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Reader SurveyStep by Step: The Fuel In BMW’s Hydrogen
Apr 1, 2008 12:00 PM, By Ellen Wolff
When the Visual Effects Society recently chose the year’s outstanding visual effects commercials, the work of Sydney, Australia-based Fuel VFX was among them. The 30 second spot that Fuel created for the BMW Hydrogen 7 automobile features 25 seconds of CG animation—and not easy-to-do CG, but fluid simulation. The commercial was designed to visualize the green technology that produces emissions of pure water vapor instead of CO2, so Fuel’s assignment was to literally build a car out of water. The sedan takes shape as stylized blue water fills the chassis, the engine, the wheels, and the headlights. Then, in a dramatic finale, the fully assembled water-car collapses into a puddle and evaporates.
Director Andrew van der Westhuyzen of the design firm Collider, along with BMW's agency GDS&M in Austin, Texas, provided a brief for Fuel VFX Supervisor Simon Maddison to start with. “The intention was for the car to be magically formed out of water, without anything holding it in,” Maddison says. “So philosophically, there couldn’t be anything in there but water.”
The flow of the water had to feel uninterrupted as it poured into the car’s cavities, and the camera had to move into a final reveal of the finished car. The nine shots of high-def fluid simulation that Fuel created for this spot had to be completed in a matter of weeks, which added to the degree of difficulty.
Maddison’s team brought some experience to the task, since Fuel had been working for the past five years with Next Limit RealFlow simulation software (a recent honoree at the Academy of Motion Picture Arts and Sciences’ Scientific and Technical Awards). “We had a pipeline that could handle the rendering and the scale of this project, and we knew what the challenges were and where the software was at. But this was the biggest fluid job we’d taken on, so there was a leap of faith,” Maddison says.
The x-factor of creating fluid sims on deadline is the inherent uncontrollability of the basic process, which requires that you let it run and see what happens. “It’s very hard to direct procedural real-world dynamics very specifically. We could craft it to a certain point, but then it was up to the physicality of fluids. When you fill a car full of water and drop it, it’s going to do what it does,” Maddison says. “We could change some things to a point—like how viscous the fluid was and how fast it flowed. But in the end, it’s a big body of water falling on the ground.”
“We started with some viscosity and speed tests of fluids moving from one vessel to another, just to get a scale for it. That was important because it needed to match the scale of actual pieces of the car as we built it up.” Except for a single live-action shot of the actual vehicle at the end of the commercial, the car would be a pure CG representation, so accuracy was key. To correctly model the car’s components in Autodesk Maya, Fuel used BMW’s CAD/CAM files as a foundation. “They sent us 30 gigs of CAD files, right down to the workings of the suspension,” Maddison says. “That data was good for CAD/CAM but pretty ugly for what we needed. We had to rebuild everything to be usable vessels for the water sims.”
Fortunately, Fuel was given the freedom to choose the components that would look most interesting when filled with water—except for the car’s signature headlights, which BMW wanted in the spot. “The director and I then worked on narrowing our choices down. Then we basically built an animatic in Maya from those shapes before we ran any fluid through them. We wanted to make sure that we had things with the right design properties that would fill the screen in ways the director wanted,” Maddison says.
“There were two stages to the edit. We gave the editors several rushes of empty 3D objects with cameras moving over them. That process gave us a shot list. The tight deadline meant that we had to be really efficient with how many simulations we generated. Then we started running the fluid through those objects. We also applied multiple camera views around those same objects, which opened up even more interesting avenues for shots, Maddison says. “We started designing the cameras based on the movement of the fluid through the objects, so for some simulations we had five or six different camera views. We delivered those to the editor as rushes, and that’s what they cut together.”
The necessity of having the water flow through actual car components put some unique constraints on the RealFlow simulation process. “The fluid sometimes got stuck in places where holes weren’t big enough for it to flow very fast,” Maddison says. “For example, we needed to put a waterfall behind the headlights to get water flowing into them. So there was a bit of manipulation, especially when there were no entry points in the actual geometry.” Sometimes that meant drilling virtual holes to facilitate the water flow, while at other times, the car components would spring leaks in unexpected places that had to be plugged. And though the fluid starts flowing from different points to fill up the car, it had to appear as though it was all flowing from one source.
When the camera finally moves around to reveal the whole car collapsing in a splash, the fluid had to be able to run out and disappear very quickly. “We built a floor that this fluid would drain into. If you could look at it from above, it would appear like spokes of a wheel. There are about nine gutters radiating out from the center to form different grooves, Maddison says. “At the end of the spot, you’re looking straight along one of those grooves. The ground had to react to the water, but didn’t render as anything. When the fluid car collapses, it runs across an invisible floor.”
Maddison credits the reliability of RealFlow for helping Fuel get the job done. “I was surprised by its ability to turn around simulations without crashing. It generates particles that have volume and viscosity, and then it builds a mesh around that. It’s not fast. The sim is a one-machine process, but you can farm out the meshing of it. We’d run some simulations overnight, particularly the final shot of the car collapsing. The larger the scale of the water, the more polygons you need to describe it. You can end up with billions of polygons,” Maddison says. The shot of the car collapsing contains at least nine simulations, including the wheels, the chassis, the engine, the seats, the steering column, and the drive chain.
While the final shot was a composite of multiple simulations (2D work was done with Apple Shake and Autodesk Flame), it needed to be rendered as a piece, because the surfaces needed to refract light. A prime challenge was creating a shader for Mental Images mental ray renderer that would make the fluid appear uniformly blue and still read as water. “We were pretty careful that nothing got too thick next to something that was too thin,” Maddison says. “We worked out a way to almost average the color so that it never got too light or too dark. When fluids doubled up, it got heavy very quickly, and we needed to solve that with the shader. There was a fine balance we had to strike to maintain the look of a fluid but also be able to show the inner workings of the engine and the seats behind it. When we rendered this with shaders that had real-world fluid properties, the center of the car just went black, and the edges went completely white. So it was a real technical challenge to see the surfaces of this car and see it as a whole.”
“We wrote the shader so it didn’t take long to render,” says Maddison. “At the end of the day, it was two passes that were comped, plus a slight color grade.” He admits that the process of running fluid sims can be hypnotic. “Some of them were so mesmerizing we’d watch them over and over again. We’d realize we’d been sitting there for 10 minutes and not talking. It was a big thing to see this car disappear without a trace.”
Credit Roll:
Director: Andrew van der Westhuyzen for Collider
Editor: Stewart Reeves for Guillotine
For Fuel VFX:
VFX Supervisor: Simon Maddison
Modelling: Michael Orbing, Grant Warwick
RealFlow Artists: Michael Bain, Romain Buignet, Matthew Hermans
Shake Artists: Sam Cole, Danielle Hession
Shading and Lighting: Pawel Olas
Flame Artists: Karen Fabling, Zelko Dejanovic
VFX Producer: Dave Kelly
Fluid Simulation Technical Directors: Mike Bain, Sam Cole
For GDS&M:
Creative Directors: Mark Ray, David Crawford, Jay Russell, Wade Alger
Art Director: Bill Marceau
Producer: Paul Golubovich
For Collider:
Executive Producer: Sam Zalaiskalns
Postproduction Producer: Pat Dedal
