A playable version of F-Stop, Valve's unreleased Portal prequel, is now fully accessible after assets and source code leaked online. The Valve Cut Content team and leakers assembled the build, which mechanics work by photographing objects to make them disappear into photos, then rematerializing them at different scales to solve puzzles. Gabe Newell reportedly liked an early version, though it tested poorly with audiences. Elements of F-Stop were later incorporated into Portal 2. An August leak of 12TB of Steam data contained F-Stop assets, but the mapsrc source code was withheld until someone paid $1000 for it. The person who held the code eventually released it.