DreamWorks Animation's newest adventure, Rise of the Guardians, set the studio several new effects challenges. Each major character - the Sandman, Jack Frost, Santa Claus, the Tooth Fairy, the Easter Bunny and the evil Boogeyman - had their own unique power and therefore required distinct effects personalities. We talk to Head of Effects Yancy Lindquist.
The Guardian of Dreams, Sandy (the Sandman) does not speak at all during the movie but instead conjures up his thoughts, and dreams, via golden sand objects and 'sand streams'. These range from dinosaurs, to unicorns, airplanes and candy canes - all needing to be crafted by the effects department.
After some early R&D, the process of creating the sand shapes involved conceptualizing the shapes, then modeling an enclosed version using sub-division surfaces that would phase volumetrically. That would then be filled in with points inside Side Effects Houdini. One particular challenge with the sand shapes arose when the shapes moved into another shape, which the effects team found hard to achieve via fluid simulations alone, and so several techniques were ultimately relied upon.
"For example, in the movie there is a sand unicorn that a child is dreaming about," notes Lindquist. "That involved binding the points to the deforming unicorn, then when that needed to break apart and start flowing, we'd pull that apart with a combination of particle and fluid simulation. We let the fluid simulation pull it along, and then when we finally get to the other shape we would then use a little bit of particle sim to pull it closer. We'd then do a little trick as the particles get really close where they would start filling in the shape. From that we were able to get sims that felt natural. We even had some flocking simulations in there."
A number of 'sand tools' were developed to help artists achieve the desired look. "The workflow inside Houdini is based around dropping down operators to perform tasks," explains Lindquist. "We could configure them in different ways to achieve various effects." The sand effects were rendered in Mantra, with the DreamWorks Animation-developed OpenVDB format relied on to create high resolution sand volumes. (OpenVDB is an open source C++ library comprising a novel hierarchical data structure and a suite of tools for the efficient storage and manipulation of sparse volumetric data discretized on three-dimensional grids - see http://www.openvdb.org).
The film's villain, Pitch (the Boogeyman), finds a way to turn Sandy's sand streams into nightmares. The now black particles of sand also took on different shapes and moved in new, gruesome ways. DreamWorks also made the nightmarish shapes more pointed and angular but, according to Lindquist, "we still needed something to make the motion feel that it wasn't the gold sand - that there's something evil or creepy."
After discussing the issue with visual effects supervisor David Prescott, Lindquist and the effects team came up with the idea of running the sand simulation backwards. "It created a really creepy feel to the look," he says. "It's not that all the sim is running backwards, but parts of it are. The black sand will start a little bit away and then suck in almost like a vacuum cleaner - it had an eerie feel to it."
Another system was also developed for the trails that wisp off the black sand nightmares. "The developer on that would take curve trails off the back of the nightmares and extrude or run some ribbons from those trails," says Lindquist. "We wanted them to break off as the nightmares were running, so when the nightmares' legs started moving fast enough they actually went transparent and we replaced the legs with particle hits. So what you would actually see is the legs would be completely represented by just particles coming off the hooves. And it created this interesting ethereal feel to the nightmares when they were running or when they were in the air."
A freeze is coming
Jack Frost is the spirit of winter and the Guardian of Fun. To depict his power and show frost slowly taking over an object, the effects team sought out a very naturalistic approach. "Our developer on that would take a model, whatever the frost needed to grow on, and he would run a cellular automata simulation across it," says Lindquist. "The cellular automata process lets you cover a surface in particles and then you turn one particle on. What it does then is say, 'My neighbors on the next frame or next step in the simulation - they can only turn on if I'm turned on, and then those neighbors can turn on.'"
"It's kind of like at a sporting event where they have The Wave," adds Lindquist. "When it's my turn to do my portion of The Wave I can't stand up until my neighbor stands up first. That's kind of how cellular automata works where a particle can't turn on until one of its neighbors has turned on, in effect creating a wave of particles turning on across the surface."
The rules for the sim were based on paintings on the surface. There would be a starting point and the painting over the frost surface would be a guide as to where it would grow. "After you have that wave crawling across the surface," says Lindquist, "we then traced millions of curves through that and they were then rendered on the surface with our in-house renderer to generate the final frost look."
Santa Claus (North) - the Guardian of Wonder - travels the world via a a magical snow globe portal. The effects department designed the look directly, adding snow elements and bursts and also working to ensure the portals appeared to be deep in the frame.
"If the portals were thrown in front of a wall, say, and the front of the portal was ten feet in front of the wall, we wanted it to actually feel like it was going back 100 feet," says Lindquist. "We had our effects developer working with a Nuke artist and they rendered a depth map of the portal. They could offset the depth map even further, so all that depth in the portal was created just by adjusting the depth information just in one eye."
There were, of course, so many other kinds of effects required for Rise of the Guardians, including fire, smoke, a river made of paint and giant black clouds for the climatic finale. For the clouds, in particular, DreamWorks Animation leveraged off its recent work for the beanstalk scenes ktunnel in Puss in Boots, this time looking to make the tools even more artist friendly by controlling more detail in the shader at render time (in Mantra) rather than in the composite.
All images and clips copyright © 2012 DreamWorks.
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