Developer Commentary (Left 4 Dead 2)

The Developer Commentary is an extra mode in Left 4 Dead 2. 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 Parish 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 Waterfront
Number of Nodes: 11

Welcome
[ Node next to the starting point ]

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

Campaigns
[ Node in front of the fence ]

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

Wound System
[ Node in front of the market entrance ]

Note: This node repeatedly spawns Common Infected.

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

Body Piles
[ Node over a body pile ]

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

Sound Design
[ Node on a balcony overlooking the street ]

[Mike Morasky] The guiding sound design principles in Left 4 Dead 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 vocalisations 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 unitelligibly to himself; the Spitter is trying to screech out her bile hairball; and the Jockey...well, who knows what he's on about?

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

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

[Ricardo Ariza] We wanted to improve the variation on the infected while keeping their memory footprint identical. The entire horde is never comprised of more than five head textures and five body textures. We use 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 details 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.

Music
[ Node next to the jukebox in the bar ]

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

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

Note: This node spawns a Common Infected.

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

Glowing Details
[ Node in the back of the kitchen ]

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

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

Explosion Debris
[ Node in the restaurant ]

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

Pipe Bomb
[ Node outside of the safe room ]

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

The Park
Number of Nodes: 13

Narrative
[ Node in the safe room ]

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

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

Note: This node spawns a Charger, Spitter, and Jockey.

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

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

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

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

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

[Matt Boone] With the added variety of weapons in Left 4 Dead 2, we wanted to encourage players to scavenge weapons and switch between them frequently. We found that placing ammo piles scattered through the level fought against this goal as players tended to 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 percieved as being less valuable than carrying a healthkit though, and many players bypassed the item, counting on finding another gun along the way. We found it was important, however, to continue to place ammo piles in the finale areas, to give players a home base to fight from in the longer set-piece battles.

Animation
[ Node at the exit of the park ]

Note: This node spawns a Spitter and a Jockey

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

Hallways
[ Node in the narrow alleyway ]

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

The Overpasses
[ Node under the destroyed overpass ]

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

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

Gauntlet Mode
[ Node in the army trailer ]

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

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

Weapons
[ Node next to the army tent ]

Note: This node spawns a variety of weapons.

[Tristan Reidford] Left 4 Dead 1 had a good, fictionally justifiable cast of guns--the M16 being a standard police and military weapon, for example. In 2 we had the opportunity to do something a bit less generic, and we use some more interesting guns, whilst still being aware that we had to justify their place in the game. The Desert Rifle, for example, is used to suggest that the military rerouted guns meant for the Middle East back to deal with the domestic crisis; hence the desert camoflage 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 form Left 4 Dead 1 is back, and we've added a military sniper rifle which has double the clip size and is more of a scoped semi-automatic assault rifle, rather than the classic bolt-action sniper rifle, which would be ineffective against the horde. Left 4 Dead 2 also features a variety of pistols including a custom civilian handgun, a standard issue police sidearm, and a large caliber magnum.

Player Guidance
[ Node further in the gauntlet ]

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

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

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

Special Infected Sounds
[ Node outside of the bus station]

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

Jockey Animation
[ Node outside of the safe room]

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

The Cemetery
Number of Nodes: 10

The Quarter
Number of Nodes: 13

The Bridge
Number of Nodes: 10