If a friend is trying to convince me to try a new game, the first thing I always say is, "I dunno... how dirty are the floors? The corners, especially?" I'm sure you're the same. As we all know, the best games have dirty floors and filthy corners, which is why ambient occulsion was invented.
Ambient occlusion (AO) is a technique used to make CG scenes appear more realistic by darkening surfaces near corners. Used subtly, this can add realism as corners and crevices are less accessible to light, making them darker. Used less subtly, it gives a distinctive dirty appearance.
Modelling AO the physically correct way is still too computationally intensive for general use, so if you want it in your game, you have to fake it. I'm doing it with a precarious conglomeration of textures, render passes and stencil tests. I think it's looking pretty good. And it works on moving objects, too! You'll have to take my word for it for now, but you'll see in the next video.