Developer Commentary

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. The comments are activated when the player starts a commentary node via the "use" key, and may skip them in the same way. In this mode, players go through the No Mercy campaign—with a significant lack of difficulty. Though there are enemies in the game, there are no boss 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). Due to this, no achievements may be earned, but it is a way of practicing a number of skills.

The Commentary
Note, this was taken directly from the subtitles associated with the comments, thus the double-spacing.

The Apartments
Number of Nodes: 18

[ Node over the First aid kits ] – Welcome

[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!

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

[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.

[ Node beside the rooftop door ] - Development

[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 is 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.

[ Node between the rooftop skylights ] – Griefing

[Steve Bond] Originally in Left 4 Dead there were some features that made it possible for players to cause trouble for others. This behaviour, 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.

[ Node in the first apartment kitchen doorway ] – Common Infected

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.

[ Node in the apartment bathroom ] - Extra Areas

[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, fairy 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.

[ Node in the lower apartment kitchen ] - Music

[Kelly Thornton] The most 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.

[ Node above the apartment “drop hole” ] - Playtesting

[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 exhilarations 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.

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

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 realised 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 player 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.

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

[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.

[ Node further down the alleyway ] - Francis

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 realistically, and thereby complete our horrifically rendered version of the zombie apocalypse which you, the player, now find yourself in.

[ Node in the alleyway connecting to the street ] - Visuals

[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.

[ Node beside the police car ] - AI

[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 behaviour', 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!

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

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.

[ Node between the truck and the flaming tanker ] - Lighting

[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.

[ Node beside the alarmed car ] - Car Alarms

[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.

[ Node at the dead end of the road ] - Buildings

[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.

[ Node outside the safe room ] - Teamwork

[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''

[ Node above the ammunition ] – Game Instruction System

[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.

[ Node before the escalators ] - Contextual Dialogue

[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.

[ Node beyond the escalators ] - Weapon Spawning

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.

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

[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 moveable 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.

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

[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.

[ Node before entering the subway car ] - Smoking The Set

[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.

[ Node after exiting the subway car ] - Player Outlines

[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.

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

[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.

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

[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.

[ Node at the Generator room entrance ] - Grenades

[Scott Dalton] We set out to create an interesting tradeoff between the pipebomb 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 pipebomb 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 pipebomb in any sort of intentional way was nearly impossible. You'd see a mob of zombies, whip out the pipebomb 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 pipebomb 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 pipebomb 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.

[ Node beside the minigun ] - Crescendo Events

[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.

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

[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.

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

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.

[ First node on the street ] - Dynamic Lighting

[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.

[ Node above the car ] - World Models

[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.

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

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 ok. 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.

[ Node to the left of the alarmed car ] - Colour Correction

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''

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

[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.

[ Node outside the lit doorway ] - Server Bandwidth

[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.

[ Node inside the restaurant ] - Zoey

Note: This node will spawn a non-interactive version of Zoey. However, his 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.

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

[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.

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

[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.

[ Node beside the scissor lift ] - Scissor Lift Crescendo Event

[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.

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

[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!'

[ Node in the alleyway outside of the wharehouse '] - Film Grain

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.

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

[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.

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

[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.

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

[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.

[ Node beside the ambulance ] - The Witch

Note: This node will spawn an interactive Witch. The Witch will attack you if you startle her.

[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: 

[ Node in the safe room ] - Statistics System

[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.

[ Node at the foot of the staircase ] - Death Animations

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.

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

[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.

[ 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.

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[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.