Issue link: https://digital.copcomm.com/i/225858
SIMULATION ■ ELSA'S MOOD determined the type of snow created for a scene. modeled as a continuous mass rather than discrete particles – elastic materials that return to their rest shape, and plastic materials that permanently deform. Selle and a team of mathematicians from UCLA became the first to apply this type of simulation to computer graphics. The team's effort produced the technique used in Frozen for deep and close-up snow, and resulted in a 2013 SIGGRAPH paper "A Material-Point Method for Snow Simulation" by Alexey Stomakhin, Craig Schroeder, Lawrence Chai, Joseph Teran, and Selle. The paper's subtitle is: "Combining a Lagrangian/Eulerian semi-implicitly solved material-point method with an elastoplastic constitutive model to simulate the varied phenomena of snow. " "When you pull on a clump of snow, it breaks apart, Selle " says. "If it's wet, it sticks together. We considered all these properties and found a model that allowed us to represent them. The program knows how to compute solutions in whichever is more appropriate – particles or grids – and integrate the equations. We can represent all the pieces of snow as particles, and unlike normal particle-based systems, each particle can have properties that represent its state; that is, a measure of is deformation, how much it stretches and rotates. In addition, we have parameters to control intrinsic properties such as stiffness and resistance to compressibility, and these things can change over time and space. Different parts of snow can have different properties – a top layer that freezes overnight and forms a crust over a soft interior. " In practice, the artists started with an initial preview based on Snow Batcher. "The first thing we needed to do, Selle " says, "was to make sure the animators could see where the snow would be even if it wasn't final snow. Snow Batcher gave us that quick preview. " If, for example, a character was stepping into knee-deep snow, Snow Batcher would carve out part of the snow. "It deforms the base-level surface and creates a cavity, Selle " explains. "In this hole, we seed the active snow. We make an 14 ■ CGW Novem ber / Dec em ber 2013 implicit surface of that space. That gives us our initial material points. We set the material properties based on what the snow needs to be – powdery or stiff. Then, we bring in the character as a collision object into the simulator, hit Run, and get snow interacting with the character. " At the end of the simulation, the team would sometimes create an implicit surface and mesh from the particles for rendering; other times they'd produce a density field and render the result as a volume. "One of the interesting things about this simulation is that the properties stayed the same whether we ran it at coarse resolution or higher resolution, Selle says. " "Higher resolution produced more interesting chunks, more interesting pieces that resolved more, but we could get a good preview at low resolution. " That meant the team could run quick sims in 20 or 30 seconds a frame to have a good idea what higher-resolution results that might take overnight or longer would produce. "We were impressed that it scaled really well, Mayeda " says. "We could run the simulation and it would give us the results we wanted to see. " Snow Blind Mohit Kallianpur, director of cinematography for lighting, led the team of 68 artists who created the look of the snow that the audiences see in the film. As with many on the crew of Frozen, Kallianpur moved onto this film after finishing work on Tangled. "We knew that one of our big challenges would be large-scale environments with snow, he says. " Disney uses Pixar's PR RenderMan and had moved to Version 17 for this film. "We knew we had to raytrace a chunk of the show, but we didn't want to raytrace the entire show, " Kallianpur says. "We used raytracing for the large ice-palace environments, which were very, very expensive. For the snow, we generated large point clouds for subsurface scattering and used deep shadow maps. " To create new snow shaders, the team in Burbank worked