Towards virtual clay

Providing the user with an intuitive sculpting system similar to real clay is one of the most challenging goals of interactive shape modeling. A user should ideally be able to deform, add and remove material freely in real time, without any geometric or topological constrains on the modeled shape. This chapter reviews and compares three techniques that bring virtual shape modeling closer to this objective. The two first ones rely on a specific representation of the sculpted shape, namely the iso-surface of a scalar field stored in a grid. Because this representation conveniently captures topological changes, adding and removing material is straightforward. Based on this representation, we compare a geometric versus a physically-based method for handling local and global shape deformations that make the model closer to virtual clay. We also discuss solutions for providing users with an intuitive interface with haptic feedback. A third and very different approach is to define the deformations of the sculpted model as spatial deformations, such that the operation is applicable to a wide range of shape representations. We show that the extension of sweepers (presented in the space deformation chapter of this tutorial) to constant volume "swirling-sweepers" produces an intuitive clay-like behavior of the modeled shape. This technique provides a very good alternative to physically-based virtual clay when preserving the shape's topology is desirable.