Without bounce light, a computer room looks flat, and tracing every bounce with rays is slow and, with too few rays, grainy. Per Christensen at Pixar built a shortcut on a disc trick from graphics chips: light the scene once and store it as a cloud of tiny coloured discs. "Basic idea: add up color from all other points!" But that also counts the discs hidden behind things. The fix, in his slides: "rasterize colors contributing to a point - world as seen by that point". Every point draws its own tiny picture of the discs it can see, near ones covering far ones, and adds up its colours. It was "4-10 times faster than ray tracing" with "no noise", and on Up and Toy Story 3 "1-bounce PBGI was used in addition to all the traditional lights." In Christensen's words: "It's as if you have a fish-eye lens at each point looking at the world and gathering all that light."

Our own room, discs and cast, computed here; no frames from any film.

Where have you seen colour bleed from one wall onto the next?

Sources: Per H. Christensen, "Point-Based Approximate Color Bleeding", Pixar Technical Memo 08-01, 2008; Christensen, "Point-Based Global Illumination for Movie Production", SIGGRAPH 2010 course; Barbara Robertson, "Bleeding Edge", Computer Graphics World, March 2010.
Sounds: our own, made in code (earshot, CC0).