Assisted Articulation of Closed Polygonal Models

Abstract

Creating articulated geometric models is a common task in animation systems. In some instances, models are procedurally instanced, and articulated degrees of freedom are designed into the model. In other instances, the model is some geometric assemblage, and an articulated skeleton (sometimes called an “I-K skeleton”) is bound to the model by the user, typically by manual indication of a correspondence between elements of each structure. In either case, some binding must be made to couple boundary motions to those of the skeleton; this can be done for example by generating spring networks or spatial deformation fields. Both processes can be tedious in the ordinary case, especially when the model to be articulated is given only as a boundary representation, for example a polygonal mesh representing a character’s skin or clothing, or an object’s surface.

We present a simple method for assisted articulation of geometric models. Given a 3D polygonal mesh representing an object, an approximation to the mesh’s medial axis is efficiently computed using a 3D Voronoi diagram of the mesh vertices, and connectivity information within the mesh. The medial axis is then simplified; the resulting tree structure has chains of edges and nodes. We interpret selected nodes as joints of an I-K skeleton, and the chains connecting them as its links. A spring network is then generated to bind the I-K skeleton to the object boundary, so that skeletal motions will affect the boundary in a reasonable way, as specified by the animator.

We show a user interface that allows interactive editing of the automatically constructed skeleton, and demonstrate the import, and mapping of key-framed motion capture data to, a variety of initially static polygonal objects.