Valve has revealed that lessons learnt during the creation of zombie shooter Left 4 Dead have heavily influenced the making of Portal 2.
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Speaking in an extensive preview in the latest edition of Edge (E225), the studio discusses how the unlikely inspiration has bled into both the campaign and co-op elements of its first person puzzle sequel.
The vocal styling of Brit comedy hero Stephen Merchant deliberately contrasts with the emotionless drone of nemesis GLaDOS, according to writer Erik Wolpaw.
"We were thinking we'd have this other character you'd be seeing a lot," he said. "You have GLaDOS and she's robotic; do you really want to listen to another? We need vocal silhouettes, which we had in Left4Dead too.
"GLaDOS is slower-speaking and more deliberate, so we wanted something to offset that - a frantic person. Stephen Merchant does that really well."
Edge itself says that Portal 2 is "clearly the result of a few lessons learnt by Left 4 Dead, with the HUD displaying the position of your Portal through walls".
But perhaps the biggest influence from L4D 1&2 on Portal 2 relates to the co-op mode - which left CVG so impressed the other week.
Writer Chet Faliszek says that the teamwork of the two players and in-game conversation have both taken their queues from Valve's 18-rated gore fest.
"You're going to end up talking about what you're doing with your friend," he explains. "It's one of the things we learned with Left 4 Dead, where [there's] this base story, but we want to pull it back a little bit."
Edge says that even the puzzles in the game show influence from L4D, as well as new mechanics such as energy balls and 'light bridges'.
It reports: "The result, a product of Valve's learning from how players approached the original as well as Left4Dead is more immediate than the original [Portal], even more tuned and even more witty."
The new issue of Edge is with subscribers now and will be in stores next week. Order it here and have it delivered to your door.