The "Effects Omelet" presentation at SIGGRAPH is always a great source for inspired creativity on the ground by VFX artists and TDs. David Lipton, Head of Effects at Dreamworks Animation, gave a particularly interesting talk about he achieved the Jack Frost frost effect in DWA's "Rise of the Guardians".
Interesting use of old school approaches to get more controllable artistic results in the expressive effect of Jack Frost's frost in DWA's Rise of the Guardians. The frost needed to be a highly stylized, very art directable and expressive effect, where Jack's staff would freeze objects by propagating elegant, icy arabesques that skated across surfaces, covering them in stylized frost patterns.
Lipton said that they were helped immensely by the copious notes, reference images and concept art prepared by the Art Department. This gave him and his team a very distinct target to aim for, and helped to narrow the problem at hand.
The first approaches were simulation based, but proved to be hard to control, especially because the effect itself needed to be an expressive actor in the film, with its performance often leading directing the eye through key story moments. The winning approach was to look far back into the history of computer graphics to an old standby of cellular automata. These are systems in which cells of a grid, like pieces on checker board, follow simple rules that determine how each cell becomes filled by its neighbors. In this case the rules would determine how ice would grow from square to square as time progresses. The speed at which the squares were filled defined paths, like roadways, along which the delicate and stylized crystal patterns would be constructed. Because the automata exist in a grid, the rules could be "painted" in like pixels in a digital photo providing a high degree of control. The end result was a controllable, yet very organic looking crystal propagation that added a sense of magic and expressiveness to the scenes.