Everyone is familiar with RNG, or Random Number Generation. It's often used to place random items in different locations of various difficulty or use. In Left 4 Dead 2, the game uses a Director for every map, including Workshop campaigns and maps.

Today, we'll focus on the Vanilla Director. The "Director" is the AI program that tells the level what to spawn in, when to spawn it in, and what Infected spawns when and where. Understand that there's a difference between a set spawn and a Director spawn. This will tell you how to spot the difference.

Weapons
Lets start easy. The easiest example for the weapon spawns is Dead Center, as the weapon spawns are usually pretty coherent. C1M1 (Dead Center Apartments), the survivors spawn on the rooftop with 2 fireaxes, 2 crowbars, infinite pistols and 4 medkits. Throughout the level there many rooms with different tools of trade, from the Magnum pistol, a Baseball Bat, Cricket Bat, Chainsaw, Kitana, CSS Knife, Pipe Bomb or Bile Jar. What spawns is dependent on the Director and what it wants to give the survivor team.

Allow me to use an example

1. The team is doing well, and managing both health and killcount. Less health items spawn while more weapons, ammo packs or grenades DO spawn.

2. The team is slacking off in health management and are often found shooting each-other by accident. More Health items, like Pain Pills, Defibrilators, Medkits and Adrenaline shots spawn, while weapons spawn less.

By reading above, you'd probably start to see a pattern. Yes, the Map Director is smart. It pays attention to how the Survivor team is doing all the time to figure out how the oncoming trials of maps are played. Understand however the Director does not carry through the maps. The Director is only in-charge of the active map, not all of them at once.

If you've caught on, you probably think "Well if it does weapons, it has to do enemy spawns too," and you'd be exactly right. Say, the team is keen-eyed and manage to mow down the Special Infected as they spawn. This tells the Director to kick it up a notch, or two, or even three. The better the team, the more frequent the Special Infected spawn in.

It behaves this way with hordes too. There's three (3) types of horde. The Panic/Crescendo Event Horde, The Car-Alarm Horde, and the Random Horde. Panic Hordes are always scripted to appear (unless either the map was tampered with via some sort of mod, cheat or file corruption). Car Alarm Hordes will always spawn, per car alarm activated (Example, the Car Lot in The Parish, each car that is activated spawns its own horde), and the third type, the Random Horde. This horde is the only one managed by the Director, as they can spawn whenever they want, or rather, whenever the Director wants. They're more or less a "Stay on your toes" challenge, compared to the carelessness of a Car-Alarm horde, or the preparedness check of a Panic/Crescendo Horde.

One more Director attribute to talk about. The Director placement of the Tank and Witch. The tank is a wild-card, however, can only appear once per Chapter, but always multiple times during a Finale. Sometimes, the Director is Generous and doesn't spawn in a Tank at all. Witches however, can always appear more than once, except during Survival or Scavenge, where she never spawns.

(Pictures will be added later.)

Everyone should be familiar with Difficulty Parameters. They're nothing new, and Left 4 Dead 2 is an exception, however, the difficulty has an impact on how people play, or even how they learn how to play.

Easy Difficulty

Easy Difficulty is essentially a training wheels difficulty. There's no friendly fire penalty or a high-damage penalty from Common and Special Infected alike. Common Infected do only 1HP of damage per hit, while the Special Infected damage varies depending on what type of Special it is.

Normal Difficulty

Second option for Difficulty, and the first that applies Friendly Fire damage, so watch your fire. While you can damage your team-mates by accident, there isnt as big of a health penalty per accidental shot, so you'd actually have to try to kill the Rochelle that still hasn't moved from spawn. Common Infected now do 2HP worth of damage per hit, while once again, Special Infected damage varies.

Advanced

Congratulations, you've graduated basic Survivor Training. Advanced is where the challenge begins and the training wheels fall off mid-ride. Friendly Fire is much more deadly, and both Common and Special Infected now do even more damage, with Commons doing 5HP per hit, and Specials can deal deadly blows within minutes.

Expert

Hope you've brought a stress ball, Rookie, because this is where the fun stops and the hard work begins. Expert, next to Realism Mode, is where you have to WORK as hard as the survivors themselves should. Friendly Fire is devastating, and damage dealt by Infected is much higher and deadlier. Common Infected deal 10HP worth of damage, being able to down a survivor in just 10 hits, faster if the survivor is surrounded. Special Infected is where the ball drops. They hit even harder and can even wipe a whole team in seconds if unchecked.

[See "TANK" & "The Witch" Sections for difficulty variations on the Tank and Witch]

The Tank needs no introduction. The booming music and heavy footfall should say it all. The Tank, is no joke. You're definitely going to need a bigger gun, and maybe a bigger boat.

The Tank, the Hulk-looking mass of muscle thats charging at your team, throwing rocks, and tossing cars with no effort, is the only "Boss" that the Survivor team will often find themselves engaging without option.

The Tank is the Director's way of testing the Survivor's teamwork and individual skill as a team member. Having the most health of all the Special infected, (See below for difficulty healthpools), the survivors need to drop everything they're doing to shred the tank faster than a meat grinder being force-fed. The survivors can utilize any tool they have in their possession, or nearby items to help combat the walking, yelling, screaming behemoth of a meat-wall. Be careful, the tank moves just as fast as a Positive Health Survivor (40+ HP), so don't get backed into a corner.

As stated in the "Director" section, the Tank may not always spawn, and can only spawn once per chapter, but always multiple times a Finale, but the Tank always does have a guaranteed spawn during the first Chapter of "The Sacrifice", where the boxcar is.

Tank HP
On Easy, the Tank only has 2000 HP, which can be dispatched by 2 survivors easy without error. The Easy Tank holds the least amount of threat to the survivor team, but do not ignore it because of that.

On Normal the gloves come off a little early. The tank's HP is doubled to 4000, and now requires all 4 team members to focus down. He's slightly more dangerous and can take some time to take down if unprepared.

On Advanced, the Tank is now much deadlier. Boasting 6000 HP and a 15-20HP Damage hit on Rock Throw or Punch, the tank can single handedly down a survivor without any Special Infected AI at his side. Tank will need to be number one target priority.

On Expert, The Tank is now much more like Resident Evil's "Mr X". At a whopping 8000 HP, the Tank needs to be focus number one at all times until it's dropped. This difficulty now boasts a on-hit Incap via direct punch and heavy damage by rock throw, meaning the tank can single-handedly wipe an entire Survivor Team in seconds if careless.

The Witch. You may have seen her, or you may have just heard her.

The witch is an icon of the Left 4 Dead 2 universe, being the only female Special Infected, besides the Spitter. The Witch is more or less a choking hazard that the survivors have to be very careful to avoid swallowing.

The Witch, or "The ♥♥♥♥♥" can be identified by her trademark crying, partnered with what sounds like a choir. There are two types of witches, but which ones spawn are both dependent on the Director AND Map. More often than not, in Left 4 Dead 1 Maps (No Mercy - Blood Harvest), you'll encounter the Weeping Witch, casually down on the ground crying, accompanied by a constant choir, vocalizing a tune that you'd associate with rain. In Left 4 Dead 2 (with the exception of Dead Center Chapter 1), you'll often come across the Wandering Witch, where she actively moves around. She still sobs, but the choir is much quieter and slower.

On each difficulty, much like the Tank, the Witch has various health pools that change, while not as much HP as the Tank itself, the Witch is still a formidable foe, and just as dangerous despite a lesser HP pool.

The witch, if provoked, will rush at the perpetrating survivor and immediately incapacitate them if she is allowed to reach her target. However, on Expert, and by extension, Realism, the Witch's incap is changed to an instant death. If left alone to maul her target, she can kill a survivor with little effort in seconds on all difficulties, however you'd have to let her kill a team-mate on Easy, or literally just not care about the guy having his skin shredded fine enough to rival confetti.

Understand this though, the Witch can be easily killed with little effort, but very careful execution.

Certain Campaign Chapters have various paths that can be taken dependent on the Director, however they are not plentiful unless certain mods are downloaded to increase the Chapter RNG.

In Total, there is nine areas that can end up being RNG based dependent on the Director's choice.

Dead Center (Chapter 3, The Mall)
Chapter 3 takes place inside Liberty Mall. The survivor team will end up going up, then down, then back up. Where the RNG is a roll-of-the-dice, is the knocked over door atop the two escalators after the food court. The RNG is as such.

Path A. To the left, through the opened double doors that take the survivors to the stuffed animal toy-shop on the second floor

Path B. To the right, through the single door that takes the survivors back down a floor, taking them to a Fire Exit door.

Dead Center (Chapter 4, Atrium Finale)

The Atrium has perhaps the most obvious RNG of the game, as the 13 gas cans needed can spawn in many different locations across the three accessible floors, however no more than 13 will spawn.

(If feeling confident, the survivors can carry a gas-can all the way from Chapter 2 to save a trip on a gas-can)

The Passing

RNG Item A - Weapon Room. After leaving the Jimmy Gibbs Jr at the bridge, the survivors are only given starting pistols and Melee weapons like that of the Dead Center first chapter, however, just a little walk through the river-edge park, they'll find two shops that are opened. One of these shops, looking to be a convenience store, or a internet cafe, will house the Shotgun or Machine Gun of the Director's choosing.

RNG Item B - Item Lockers. Various side-rooms in the entire campaign, minus Finale Chapter 3, will have footlockers filled with items, however each locker will have different items, and will always be found in different rooms. The lockers can have anything from Pipe Bombs, Molotovs, Adrenaline Shots, or Pain Pills. They'll never house Tier 1, 2 or 3 weapons, nor will they house melee weapons.

RNG Item C - Midnight Riders Tour Bus. After exiting the bar in the Second Chapter, the survivors will climb up out of a landfill where the city was working on the drainage system, and just behind the cement mixing truck, beyond the chainlink fence, theres a chance the Tour Bus of the Midnight Riders rock band will drive by. Depending on what survivors were around to see it pass will comment on the sight, albeit all in disbelief or confusion

Dark Carnival, Chapter 4 - The Barns

After exiting the safe house, the survivors find themselves able to access the rest of the fairegrounds outside the Peach Bowl Stadium, however, its right outside the saferoom is where the RNG has its effect.

Path A. The aisle through the food stands to the Moustachio Strongman Minigame has been fenced off and the survivors are driven to the left into the picnic tables and into the following area to access the bumpercars

Path B. The aisle leading to the Strongman and Bumpercars is opened, however the left path to the picnic tables now has the fence, allowing the survivors to take a narrower path over a more open one.

Path C. Neither path is blocked by fencing and any of the survivors can take their preffered path of trade to access the Strongman and Bumpercars.

Swamp Fever - Chapter 5, Plantation House Finale

Weapon Spawn A - Inside the Plantation House on the top floor, positioned at the railing behind the M2 Mounted Machine Gun.

Weapon Spawn B - Center Hedge Aisle on the way to the gate and radio, with weapons and ammo leaning against the hedges on the right.

Weapon Spawn C - To the right in the back yard, at a green crate. Ammo on top with weapons around it.

The Parish - Chapter 3, The Graveyard

After witnessing the destruction of the highway to the main bridge during a Military Bombing, the Survivors need to go through a graveyard. The graveyard has the most variations of what paths are opened and closed, meant to act like a maze rather than a simple detour.

No Mercy - Chapter 3, The Sewer

Upon leaving the sewers right outside Mercy Hospital, there's a chance that a Horde will await the survivors at the surface. Other times, this does not happen, leaving the survivors a clear path to the saferoom inside the hospital's front door.

No Mercy - Chapter 5, Rooftop Finale

With the many platforms the survivors can perch on to have a nice view of the battlefield, 3 of those platforms can spawn throwables, being either molotovs or pipe bombs, however, not every run of the finale will have the same grenades spawn, or have them spawn in the same place.

(If I missed any please let me know in the comments. Again, Photos will be added later.)

If in Singleplayer, and up until the point in Multiplayer that people join, there's three survivor bots that the player has as "Team Mates".

Rochelle jokes aside, the AI are pretty responsive for what they are, however they are quite limited to what they can and cannot do.

In Singleplayer, the AI always respond to the Priority Player (The player in SP) as long as the AI Navigation says they can get to them, whether it be helping them from an infected, healing them or giving them items. However, they cannot do as much as the player. As, should the player themselves, die, the campaign is considered failed and forces a chapter restart.

The AI cannot use Defibrilators, Melee Weapons, Deployable Ammo Boxes, Grenade Launchers, M60s, Mounted Guns, scavenge items (Gas Cans / Generators), grenades, or Finale Start objects. This can be altered with certain mods that overhaul the AI's code and programming, but as it stands in Vanilla, they cannot do this at all.

Their response to Special Infected attacks aren't as you'd expect though. Allow me.

Should the player be pinned by a Smoker, Jockey or Hunter, the Survivors will first prioritize freeing the player before killing the aforementioned Special Infected culprit. If pinned by a Charger, the survivors will kill it to help the player. If the player is downed by a witch, the AI will work as hard as their code allows them to kill the Witch. If downed by a Tank, and the Tank is giving chase, chance is a survivor will try to break from the pursuit to help the player. Rarely, the AI will help the player up even if the Tank is ontop of the player.

Something some may not know is that the AI adapts to how the player, or players, treat them. the more harm a player does to a Survivor bot, the more negligent they'll be in helping that player, or players due to the team-harm the player or players have caused

Every Campaign ends with a Finale, a stage that puts the survivors up against at the least, four hordes and two tanks.

Each finale is different, as they take place in a different area and require some setup or arrival time before the Survivors are allowed to live to fight another day. The order and prep for each finale are as follows...

No Mercy
Radio the News Helicopter #5 pilot and say you're ready for pickup. After the pilot wishes you good luck, the survivors will have to fight two hordes before the first tank, and then two more hordes before the second tank. The Helicopter arrives after the Second Tank is killed. However, beware, that even during the rescue arrival, a third tank can spawn along with a constant horde.

Crash Course
After reading about a truck yard with a zombie proof truck, the survivors make their way there to lower it and use it to get to Riverside. The truck yard consists of a Generator that needs to be started in order to power the lift that the truck is resting on. Same formula. Two hordes, then the first tank. The Generator will cut off a little after the first tank is killed, and will require a survivor to restart it. Common Infected will be constant during this period, but can still trigger a tank spawn if waiting too long. If started up in time, the survivors will end up in the truck by the end of the second tank.

Death Toll
After finding that Riverside is abandoned, the Survivors make their way to the river and phone in a boat, owned by the surviving couple John Slater and Amanda Slater. This finale shares similar code to the No Mercy Finale.

Dead Air
The Survivors make their way to the airport after seeing a C130 plane fly overhead. Upon arrival to the airstrip, the survivors need to fuel the plane for the pilot then climb aboard. Similar formula to the previous finales, only with the added bonus like No Mercy on knowing how much further they have before they can escape.

Blood Harvest
Having little to no options left, the Survivors bite the bullet and decide to ask the Military for help at a farm house. Nothing is different about the finale, minus the much more open area to utilize to the Survivors advantage. The military arrives in a APC after all four hordes and two tanks are dispatched.

The Sacrifice
After a debacle at the Military Outpost the survivors were held at, they decide to turn their back on the rest of humanity and decide to fend for themselves from here on. Bill, having the idea to use a boat to get to the Florida keys. The port finale proves a challenge if the players wish, as it requires 3 generators to be started to raise a bridge in the way of a sailboat that fits the role of what they need. The survivors can tackle all 3 generators at once, or one at a time. Doing all 3 at once results in the "Chaos Generator" achievement, and also brings a party of three tanks at once for the ultimate challenge. After making it to the bridge, the generator will fail and needing a restart. Whatever survivor jumps off the bridge will end up being the one left 4 dead, although it will always result in it being Bill in The Passing.

Dead Center
Finding that CEDA killed a lot of people at the Mall's evacuation center, they decide to save themselves, and Ellis has the idea of using the Stock Car of one Jimmy Gibbs Jr, as it was at the mall for a car show. Depending on the gamemode (Singleplayer/Multiplayer) the survivors will need either 8 or 13 gas cans to fill the car and escape. Tanks and Hordes will be frequent until the car is filled with all 13 cans of gas.

The Passing
Being the same port town from The Sacrifice, the survivors, instead of needing to start all the generators, they just need to fill one, using 16 cans of gasoline to be sufficient enough to kick the motor on. However, this time, the surviving 3 from the previous group help the new group of survivors live and fight off the infected. Finale follows the same rule as Dead Centers

Dark Carnival
Perhaps the best finale in the game, killing Infected as the survivors rock out on a stage in a amusement park. The survivors spotted a helicopter in a previous chapter, and Coach, has the idea to use the Midnight Riders stage show to get his attention, due to the band having the biggest pyrotechnics show. The survivors need to fight off 4 hordes and 2 tanks. If the player wishes, they can pause the game to hear the songs in their entirety. The helicopter will only arrive after the death of the second tank, much like most of the finales.

Swamp Fever
Throughout the Swamp, the survivors see messages on the once-alive townsfolk that made it to the old plantation house near the river for rescue. They learn of another survivor, Virgil, a Boat Captain, that will help them get to where they're going, as long as they can provide a clean escape at the house. Common Finale rules apply.

Hard Rain
Virgil drops the survivors off at a town on the way to New Orleans to get him some diesel for the boat's fuel. Being the only campaign that takes the survivors to the same place they arrived for rescue, the survivors need to activate the sign at the Burger Tank restaurant to get Virgils attention to come get them and his gasoline. Common Finale Rules apply.

The Parish
The Bridge finale is the only Crescendo-To-Rescue in the canon timeline, however does not seem to have a timelimit albeit the game wanting you to think that. The survivors need to use a radio found on a dead civilian to tell the military they need help to escape. The Military, being equipped to handle Carriers, agrees to help, however says the survivors have their last chance leaving in 10 minutes. They need to run the gauntlet that is the bridge to make it there. The finale only ends when all survivors make it to the active rescue.

The Last Stand
The finale of TLS, is a mix of No Mercy, Crash Course and Dead Center, where the first half, the survivors need to start a generator. In the second half, the generator will cut off and the survivors now need to scavenge for gas cans to start it up again. Difficulty will vary how many gas cans are needed to start the generator again. Rescue is similar to Death Toll.

Cold Stream
The Cold Stream finale is no different from The Parish finale. Just run and shoot. Rescue again, only ends after all living survivors make it to the rescue.

Hello yes, you may have noticed some campaigns, like No Mercy, Dead Air, Blood Harvest, The Passing, Dark Carnival, The Parish, and Cold Stream have some time before the Credits Crawl starts. During this time, any survivor that leaves will be counted as deceased, which in turn can prompt a restart if all living survivors leave with one dead. Be careful before leaving the game. Be a sport and try to stick around until the credits start rolling, as nobody can die after that.