F-Stop, the secret mechanic of Valve’s unreleased Portal prequel, has finally been revealed
Just after Portal 2 was released, Valve and Geoff Keighley also released The Final Hours of Portal 2, an interactive book about the making of 2011’s best potato-based puzzle game. The most revealing story in the book was about how Valve had spent some time making a game that was Portal’s follow-up in name only. It was set in the past, without Chell or the…