Issue link: https://digital.copcomm.com/i/225858
pixie-dust language, the snowflakes don't pop on and off. They grow like snowflakes in nature. Our snowflake simulator created 2,500 unique snowflakes. " Snow Tech All those falling snowflakes created ankle-deep powder through which the characters in the film walk, and which the effects team created. Only one sequence, an action sequence with a giant snowman, put the characters on hard-packed snow. The rest of the time, they move through softer snow in various depths. "When characters walk through the snow, the only limit should be on how fast they move, and that is dictated by the story, Goldberg says. "How fast they move tells us how deep " the snow should be. It freaked people out a little at first. There are some shots with snow up to the characters' mid-thighs and in those shots Anna has to hold her dress up. " To research shots such at those, Disney sent the animators and effects artists on a field trip to Jackson Hole, Wyoming. "We had everyone wear a dress just to get their minds around the hard work it is walking through the stuff, Goldberg says. " "We're in Burbank, California. For us, snow is magical. " To create the snow and manage the interaction between snow and characters, the team developed two systems: Snow Batcher for shallow snow, and Matterhorn for deep snow and close-ups. "We knew a good portion of the film would be outdoors and that the characters would walk through ankle-deep and deeper snow, Mayeda says. "The standard slight [foot] impressions " wouldn't work, so we created our Snow Batcher pipeline. It could define which characters disturbed the snow and how deep, automatically create foot impressions, add additional snow, and kick it up. We put a lot of information into the database for each shot. " Snow Batcher worked for shallow snow, but not for the knee-deep and deeper snow the characters trudge through in some scenes, for close-up shots, and for scenes in which a character moves a hand through the snow. "We looked at all the tools on the market, but nothing really does snow that looks like snow, Mayeda says. "So, Andrew " Selle and his team spent a good amount of time creating Matterhorn, a snow solver. We thought we'd use it on only a few shots, but they worked with [Effects Animator] David Hutchins to productize it, and we used it for 40 shots in the film. It's the most amazing stuff I've seen in CG. " West provides some examples: "We used Matterhorn in a sequence where Anna walks through almost waist-deep show, a blizzard when a ship tips over and dumps tons of snow, and other shots, he says. "We sprinkled it on Kristoff's feet. " " Snow can be both solid and pliable – it can clump and break apart, or cling to itself when wet. It's neither fluid nor rigid body, so neither fluid or rigid-body simulations would do. Selle and his team needed another type of simulator, one that could handle "elastic to plastic" materials. Clothes Encounters With 114 characters in the film wearing winter clothes, the CG character team had a great excuse for not simulating every costume in the film. "But, we didn't go down that road," says Frank Hanner, character CG supervisor. "Rather than do little CG cheats, we decided to simulate every piece of clothing. We wanted fully dynamic, fully simulated wardrobes." The design comes from bunad, a style of clothing based on traditional Scandinavian folk costumes. "It features a lot of heavy wools, multi-layered, pleated costumes with intricate embroidering, and the Norwegian decorative trim pattern called rosemaling," Hanner says. "There are so many elements in these bunad designs that the artists couldn't approach them in a traditional CG sense, so they learned how to tailor a real-world costume. We started with a sculpted shape and then cut it into flat patterns that we put into the simulator." Disney uses a custom cloth simulator called Fabric that they updated to handle the bunad costumes. "We needed to implement a distinction between warp and weft stretch forces and sheering forces, and to support pattern-based designs where we cut fabric along the bias, which is important with tight-fitting, stretchy fabric," Hanner says. By the end of the film, the team had created 245 simulation rigs for the clothing, more than double the number used for all their previous films combined. – Barbara Robertson "Snow Batcher allowed us to do shots farther away where we couldn't tell it was an approximation, Selle says. "But up " close, the snow looked like packing peanuts. So we stepped back and looked for research on snow simulation. We couldn't find any papers. " What they did find, however, was research into material point methods (MPM) of simulation. "The material point method was the basis, Selle says. "Then we determined rules " for the continuum mechanics. " Continuum mechanics considers the physics of materials C G W N ove mb e r / D e c e mb e r 2 013 ■ 13