When Left 4 Dead released late last year, it took everyone by surprise. Well, except those of us who'd been waiting for a decent co-operative first person shooter since, oh, let's say Doom (which, you might remember, came out in 1993). But it really feels like we've been clamouring for years for some good co-op action, like the proverbial zombie trying to knock down a door to get the fresh juicy brains that lie beyond.

Well, prayers were answered when Left 4 Dead came out. Not only did it have an awesome co-operative campaign, it was actually created in such a way that you could play it again and again; getting a new experience each time. And that's not even mentioning versus mode.

Just like Blizzard's success in creating engaging multiplayer games (which we covered at last year's GDC), Valve didn't just release a game and hope people liked. It was the result of a keen awareness of how people like playing games, both single player and multiplayer, derived from their experience in developing titles like Half-Life and Counter-Strike. It was the result of a long development process with serious planning and extensive play-testing.



Today at GDC 2009, Michael Booth from Valve Software presented a lecture titled "From Counter-Strike to Left 4 Dead: Creating Replayable Co-operative Experiences" to a packed room of developers, all keen to gather some insight into the thought processes that helped them create a game that has been a smash hit all around the world.

The lecture began with a quick analysis about why they decided to make something like Left 4 Dead in the first place. It was clear that there was some demand for a co-operative game - but it also seemed obvious that it was a risky proposition. However, they have learned from experience (sooner and better than many other PC games developers) that creating titles with a strong multiplayer component often means a big online community spawns - which translates directly to retail sales (and longevity). Thus, the goal was to combine all the lessons learned from previous forays in both multiplayer and single-player games and work on a co-operative title.

Co-operative play was a requirement. This might seem like a bit of a no-brainer,bBut they didn't want to create a game where "co-op" meant you were just playing it together. Left 4 Dead was structured from the ground up so that players wanted to play together, using a series of carefully-considered and well-integrated design elements to make sure proper co-operative play was the only winning strategy.

One such way was the psychological effect of the horde - the feeling of helplessness when alone and confronted by zillions of bloodthirsty creatures trying to rip your guts out. It's clear after a bit of playing that there's just too many to handle by yourself - even if you can shoot a few of them and run from some others, a single hit will stop you dead, meaning you'll quickly get overwhelmed. There's also the subtle reminder from horror movies that the Good Guys always team together to survive, and the Bad Guys are the ones that run off and try to make it alone (remember Burke from Aliens? Yeh you do.).

Of course, it's possible to artificially force co-operative play through a series of simple in-game mechanics. For example, you could force players to stay in a close group through a system that simply stops them walking too far apart. But clearly, if you can encourage people to do it properly by rewarding them (ie, with survival) when they do it right, and punishing them when they do it wrong (for example, wandering away from the group), then that's better all-round.



Now, it's almost inevitable that - given the sort of endless time, creativity and patience they have - gamers would have figured out a way around even a fairly clever system of co-operative incentive. Fortunately, Valve play-tested the crap out of Left 4 Dead, and they found a bunch of places where their system did fall down. As a result of this, the Special Infected were born, both to handle specific issues with gameplay and to help create more dramatic tension.

The incapacitating attacks that the Special Infected have were similarly carefully planned. A major reason was to enhance the fear when a survivor was alone - if you're alone and a Special gets you, you're in big trouble. Further, it gives other players a “hero" motivator if they rescue a survivor who is getting mauled - gamers, it seems, like having the ability to help each other out especially when it’s presented in the very concrete manner that Left 4 Dead uses.

There were a host of other techniques used to encourage co-operative play dropped into the game. Character vocalisations improve situational awareness, helping everyone know what is going on and establishing a feeling of camaraderie between the players. Limiting critical resources like health ensures the group works together to keep each other alive - losing one player at a critical juncture can mean the difference between survival and... not survival.

Replayability was another key goal of the game as well, continuing Valve's ethos of "entertainment as a service". The plan was always to expand on the game through the usual series of patches and DLC (free for PC and Xbox 360 users). Making the game fun to play through again was not a simple task, and Valve had to create a series of new techniques and systems to make it work as desired.

"Adaptive Dramatic Pacing" was one way this was accomplished - algorithmically adjusting the pace of the game to maximise the drama. Lessons learned from years of Counter-Strike showed that constant, relentless action is wearing for the gamer - but too many long, slow gaps and the player gets bored. The solution is unpredictability; the AI Director dynamically changes the pace of the game by dropping in hordes of zombies when it’s quiet, and letting the player relax after a prolonged battle, tracking the estimated “emotional intensity" of the survivors (which fluctuates based on factors like injuries, being incapacitated or dangling from a ledge).



Manual triggers for events were avoided like the replay-killers they are. After a couple of rounds, players simply learn where all the manual triggers are and there's no tension, no heightened sense of drama for them - they just react based on their learned responses, which quickly gets boring. Procedurally populating the environment and using layers of structured unpredictability based on actual in-game events ensures that each time you play Left 4 Dead, it is a distinctly unique experience.

One last aspect that was positive for both co-operative play and replayability was the constant attention to the dramatic elements in the game. Dramatic tension is sprayed liberally throughout the game - for example, when you get Boomed, you know that the horde is going to spring on you in a few seconds (and you're probably blinded, to make it even worse). The appearance of the Tank is preceded by a change in music and the shouts of the survivors. The eerie crying of the Witch means you know trouble is going to come if your teammates aren't careful.

Clearly, the development team of Left 4 Dead have succeeded creating a co-operative title. You only have to look at the vast amount of heavily populated online servers to know there're a lot of people out there playing it. If you need another metric to measure success - we also discovered that it has sold 2.5 million copies in six months. Not too shabby.

Hopefully as a result of the success of Left 4 Dead and Valve's willingness to share this sort of information we'll see more co-op first person shooters emerge. At long last.