Giant Moles, Rushing Rivers, and "Pandemoniun" Reign in "City of Ember"
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Water, Water…Everywhere!

Luma Pictures' VFX Supervisor, Vince Cirelli, would wholeheartedly agree. His company was tasked by Durst with creating not only a "natural-looking" giant, star-nosed mole that chases Doon and Lina through the City of Ember, but also the realistic-looking river and the generator powered by it.

When a CG object like flowing water hits an obstruction, like a river bank, it breaks into thousands of pieces, making the image exponentially more complex to render. Image courtesy of Walden Media and Luma Pictures.

"They should add 'water' to the list of things that are difficult to work with in film, like animals and children," laughs Durst, who says the filmmakers originally considered using millions of gallons of actual H2O before deciding that working with real water was prohibitive. "But CG water is notoriously complex to render," says Durst. There are many different kinds of water—clear, muddy, gently flowing, cascading—and, because of its reflective and transparent properties, its look is influenced by its environment and all available light sources, both natural and artificial. Making matters more difficult, when a CG object like flowing water hits an obstruction, like a river bank, it breaks into thousands of pieces, making the image exponentially more complex to render. Now each one of these thousands of objects must reflect and refract light, whereas, originally, there was just one.

"We believe effects development is one part hardware and software, and three parts ingenuity," says Cirelli, whose company's strategy was to deliver as many iterations, often low-res simulations, of the "rushing river" effect as possible to Durst and Kenan for their feedback, before rendering the high-resolution versions, which would take weeks to render on a dedicated render farm.

One of the most difficult water "set pieces" was a scene in which Doon and Lina are chased on a boat speeding through a labyrinth of underground rivers and rapids. Using fluid-simulating software RealFlow, the Luma team started by submitting low-resolution "roughs" for Durst and Kenan's approval. "At this early stage, the water looks like gelatinous goo," Cirelli explains. "The main purpose of these low-res simulations is for the director to get a sense of the speed, shape, and density of the water."

Luma programmers then wrote scripts (in C++) to partition the simulation into much smaller portions, chunks of the cave, so to speak, that greatly sped up the revision process. When all of the smaller, diced-up sections of the simulation were complete, Luma simulated them together so that the various elements would properly react to each other. As Cirelli notes: "When running simulations based on real-world physics, it can be difficult to trigger specific events throughout a timeline. We took an inverted approach to our simulations and 'sculpted' the way the water thrashed down the cave by modifying the virtual cave floor."

To make the rushing water slow down, for instance, a series of horizontal divots, channels and ramps were created on the virtual cave floor to make it "lap up and roll on itself during a tight turn." Initially, Kenan wanted the river rushing at a speed of approximately 70 mph, but "in the real world, water would explode up the sides of the cave wall, as if blasting from a hose," says Cirelli, "so we amplified the property of gravity by six times in order to 'choreograph' the rushing water to Gil's specifications."

The Biggest Star in the Picture

Kenan had very interesting specifications for the giant, 6-foot star-nosed mole that chases Doon and Lina through a maze. "If you've ever seen a real star-nosed mole, it's quite a creepy creature," says Cirelli, "with these tentacles that extend from its snout." What the director wanted is for the mole's nose "to react first, then its body to follow." As it got closer to Doon and Lina, for instance, the mole's nose would start "twitching with rage, or, in its more inquisitive moments, would feel around and explore with its tentacles."

The real star-nosed mole is one of nature's creepiest creatures...

...but the virtual star-nosed mole is no pageant winner either. Image courtesy of Walden Media and Luma Pictures.

Of course, the team responsible for creating the star-nosed mole studied the real thing. In fact, says Cirelli, "Our head modeler was lucky enough to find an actual star-nosed mole on his property in Pennsylvania." The biggest challenge in creating a furry CG creature like the star-nosed mole is getting its skin and fur to "roll up and bunch and jiggle," as it would in real life.

"We made two extreme, but very rudimentary, versions of what would become the mole in Maya," says Cirelli, "one with very soft, pliant properties, almost like a pillow, and one that was more rigid. We painted the areas we wanted to 'bunch up,' and in a very short time, we were able to sculpt realistic muscles and skin." Another challenge: "scaling up" the mole. As Cirelli explains, "You'd think a large, fat, furry creature would sort of lumber along, like a bear, but Gil wanted the CG giant star-nosed mole to be fast and quick enough to give the kids a run for their money which, of course, ratcheted up the drama during the chase."

"We're excited for audiences to get out and see the film," says Durst. "Hopefully, if we on the effects team have done our jobs right, they won't even notice us."