When Disney Animation Studios' Frozen opens on Wednesday, you’ll probably laugh and maybe cry. But it's unlikely you’ll notice the effort that went into making the star of the show — snow — look and act like the real thing.
The tale is about a band of unlikely friends who embark on a quest to save a kingdom, but snow is everywhere in Frozen. One of the main characters is even made of the stuff.
Snow, like any other particle in animation, like rain, fog or leaves, is a complex material. Convincing viewers that it's real on screen is more math than animation magic.
In a blog post, Disney wrote:
“Snow is a challenging natural phenomenon to visually simulate. While the graphics community has previously considered accumulation and rendering of snow, animation of snow dynamics has not been fully addressed. Additionally, existing techniques for solids and fluids have difficulty producing convincing snow results."
Disney animators created something new to achieve the necessary effect. The technology, which is technically known as a material point method (MPM) for snow dynamics simulation, addresses particle volumes, grid forces and collision response to help animators create shockingly realistic snowball throws, drops and collision.
Disney animators Alexey Stomakhin, Craig Schroeder, Lawrence Chai, Joseph Teran and Andrew Selle wrote that it utilizes “a user controllable elasto-plastic constitutive model integrated with a hybrid Eulerian/Lagrangian Material Point Method.” Now try saying that 15 times, fast.
In a video (above) unveiled earlier this year at the computer graphics conference Siggraph, animators explained the algorithms behind the complex particle response and showed some nifty and very realistic demonstrations of their “snow” in action. (All the geniuses out there can view this PDF with all of the formulae.)

Disney’s animated snowballs not only look good hitting a wall and colliding with other snowballs, but also mimic different forms of snow, like soft, powderery, crunchy, and chunky. The animation studios also recreated packing snow, a wintertime favorite.
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Image: Disney