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viscous fluid simulation. "The metal starts to sag and embers spark," Dessero says. For the tower destruction, modelers pre-scored the geometry and then used rigid-body dynamics from within Houdini to break it apart when it crashed to the ground. "Chris filmed some beautiful water tank shots with multiple cameras," Dessero says. "We added debris and part of the construction trailer in the water. The water under the camera is all-CG, simulated with Houdini." Extreme Weta These shots all lead to the finale created at Weta Digital, where an army of CG iron men fight CG digital doubles filled with the extremis effect. "When we talked to Weta, they were finishing The Hobbit, so they had hundreds of people available," Townsend says. "We gave them the least-developed 500 shots and the last we would shoot. Their deadline was the end of March. So we needed a heavy hitter. They had a tremendous amount of work." The big battle takes place in a seaport around giant container cranes. The evil extremis mercenaries have captured the president and plan to sacrifice him during a television broadcast. Tony Stark arrives with his own army of iron suits. "We had to hit the ground running," says Matt Aitken, who, along .com Video: Go to "Extras" in the May/June 2013 issue box ■ WETA DIGITAL built 36 iron suits for the final battle from 10 hero suits. with Guy Williams, supervised the visual effects at Weta. "We have suits flying around, taking on the extremis mercenaries. We have a lot of destruction. And, I think we might have had the tightest deadline ever. We didn't have plates until mid-December." To lace the digital doubles with the orange, glowing extremis effect, Weta relied on their "genman" rigs (see "Of Gollum and Wargs and Goblins, Oh My!," January/February 2013) configured with a skeleton, muscles that drive the skin, networks of veins, more finely detailed muscles, and organs. "We needed a correct representation of what it looked like to have an internal glowing energy source," Aitken says. "We used a technically correct approach to lighting, raytracing with interior light sources, which really paid off. The light sources are just like another light. The geometry provides the complexity and visual detail. We didn't get too anatomically correct – we didn't want it to look gruesome – but, we illuminate biological detail: the muscles, veins, layers of skin, the interior of the body. We had the extremis effect in hundreds of shots, and it was largely automated. If we had created it by hand, we'd still be compositing." For Tony's remotely controlled iron suits, the artists began with 10 hero models from Digital Domain, and then derived additional suits from hero components, on which they varied the textures to create a total of 36 suits. A new shading system dubbed "Gen2" provided energy conservation between layers on the surface so that the aluminum, emulsion, and clear coat have a different response based on the effect each has on light passing through the layers. The lighting artists also called on Weta's new real-time "pre-lighting" tool called Gazebo (see "Shaping Middle-Earth," January/February 2013) to help speed animation approvals. "We used Gazebo to render animation for our client reviews," Aitken says. "We could render shots in stereo while we were still developing final lighting for the shots." Final rendering was through Pixar's RenderMan. Aaron Gilman led a team of animators who performed the suits, the army of extremis mercenaries, the moving cranes, and even the destruction. "It was a massive aerial battle with mercenaries jumping off cranes, hand-to-hand combat, blaster fire, suits thrusting all over the background," Gilman says, "32 suits beating the crap out of mercenaries and getting the crap beaten out of them. We had previs for shots with hero performances, but nothing that really defined the nature of the aerial battle – how many suits, where they'd fly, how fast. We needed to choreograph those performances." Once the battle begins, everything is digital, and within this chaos, the animators had several basic challenges. They needed to make sure the audience knew the mercenaries could win. They needed to have interactive performances between the mercenaries and the iron suits; they couldn't just have suits flying around. And, they needed to create a battle that didn't detract from the star performances. "We needed to do the opposite of hero performances," Gilman says. "We had to learn how to design performances that didn't take center stage. Guy Williams told us to take a camera, stick it on a tripod, and put it in the middle of a school courtyard at lunchtime. That's what we wanted; we wanted the viewer in the middle of the fight. And, we wanted to tell the story without drawing the viewer's eye, so we broke a lot of rules in terms of composition, staging, and timing. We broke the rule of thirds. Sometimes you see only the thruster boots of a suit." The fundamental challenge, though, was that Tony Stark would jump from suit to suit. That is, as he ran, jumped, and fought, various suits formed around him in turn. "He'd do these skydiving moves and a suit would fly around him," Gilman says. That created a pipeline problem: The animators had to break apart the suit model – change the geometry – to fit it around Downey Jr. in the transformation shots. But, traditionally, animators work with a puppet that references a high-res model and textures that live elsewhere in the pipeline. "We can't change the topology on the global model," Gilman CG W M ay / June 2013 ■ 25