Issue: Volume 36 Issue 7: (Nov/Dec 2013)

Winter Wonderland

By: Barbara Robertson

The theme of Disney’s 53rd feature animation is the power of love over fear, and it’s tempting to extend that metaphor to the studio itself. With this film, Disney Animation has fully embraced its past and skillfully incorporated the beauty and magic of traditionally animated fairy tales within a truly modern feature film.

Based very loosely on Hans Christian Andersen’s “The Snow Queen,” the new Disney classic gives Andersen’s villainous queen a more nuanced role, replaces the little girl on a rescue mission with an older, spunky princess, gives the princess a good-natured guide, and introduces a magical snowman.

“I remember when we were all talking about making the snow queen more three-dimensional,” says Jennifer Lee, who wrote and directed the film with Chris Buck. “Someone said, ‘What if they [the girl and the queen] are sisters?’ And, everyone felt something. I thought, ‘Oh, gosh. I love this now.’”

The sisters are Elsa, voiced by Idina Menzel, and Anna, voiced by Kristen Bell. “Anna is an 18-year-old girl who calls herself ‘ordinary,’” Lee says. “She has a big heart, and she’s fearless. She’s also messy, talks before she thinks, and is funny and quirky. Elsa was born with the power to create snow and ice out of nothing. When the sisters were small, they used to sneak out at night and play with Elsa’s magic with such joy. But Anna is too fearless, and she pushes too far. She gets in the way of the magic and is hurt. The trolls save her, but they remove her memory of Elsa’s magic, and Elsa lives her life hiding her powers as best she can. Her fear is that her powers will come out.”

Frozen

DISNEY’S EFFECTS and R&D teams developed new technology to create magical yet believable CG snow and ice. At far left, Anna, Olaf, Kristoff, and Sven the reindeer travel through the result.

And then one day, Elsa becomes queen. At her coronation, the teenaged Anna falls in love at first sight with handsome prince Hans (Santino Fontana). After knowing Hans only one day, she agrees to marry him. Elsa’s emotional reaction causes her to lose control over her powers, and she flees. “The fear she feels has created an eternal winter,” Lee says. “After Elsa accidentally turns the world cold, Anna sets out after her sister.”

Anna seeks to melt Elsa’s frozen heart and bring summer back to the kingdom. And with that, the movie begins in earnest. On Anna’s journey through the snowy mountain landscape, she meets Kristoff (Jonathan Groff), an ice harvester who lives with his reindeer. He offers to help her navigate through the mountains. Along the way, they meet the magical snowman named Olaf (Josh Gad).

“What a lot of people don’t know is that Elsa and Anna had created Olaf when they were little by hand-rolling him out of Elsa’s snow,” Lee says. “He represents their beautiful innocence. When Elsa thinks she’s a safe distance away, she builds an ice palace, and in that freedom re-creates the Olaf from her childhood in a song, the song ‘Let It Go.’ In this film, we never start and stop for a song. Every song is a continuation of the plot; they drive the plot forward.”

Olaf represents the love between the sisters, and in doing so, has an emotional role. But, he also provides slapstick comic relief in the film. “He’s obviously made of snow, so we had to respect truth in materials in how snow moves,” says Lino DiSalvo, head of animation. “And, his arms are sticks, so we couldn’t bend them in the film. The beauty of that is it put him in interesting situations. Having him scratch his head or reach for an object becomes a complex, fun situation. How do you get a character with straight arms on Sven [the reindeer]? How does he poke up over an ice bluff to see what’s going on?”

The answer to the latter question is: He pops off his head and holds it up with his arm.

Frozen

AT TOP, OLAF CAN bend his stick arms (and not melt) only in his dreams of summer. At bottom, lighting artists used full raytracing to render Elsa’s ice palace.

Truth in Acting

Olaf is a bipedal character with the structure of a human, but animators could disconnect and reconnect all his body parts. “The things that made him special were out of the ordinary for us,” says Frank Hanner, character CG supervisor. “Luckily, we knew those things early on. He was always going to be magical, and from day one or two we had reference designs of Olaf pulling his head off, or melting in a corner and rolling a new body. They were fun, cool ideas. But, it was a bit of a challenge to construct a system that allowed any part of this little guy to disconnect and arbitrarily reconnect.”

The rigging team devised a tool they named Spaces that gave animators a convenient way to reconfigure the rig. “He has one rig with mechanisms for connecting and disconnecting,” Hanner says. Working in Autodesk’s Maya, an animator could click a button to have Olaf’s head fall off and still animate his body walking away.

“His body parts could be in world space or local space,” DiSalvo says. “So, we could pull his arm off and pop it into world space and then continue working on his body.”

DiSalvo led a crew of 70 animators by casting supervisors for each of the hero characters and then finding pockets of animators for specific moments. “Our animation department is so fine-tuned, I felt like a conductor of an amazing orchestra,” he says. “We’d be in review sessions and nine out of 10 times we’d find ourselves involved in the movie rather than critiquing lighting or whatever. We’d all be leaning forward watching the film, and I’d think, ‘Oh my gosh, I think we have something special.’”

Although all the humans, the reindeer, and, of course, Olaf, are caricatures, the animation team strived for what they call “truth in acting.” “The first order of business for us was bringing in the voice actors,” DiSalvo says. “I moderated a session like ‘Inside the Actor’s Studio,’ with the animators sitting around. And, an acting coach came here early in the process and we went through pages of the script. We wanted truth in acting, truth in the emotion. We wanted to make sure emotions crescendo at the right time. It’s easy in an animated film to put the volume on 12 in every shot, with every animator trying to outdo the other. But, we didn’t want clichés.”

Before committing to CG animation, the animators spent time looking at hand-drawn animation. “We went through explorations of the hero characters for a year and a half,” DiSalvo says. They also shot live-action reference, drew thumbnails, and pulled from their own life experiences to dig into that truth. DiSalvo provides an example from his life experience that informed the animators’ performance of Kristoff in an emotional scene.

In the sequence, Kristoff and his reindeer, Sven, are travelling through frozen tundra. Sven slips on the ice and falls into the water, and DiSalvo remembered a similar situation. “My dog was walking around my parents’ backyard and fell through the ice in a koi pond,” he says. “I had to run across the yard and jump into the frozen lake to save him. We discussed that moment and knew that Kristoff wouldn’t be just surprised or sad. You see fear on his face. He potentially lost his best friend. We didn’t over-animate. We didn’t caricature. He has a specific expression of fear.”

The crew even had a reindeer come to the studio so the animators could research Sven’s movements. “We got all the 70 animators outside together,” DiSalvo says. “We brought in the reindeer. And he just stood there eating grass. We realized reindeer don’t do much. So John Lasseter and I started talking about what we could caricature. I talked about my French bulldog, and John talked about his two dogs, and we started exploring Sven as if he were a dog. Sven became everyone’s favorite fun moments with their pets.”

As for the lead characters: “The most important thing was bringing the nuances and subtleties of the hand-drawn characters to the CG characters,” DiSalvo says. “To be able to animate a film about love, about someone who is driven by fear, and someone who is fearless, to have one sister wanting to get her sister back – we couldn’t wait to get a shot with these characters. Acting-wise, there is nothing more satisfying than animating characters with so much subtext. So many of us here wanted to get into animation because of Beauty and the Beast, The Little Mermaid, The Lion King. And all of a sudden, here we are about to animate Elsa.”

Elsa’s Magic

Elsa’s conflicting emotions, which provided such depth for the animators, created a flurry of challenges for an effects team that needed to produce her magical snow and ice. “Elsa’s magic is emotional,” Lee says. “We wanted it to have a language that was part of the storytelling and help express when she’s feeling joy, when she’s gnarly, when she’s grieving.”

That intent was so strong the effects team became involved with the story and animation departments early in the process. “We put the film up in storyboards every 12 weeks to get notes from our colleagues,” Lee says. “We wanted to know how they felt and what was possible. We had them hand-draw what Elsa’s magic would look like, and the drawings were stunning. Some of these hand-drawings are still in the film because they fit so well in this magical, not real, world.”

Visual Effects Supervisor Steve Goldberg, who came onto the show soon after finishing Tangled, led the teams of artists who would create the stunning effects. “Michael Giaimo [art director] and Chris Buck [director], who was solo on Frozen at the time, took me to lunch two and a half years ago, told me about the show, and asked me to be a Sherpa guide for all things CG. We had all worked together on Pocahontas – Chris was an animator and Mike was the art director. I thought it sounded intriguing, but I was still recovering from Tangled.”

The more Goldberg talked with Giaimo, though, the more smitten with the project he became. “The studio was interested in what it calls ‘creative R&D,’ and asked me to join with Mike on Frozen,” Goldberg says. “They were having traditionally-trained artists with little CG background do animation tests in paper and pencil – all 2D exploration. I got to work with this group and point them in directions that might influence the film.”

Designer Dan Lund, for example, began drawing 2D snow flurries. “We wanted our snow flurries and gusts to have a lyrical quality, not like someone ran a particle simulation with occasional gusts,” Goldberg says. “Dan came up with gorgeous tests in 2D, experimenting with positive and negative shapes. Elsa has a signature snowflake design that shows up during the film. And Dan’s snowflakes not only had this design, but his snow flurries formed negative shapes that had her signature design. I don’t think we would ever have come across that if someone had just said, ‘We need snow flurries.’”

Working with Goldberg were Effects Supervisors Marlon West and Dale Mayeda. West’s career as an effects artist and supervisor stretches back to Lion King, and includes Mulan, Tarzan, Fantasia 2000, and, more recently, The Princess and the Frog. And, like Goldberg, Giaimo, and Buck, he had worked on Pocahontas. Mayeda has been a 3D animator and effects supervisor on live-action films (Mission to Mars) and Disney Animation CG features (Bolt, Tangled, Chicken Little). Together, they helped bring the 2D design elements into the CG film.

“This was a rare film in which effects came into the process early on,” West says. “We did a lot of 2D tests to suggest things Elsa could do with her magic. We did proof-of-concept tests. We had an effects artist embedded in layout who did previs. We put 2D drawings on cards, and character animators acted to some of them. And, we did 2D animation that drove CG geometry. Every time Elsa freezes a surface, it’s 2D artwork that we took into [Side Effects] Houdini and made ice with it. We had to show who she is at different times of her life, when she is happy, sad, angry. She creates mini-weather systems from her being. We didn’t want snow shooting out of her hands.”

Frozen Frozen

AT TOP, ANIMATORS REFERENCED their pets to give Sven an appealing personality. At bottom, a fight with a giant snowman is the only scene in which characters move on hard-packed, rather than soft, snow.

In one sequence, for example, a panicky Elsa backs up against a water fountain. When she touches it, a frost pattern shoots across the edge, and the water in the fountain freezes.

“The effects artists previs’d the whole thing in 2D,” Mayeda says. “When the effects animators began looking at it, they realized they could grow it procedurally with particles, but the growth pattern would be too realistic. So instead, they used the designer-drawn 2D artwork to drive the shape of the frost patterns formed in Houdini and the timing. The timing from the 2D artwork was snappy rather than linear and procedural. We spent a lot of time on this show incorporating hand-drawn artwork.”

Emotional Effects

During the testing stage, Michael Kaschalk, a studio leader in the effects group and former effects supervisor on Tangled, played around with the idea of capturing designs that Elsa might generate. “He went to our camera capture stage, which we use for digital scouting and handheld camera work,” Goldberg says, “took a wand, and drew arcs like someone with a sparkler on the Fourth of July. We tracked those arcs in space. So, rather than having someone draw curves on the computer, we got interesting shapes and double, triple, quadruple paths. We used these lyrical shapes as forces in a particle system. We ended up not using many of the results, but they informed the simulation that we used later.”

Capturing the wand in space also helped the artists create snow flurries that wrap around Anna when she falls down a cliff and lands in a snowbank. “We had someone sit on the motion-capture stage while we wrapped the wand around and around her,” Goldberg explains. “The main path gave us an overall sense, and then Michael [Kaschalk] played around with aesthetic ideas for smaller tendrils that branch off, working back and forth with the directors and John Lasseter.”

Throughout the film, snow supports the storytelling and Elsa’s emotions. “All the snow is tied to Elsa’s mood, so it has an emotional beat to it – the turbulence as it falls, the angle,” West says. “We ended up with 20 kinds of snow falling in the film. And, all the snowflakes that fall are unique. Our snowflakes follow a path like in nature, with a particle that branches and forms plate-like crystals. Even in Elsa’s magic, which has a 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.”

Frozen

ELSA’S MOOD determined the type of snow created for a scene.

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.

“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 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 elasto-plastic 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 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 with Disney researchers in Zurich, Switzerland – “where it’s easier to get snow,” Kallianpur laughs. There, the team measured the diffusion profile of snow with lasers.

“We had weekly meetings with them and our shading department to talk about the technical aspects of rendering snow,” Kallianpur says. “We ended up shaping shallow and deep subsurface scattering lobes according to real data and then combining the two different effects. It isn’t raytracing through a volume; it’s an approximation. But, we got a nice lighting effect.”

All the hero characters benefited from the R&D into subsurface scattering, as well. “We used a similar technique for their skin with shallow and deep subsurface scattering, but with different lobes than the snow,” Kallianpur says. For the deep snow and snow that the characters interact with, the lighting team used a completely different shading system that lit the snow as if it were a volume.

Olaf, however, provided a unique challenge: He’s a snowman in snow. “We used a cheat to make him stand out,” Kallianpur says. “We always made him pop by using value – he’s a little brighter than the background behind him, or we placed a shadow behind him. Or, if he’s in a saturated environment, he might be slightly more neutral. And, we always had rim light on him.”

Throughout the film, the lighting artists took care to use hues and saturations that kept the film from looking too white, and at the same time, made sure the snow never turned gray. “Even in our bleak scenes when the directors wanted the mood dark and gray, we play our grays slightly blue-violet,” Kallianpur says.

Kallianpur stayed involved with the look of the film all the way through color timing, and is thrilled with the result. “I have seen this film I don’t know how many times, and it’s just amazing,” he says. “I’m biased, but it’s really the most beautiful CG film I have ever seen.”

By successfully integrating 2D traditions into a CG world, one filled with technical challenges, the Disney artists have created a unique film and a certain classic.

“I felt like a gift had landed in my lap,” Goldberg says. “One of the reasons I came to Disney was the promise of applying and blending the fantastic 2D aesthetic and design, those design principles, in a CG 3D medium. I felt we achieved that on this show.”

Barbara Robertson is an award-winning writer and a contributing editor for CGW. She can be reached at BarbaraRR@comcast.net.

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