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 GlaDOS that we know, and had a new mechanic called F-Stop . It was essentially a different game and was never released.
Valve remained coy about what F-Stop was in the hope of eventually using it, but they never did. In an unlikely twist, a developer using the Source Engine for their own game were given permission to show off F-Stop s secrets. It was based around the Aperture Camera , which could copy, paste, and rescale items in the world.