The rendering system, called Hyperion, manages the huge computational volume required for ray tracing. In a ray-traced image, the graphics system tracks the behavior of light rays that interact with various kinds of surfaces before passing through the picture plane.
Any given light ray may bounce as many as 10 times, creating all sorts of secondary shadows, reflected light, or subsurface scattering. The inflatable robot character called Baymax is a perfect proof-of-concept for the rendering system because of all the internal scattering inside the vinyl skin.
Although the designers could have used this system for a photo-real image, they were very conscious of keeping to the stylized character of the animated world.
The film is set in an alternate universe of "San Fransokyo." It not only had to combine design elements of east and west, but also had to be extremely detailed and layered to allow for some fly-through sequences.
The geometry was connected to an actual street grid of San Francisco, and the assets can be reused for future films and games.
Both the rendering software and the architectural generator put immense demands on the Disney supercomputers. Tech supervisor Andy Hendrickson said "This movie is more computationally complex than our last three movies combined."
In this video, Norm from Tested interviews Mr. Hendrickson about the techniques and challenges. (link to video).
Book: The Art of Big Hero 6
Ray tracing on Wikipedia
All images ©Disney 2014
Speaking of animation, I'll be a speaker at CTN Animation Expo at Burbank in less than two weeks, giving presentations about Color and Light and Imaginative Realism. Hope to meet you there.