The team started by adjusting solid models with hollow voids in the interior. The centre of mass is also placed as low on the rotation axis as possible. This balances the asymmetrical form factor of the object, allowing it to spin, while 3D printing allowed the team to rapidly prototype their objects.
For objects where hollowing wasn't effective, the team used "cage-based deformation", deforming both the interior and exterior voids to smooth the shape into one that better allowed for hollowing. Finally, they used a technique they called "dual-density optimisation"; that is, using materials of multiple densities as fill, allowing the external shape to remain unmodified.
"Our approach is effective on a wide range of models, from characters such as an elephant balancing on its toe, or an armadillo break-dancing on its shell, to abstract shapes," said Moritz Bächer, a post-doctoral researcher at Disney Research Zurich. "It's well-suited to objects that can be produced with a 3D printer, which we used to make tops and yo-yos with unusual shapes but remarkably stable spins."
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