Computer Graphics World

July/August 2014

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62 cgw j u ly . a u g u s t 2 0 1 4 V I S U A L E F F E C T S devised a method to ramp up the tension. "We realized that a lot of shots needed to be based on the eff ects of the tornado, not the tornado itself," Rahhali says. "So I went through every shot and graded it from one to fi ve based on wind speed and the severity of the weather. Then, I provided reference for each category." Shotgun so ware helped the crew keep the materials organized per shot. "Doug [Bloom] would have eff ects dailies with me, and we'd go through the elements on the layers," Rahhali says. "We had lots of simulation layers, and they all had to talk together." To help speed approvals, Rahhali targeted a few "tent- pole" shots. "We'd start with concept art and then push the look de- velopment and R&D forward," he says. "When Steve [Quale, director] was happy, we had a target. The weather system might be more intense in subsequent shots or less in pre- vious shots, but it was a good middle ground that showed the layering, what we needed to do in eff ects, and what we would need to composite." M I L E - W I D E T O R N A D O In the fi lm, to save the people holding on inside the drain, a storm chaser uses his truck to pin a grate into the opening and keep those inside from being sucked out. When the storm calms, they believe they have survived. But, not so fast. They are inside the eye of the storm. The camera moves from dialog scenes inside to the storm outside. We see cars, trees, and the school bus sucked up into the funnel. The backside of the tornado is even more powerful. "When we fi nally see the whole tornado, we had to make it look massive," Rahhali says. "And, Steve [Quale] wanted the tornado to look real. We looked at video reference, and then spent a lot of time building atmospherics and layers and layers of reality." The spinning funnel needed to integrate with the ground below and the slowly rotating supercell above. So, to have control in compositing, the team rendered the tornado into sections. At the ground, a dust cloud twisted and wrapped particulate matter, rendered as a debris pass, up into the core to a certain height. "That gave us the tornado reaching out and pulling stuff into it," Rahhali says. "The dust cloud is nearly twice the diame- ter of the tornado." Individually animated ele- ments, such as cars, were hold- outs and shadow passes that compositors mixed into the dust cloud and into the core volume. The core was a dense volu- metric cloud surrounded by a wispy exterior and, occasionally, mini-tornadoes that extended out from the main body of the tornado. At the top, a stormy cloud environment, also vol- umetrics, formed the slowing rotating supercell. To move the mile-wide tornado across the plates, animators set a representative cylinder in motion. "We used animation to drive the rough placement, rota- tion speed, and movement," Rahhali says. "We tweaked it to maintain the scale. From that, we drove the volumetrics in Houdini, the twist, and the speed. We had lots of RBD [rigid-body dynamics] debris and quite a bit of animation. We had so many hero elements, we animated them; we didn't want to run it all through simulation." Matte painters created skies with layers of separately ani- mated clouds, and compositors fi t it all together. "Compositors had elements from lighting and eff ects that they tweaked in the comp to get the right balance and look," Rahhali says. "They also added rendering eff ects, like drops of water on the [virtual] camera lens, to add chromatic aberra- tion, as if the camera was sitting in this environment." Throughout the postpro- duction, all the members of the team had to be in constant communication in order to imitate the fi erce storm. "We didn't have a ton of time – only three or four months," Rahhali says. "And, it was very complicated work. There were so many layers and multiple people working on them. We had to be extra diligent to make sure each layer worked together in the end." One only needs to look at the 1996 fi lm Twister, which repre- sented state-of-the-art visual eff ects at the time, to see how far computer graphics and com- puter graphics artists have come. You may not be able to fool Mother Nature, but these artists come closer each year to making her believable on screen. ¢ Barbara Robertson is an award-winning writer and a contributing editor for CGW. She can be reached at VIDEO: GO TO EXTRAS IN THE JULY/AUGUST 2014 ISSUE BOX C G W. C O M ANIMATORS SET THE TORNADO IN MOTION USING A REPRESENTA- TIVE CYLINDER. A CG DUST CLOUD AT THE BOTTOM PULLED DEBRIS, RENDERED AS A SEPARATE PASS, UP TO A CERTAIN HEIGHT. COMPOSITORS LAYERED THE DEBRIS INTO THE CORE VOLUMETRIC TORNADO TO INTEGRATE THE TORNADO WITH THE GROUND.

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