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By Daniel Terdiman, 28 May 2009 17:06

NEWS

If you want to consider a difficult computational problem, try thinking of the algorithms required to animate more than 10,000 helium balloons, each with its own string, but each also interdependent on the rest, which are collectively hoisting aloft a small house.

That was the challenge the production team at Pixar faced when it set out to begin work on Up its 10th feature film, five years in the works, which hits cinemas in the US on Friday.

There was absolutely no way the team was going to hand-animate the balloons. Not with their numbers in five-figures, and especially not when you consider that within the cluster, every interaction between two balloons has a ripple effect: if one bumped another, the second would move, likely bumping a third, and so on. And every bit of this would need to be seen on screen.

In Up the story revolves around the main character, 78-year-old Carl Fredricksen, who, frustrated with his mundane life, ties the thousands of balloons to his house and sets off for adventures in South America. A small boy ends up marooned on board, and hilarity ensues.

Steve May, the supervising technical director on Up, said: "You have a movie that's about a house that flies, which is a pretty far-fetched idea. We all know, from kids' parties, how a bunch of balloons behave, so if we could animate balloons in a realistic way, the believability that the house could fly would sell."

Balloon animation
May said the animation department at Pixar never even considered hand-animating the balloons. But even standard computer animation wouldn't be up to the task, because of the N-squared complexity involved in the thousands of interdependent balloons. Instead, the studio's computer whizzes figured out a way to turn the problem over to a programmed physical simulator, which, employing Newtonian physics, was able to address the animation problem.

"These are relatively simple physical equations, so you programme them into the computer and therefore kind of let the computer animate things for you, using those physics," said May. "So in every frame of the animation, [the computer can] literally compute the forces acting on those balloons, [so] that they're buoyant, that their strings are attached, that wind is blowing through them. And based on those forces, we can compute how the balloon should move."

This process is known as procedural animation, and is described by an algorithm or set of equations, and is in stark contrast to what is known as key frame animation, in which the animators explicitly define the movement of an object or objects in every frame.

Procedural animation has been around for some time, but May suggested that even the most difficult uses of it in the past don't come close to what Pixar had to achieve in Up.

Pixar fans may remember the scenes in Cars of a stadium full of 300,000 car fans cheering on a high-speed race below, each of which was independently animated. That, too, was done with procedural animation, May said, since creating so many cars individually would have been impossible.

Getting the simulator humming properly is no easy task. May said it involves setting rules for how individual objects should behave, giving the computer these initial conditions, and then "let it run".

Oddly...

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