moving through the world in a panic, and trying to coordinate with their team, the last thing you want them to worry about is which key lets them crouch so they can scramble into a vent. Throughout the game we tried to simplify movement controls so that players could enjoy the terror of the experience with as little frustration as possible. Here, and at similar obstacles, we make the Survivor crouch automatically when they come to a vent. Wherever possible, we try to keep these decisions invisible to players, to keep the illusion that they're moving through this frightening world on instinct—without ever thinking about controls.

Rooftop Finale
[ Node on the hospital entrance rooftop ]

[Chris Ashton] The hospital rooftop is the finale that saw the most iterations; it's the first one that we ever built. The initial design had the Survivors inside of a building, with the helipad outside. You would fight inside the building until help arrived and then you'd go out to the helipad to escape. We found that it was too easy for the Survivors to back into a small room and hold out, just shooting the Infected as they came through the doorway. This killed the drama, because we'd have hundreds of Infected attacking the rooftop but you'd never see them—only the trickle that got through the doorway. We learned through playtests that Survivors have a natural tendency to climb up on things. It's like the Infected are a flood and Survivors are always trying to get to high ground. So we played to that tendency, building rooftop areas that were easily accessible, where the Survivors could navigate from one roof to another. Knowing that Survivors want to get to high ground, we can anticipate how they'll approach the map and we can design around that. For example, the Infected can scale rooftops from all sides, so the Survivors can never back into a corner where they're safe. As a bonus, Survivors get to actually see the Infected running at them from a long distance away—which is a lot more dramatic than shooting at them as they pile through a doorway.

Continuous Story
[ Node on the helipad ]

[Jess Cliffe] Left 4 Dead was originally going to have a continuous story that started with the hospital campaign. When the chopper pilot picked you up, he would start talking about how he had just done a street pickup and wasn't feeling so well. Clearly, he had just been Infected—and the chopper was about to go down. We found that this left playtesters feeling let-down, instead of rewarded for having survived the challenging campaign. We decided to cut that section of the pilot's dialogue and kept the ending upbeat - reinforcing the players' sense of accomplishment.

The Smoker
[ Node next to the rooftop satellite ]

Note: This node will spawn an interactive Smoker.

[Khanh Nguyen] Highly cooperative and experienced Survivor teams can become very good at staying together and watching all around them, to the point that it became difficult for the zombies to cause the kind of chaos and panic that makes the game exciting. The Smoker was created to forcibly pull these tight teams apart. Moving just one Survivor far out of position with the Smoker's tongue attack proved to be enough to break things up and bring the chaos back.

Smoker Effects
[ Node behind the rooftop satellite ]

[Scott Dalton] While special effects seem on the surface to be visual fluff, they also serve critical gameplay functions. To use an example of the Smoker, we can see how his various effects follow his design. The role of the Smoker is to keep his distance, lurking on rooftops, trees, and other out of the way spots to pull a Survivor out of formation and break up the group. As such, unlike other types of Infected, he prefers to keep his distance from the Survivors. In order to give him a distinct and visible silhouette in the often dark and distant locales that he frequents, he has a swarming spore cloud. We use a variety of small techniques here to help guarantee a strong read in all situations. The spores themselves are lit by the local lighting, but always maintain some degree of color, so that the Smoker retains some distinct visibility even in complete darkness. Further, we apply a technique we also use in a variety of feedback based effects such as blood and impacts where the particles have a minimal screenspace size regardless of their distance. This allows the Smoker to retain his distinctive effect at far distances or when silhouetted against a sky, while keeping the actual particle sizes small when up close. Since the Smoker is usually dealt with at a distance, often at angles which make getting a shot difficult, his death must also be clearly broadcast to players so they can tell when they've killed him. This is where the Smoker's moniker comes into play. When he is killed, he releases a large smoke cloud, which is not only a visual pay off for killing the Smoker, but a clear read that he has been defeated and won't escape to attack again. His cloud also serves as a final act of revenge, obscuring nearby players' visibility, as well as hiding Infected within.

Tank Raffle
[ Node halfway down the helipad ramp ]

Note: This node will spawn an interactive Tank. (Only Once)

[Eric Tams] The Tank is an awesome presence on the battlefield. It's the only zombie that can withstand any amount of direct fire. To successfully combat a Tank the Survivors need to work together and focus on their target, and even then the chance of catastrophe is high. With all of this power it's a huge reward when a player gets to play the Tank. We wanted to create something that rewarded player performance so it would feel fair, but we also wanted to make sure that it didn't generate a feedback loop where one player would always end up being the Tank. Often when trying to solve a game design problem like this it's good to look at mechanisms that exist outside of the gaming world that solve the same problems. That way our players will instantly understand the mechanism at play and we can tap into the same excitement that might already exist for the external mechanism. The Tank Raffle has the added benefit of announcing to the Player Zombies when the Tank is created, and peaking anticipation for the most exciting part of the game.

Tank Patience
[ Node at the end of the helipad ramp ]

[Jamaal Bradley] In early playtests, when players controlled the Tank, we had a major problem with them not behaving 'in character' - totally breaking the game balance of the Tank, and making him a game-ender almost every time. The problem was that an intelligent human player wants to win right now, and the Tank was designed to be an enraged berserker that just charges in and starts smashing stuff. Having a Tank that can take tons of damage, has an incredibly powerful melee attack, and is smart, hides in choke points and stays in cover constantly, as a smart human does, was a huge problem. One choice was to simply not allow the Tank to be human-controlled, and leave it to the AI to drive his behaviors consistently. With that as our fall-back, we worked on trying to find a 'behavior modifier' that would encourage human players to act in more Tank-like ways. Eventually we settled on a fairly simple solution: the 'patience meter'. When a human becomes a Tank, he has a certain amount of 'patience', which is shown graphically on his HUD. This 'patience' wears down whenever he can't see the Survivors. If the Tank's 'patience' runs out, the player loses control to the Tank AI and returns to spectator mode. This allows the player to decide how to engage the Survivors, but makes it clear that he needs to get in and start swinging or he'll lose his turn. By making this 'behavior contract' clear to the player, we get the kind of Tank behavior we need and human players get to enjoy causing total chaos as the biggest, baddest, boss in the game.

Radio Answering
[ Node above the radio ]

[Kim Swift] Early in the design of this rooftop scenario, the finale would start as soon as someone hit the radio button. We saw playtesters consistently run up and hit the button without preparing for the battle first. To let players know that they needed to prepare and to give them time to do it, we changed the design so that the pilot tells them to call him back when they're ready.

Dialogue Scenes
[ Node in the center of the radio-room ]

[Erik Wolpaw] Because Left 4 Dead is designed to be played over and over again, we made the dialog scenes have as many variants as we could cram in. For instance, the encounter with the crazy Church Guy halfway through the Death Toll campaign includes more than 130 lines spread across five characters that can play out about 25 different ways. The variations appear based on which Survivor initially uses the door, what other Survivors are near that Survivor, and random chance. The end result is that players can go through that encounter many times without hearing the same conversation twice.

The Survivors
[ Node on the lower radio-room roof ]

Note: This node will spawn non-interactive versions of Bill, Francis, Louis and Zoey. However, their heads and eyes will follow your movements.

[Nick Maggiore] While there are hundreds of Infected types mobbing the player in Left 4 Dead, there are only four Survivors. To make sure these four stood up to close scrutiny over many hours of playtime, we decided to use photographic reference of faces and wardrobe details. We started out by casting four individuals who fit the general design concepts we had created for our characters. These models were photographed on a turntable, under both flat and dynamic lighting conditions, in order to provide reference for modelers and texture artists. In addition, each model was photographed in a custom-built mirror box, which allows us to capture simultaneous front, right, left, top and bottom views in a single shot. The talent was guided through a matrix of facial expressions, creating a complete catalogue of movements and expressions for each character's face. This is an invaluable resource for our modelers and animators when they start to recreate these expressions in three dimensional models.

Awards
[ Second node on the lower radio-room roof ]

[Jeremy Stone] Because teamwork is so important to gameplay in Left 4 Dead, we wanted to give players feedback on how they and their teammates were working as a team. The first version of this was something we called awards, which was basically a large collection of positive and negative player actions. It was like having merits and demerits. Awards showed up as icons in the HUD and everyone could see what awards other people on the team were receiving. This turned out to be too complex and confusing. It was hard to decipher what the icons meant, and the negative awards just hurt the feelings of new players rather than educating them. Also, with so much activity going on in the game, it was hard to notice the award icons at the time the good teamwork occurred. In the next version, we drastically narrowed the list to just a few positive things a player can do, replaced the icons with very short text, and added a glow to the player icon in the HUD. This had the desired effect of reinforcing good teamwork right when it occurred and broadcasting a player's good deeds to his teammates.

Cooperative Gameplay
[ Node on the upper radio-room roof ]

[Matt Campbell] From the start, focusing on cooperation was a primary design goal of Left 4 Dead. Although other games support cooperative play, extremely few actually require it. There are many core game mechanics in our game that require cooperation to succeed. One of the first that we added was having your character collapse to the ground, incapacitated and unable to move, when your health reaches zero. You continue to lose health in this state, and the zombies will continue to attack you. Once you are incapacitated, you will die unless a friend helps you back to your feet. Reviving an incapacitated friend takes 5 seconds, so it is a risky investment as the rescuer is vulnerable and unable to defend himself during this time.

Special Vignetting
[ Second node on the upper radio-room roof ]

[Gary McTaggart] Traditional vignetting is a term from photography which refers to the slight darkening of the periphery of a photograph. In our vignetting approach, we darken only the top edge and corners. This focuses player attention down into the middle of the play-space, where it belongs, while also creating a sense of oppression or claustrophobia. This is a great example of how a visual enhancement can have a positive effect on the overall game experience as well.

Pipe bomb
[ Node on the building with crates connected by pipes to the radio-room ]

Note: It will spawn pipe bombs on the table if isn't present.

[Brandon Idol] We knew we wanted homemade weaponry for Left 4 Dead, and a pipe bomb seemed ideal. However, for a long time the pipe bomb wasn't very fun. We tried various things, such as having a pipe bomb kill a Tank. That certainly made the pipe bomb valuable, but instead of being exciting, it made the Tank battle anticlimactic. We also tried making the explosion radius much larger, but the instant explosion made it difficult to time perfectly to be effective against a mob rush. In the final months of development, one of the animators suggested the pipe bomb having a 'beeping, flashing light that attracted zombies'. That added the missing fun element – an interesting and useful tactical choice for the Survivor. We prototyped this new pipe bomb behavior and it was an instant success. There are definitely situations where you want a Molotov, but now that choice is harder to make, because both are useful and fun.

= Left 4 Dead 2 Commentary =

Welcome
[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 have 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 use 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!

Wounds
[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 combination. To create the appearance of a wound, we project a texture modified by an ellipsoid that culls the pixels of 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.

Infected Variation
[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 a luminosity lookup into a gradient texture for tinting variation, allowing us to get not only hue variation but also luminosity variation; for example we can make black or white t-shirts 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 Left 4 Dead.

Body Piles
[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.

Secret Ingredients
[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.

Furniture
[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.

Park Paths
[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.

Overpasses
[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.

Deadly Hallways
[Chris Carollo] In Left 4 Dead 1, long narrow hallways usually meant relative safety for 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.

Bus Return
[Alireza Razmpoosh] This drop onto the bus shows a good example of what we call a 'return.' If a Survivor walks on the 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.

Sneak or Fight
[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 through without setting off a single car alarm, and if they manage it, the experience of carefully sneaking through an obstacle course full of traps is itself quite rewarding.

Dynamic Paths
[Kim Swift] Something that was always sad for us to watch in Left 4 Dead was that players would always choose an optimized path through a level once they'd played the 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.

Balcony
[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 Bourbon Street, giving a nice atmospheric vista without too much immediate threat.

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.

Parade Float
[Danika 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.

Traditional Crescendo
[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.

Dynamic Alarms
[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 alarmed 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.

Elevation
[Chris Chin] Most of the region where Left 4 Dead 2 is set is quite 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 not only a visually strong set-piece; it added the much-needed element of verticality to the campaign.

Flowing Water
[Alex Vlachos] The new campaigns in Left 4 Dead 2 featured a great deal 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, 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."

Spitter Design
[Matt Charlesworth] In her original incarnation, the Spitter had a large distended stomach in which she brewed her volatile goo, and a long mutated neck which served to catapult the acid over long distances. After the first pass of the model was playtested in game, it was noted by some players that they presumed she was a pregnant woman who had been turned by the infection. Although the inflated abdomen had positive effects on her readability in game, and helped separate her from the horde and other specials, it was decided that the image of targeting pregnant women was a road that we did not want to travel down. Therefore the final iteration of the Spitter features the same long neck and corroded mouth as the original design, but has a flatter profile and more aged features."

New Survivors
[Jeremy Bennett] When we began development on Left for 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."

Straightforward Finale
[Ido Magal] This finale felt risky at first, since it involved the players just moving in a straight line almost the entire time. We wondered whether 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.

Glowing Eyes
[[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 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.

Gunplay
[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 Left 4 Dead 2 we had the opportunity to do something a bit less generic, and 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 had 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 side arm and a large caliber magnum."

Riverboat Gambler
[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.

Unique Campaigns
[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 regard 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 our journey through a variety of locations. While each location has its own particular common zombie types, we further differentiated each campaign by creating ‘Uncommon’ zombies which 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.

Spitter Effects
[Peter Konig] 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 other game colors, while playing off the 'Green Death' motif associated with the infection. The trail she leaves, which is visually similar to her projectile attack, allows players a brief chance to track her movements when she retreats to hide.

L4Dictionary
[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 game play, 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 type of 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.

Assets
[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.

Hybrid Environment Authoring
[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 a 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.

From Server Browsing to Matchmaking
[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 team 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.

Third Party Addon Campaigns
[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 new 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 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.

Sound Design Principles
[Mike Morasky] The guiding sound design principals in Left 4 Dead 1, 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 vocalizations are human performances with very little effects processing, if any at all. The challenge with the new character sounds in L4D2 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?

Temporary Sounds
[Kelly Thornton] Because sound is such a crucial gameplay element in Left 4 Dead, the designers needed temporary sounds to test out ideas for the new boss infected. Sometimes we are 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 sounds were highly popular within the team, and it was common to hear animators up and down the halls mimicking the early Charger test calls.

Weather Effects
[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 unique experience for that campaign.

Engine Optimizations
[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 now 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 the 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.

The New Special Infected
[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 close 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 to maintain 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.

Special Moves
[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 preying mantis, ready to give Survivors a big, deadly hug.

The Axe
[Gary McTaggart] One of the challenges with melee weapons is getting them to feel right and look 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 the 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.

Specials and Sonic Silhouettes
[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 both a distinct tonal range and a character attribute to keep them unique -- for instance the Boomer’s vocalizations were kind of a slow, deep, bassy gastric horror. As we added the new Specials into the mix, it became increasingly difficult to avoid impinging 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 to 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 griping bark that we all liked. Though the 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 he makes 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 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.

Bus Station
[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.

Mocap Basics
[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 in 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 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, as our Survivors do, and you will feel the burn--but that's just one of the 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 record the actor on video during the motion capture to use for extra reference. With the motions recorded, the studio processes the data, and 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 as interesting as they are.

Giddy Up! Compositing the Jockey and Survivor
[John Morello] The source engine's animation system is a powerful tool that lets us create animations 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 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 along that vector 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 authoring time as well as memory inside the game--both of which are finite and very valuable on any project."

Ammo Piles and Weapon Scavenging
[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.

Defibrillator
[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 the 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 takes 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 defibrillator immediately conveyed the purpose of the item, plus it gives the Survivors a chance to yell, 'Clear!'

Adrenaline
[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 health kits, 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.

Upgrade Packs
[Eric Strand] Setting fires 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 ammo boxes placed through the levels. These boxes could be used many times by a 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.

Laser-Sights
[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, when 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 of the item 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.

Grenade Launcher
[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.

Local Music
[Erik Wolpaw] When deciding what to do about 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 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.

Uncommon Commons
[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 include 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.

Separation Anxiety
[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 during 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.

Playtest Observation
[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 playtests--which often take several hours out of every day.

Item Placement
[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 will spawn in a given location, it also chooses what type of item will appear.

Daytime Cues
[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.

Pipe Bombs
[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.

Lobby Browser
[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 for 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 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.

Story
[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 let’s 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 naïve 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 of Left 4 Dead will notice some tie-ins to the story as both games are set in the same world.