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One Per Cent: A New Scientist Blog

How to make a fat man jiggle in 3D animation

Paul Marks, senior technology correspondent

For cartoon characters like the hapless Homer Simpson, being fat is often a matter of pride.

But for 3D animators who have to model body fat - and the way it jiggles about as a rotund character moves - it's a major problem: doing it convincingly requires large amounts of computer processing power and expensive rendering time.

Computer scientists Junggon Kim and Nancy Pollard at Carnegie Mellon University in Pittsburgh, Pennsylvania, say that instead of simply modelling a 3D cartoon character as a deformable human shape, they should additionally build a separate skeleton into the body - and model the way the deformable material (flesh and fat) behaves as the skeleton moves it.

While skeletons are widely used to animate 3D characters, driven by motion captured from a human, they have rarely interacted with the deformable material that surrounds them. Kim and Pollard's innovation was working out an algorithm that can do that with the fewest possible processing cycles - and so the shortest rendering time, making it more economically feasible for animation studios.

Their 3D graphics algorithm attaches the volume pixels ("voxels") of the deformable body to the skeleton. "So when the skeleton moves, the fixed body points also move along the bones and this leads to the passive jiggling movement of the deformable body," Kim says. But they have also geared their algorithm for processing on parallel graphics processor units (GPUs) which, in the case of the 'Fatman' animation above, sped rendering by 10 times.

"Normally, this is a very time-consuming task because of the massive computation required to capture dynamic deformation of an elastic body," says Kim. "Our technique boosts the simulation speed by orders of magnitude while preserving the quality of the simulated motion." Their work also applies to nonhuman forms - you can see how they have applied it to the animation of a trapped starfish escaping from a box (like they do) on this page, where they reveal how these animations can be controlled.

Journal reference: ACM Transactions on Graphics, DOI: 10.1145/2019627.2019640

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"While skeletons are widely used to animate 3D characters ... they have rarely interacted with the deformable material that surrounds them"
What? "Spring systems" have been in common use for modelling jiggling fatty parts of computer game characters for a very long time, and more advanced techiques are even used in some engines.


Yes, "mass-spring systems" have been used for a long time, and using more advanced techniques (e.g., finite element methods) is not new either. But, in general, if we want more accuracy or realism in the simulated deformation, we need to spend more time to get it. What we have done here is to speed up an advanced deformable body simulation interacting with the underlying skeleton system while keeping the modeling accuracy as much as possible. The rich jiggling of the fat man character were computed using nonlinear finite elements in real time on a desktop PC. Another feature, which has been cut out from the video shown above, is that the system can also handle the "two-way" coupling between the deformable body, skeleton, and an obstacle in the environment in the same way. (By the way, I agree that the word "rarely" isn't appropriate in the article and it was my mistake in communication.)

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