Far from all Valve projects make it to release. Some fans have long been waiting for a hypothetical Half-Life 3, and for a while, the company was also developing a new installment in the Portal franchise. Chronologically, this “sequel” was intended to be a prequel.
The events of F-Stop would have taken place long before the original Portal, where players already possessed the portal gun. In the prequel, the developers planned to do without this iconic device: the protagonist would instead wield an unusual Aperture Science camera. Using this device, one could take a picture of an object, turning it into a photograph. The resulting photo could then be moved and the item placed in the desired location. The puzzles were built around this mechanic.
F-Stop never made it to release, but its source code and later its game assets were leaked online. Modders examined the available maps and assembled a playable build featuring several levels and functional puzzles.
Notably, even before this leak, development had already begun on Stop The F, a fan-made reimagining of the cancelled prequel. You can add the game to your Steam wishlist via the following link.