CGSociety has a great article on the art & direction of Valve’s Portal 2, and gets into some of the programmatic issues they had to deal with in the engine. Making it run on everything from PC’s to XBox360’s presented some technical challenges, but nothing compared to the difficulties inherent in light-bending physics.
If that portal can see its paired portal, the engine is suddenly drawing the world three times over. If you add to that the two-player co-op, which can have four portals simultaneously, plus reflective water surfaces and split-screen rendering, the engine can very quickly be drawing the world up to sixteen times per frame.
This required the Valve Software team to do more graphical optimization that any game they’ve created previously, including finding methods for creating ‘world imposters’ which baked portions of the scene into rough model geometry that can could selectively turn on in graphically intense scenes.
