Direct manipulation of interactive character skins

Abstract

Geometry deformations for interactive animated characters are most commonly achieved using a skeleton-driven deformation technique called linear blend skinning. To deform a vertex, linear blend skinning computes a weighted average of that vertex rigidly transformed by each bone that influences it. Authoring a character for linear blend skinning involves explicitly setting the weights used to compute deformed vertex positions. This process is tedious, repetitive, and frustrating not only because the deformed vertex positions are not intuitively related to the vertex weights, but also because the range of possible deformations is unclear. In this paper, we present a method that lets users directly manipulate the deformed vertex positions in a linear blend skin. We compute the subspace of possible deformed vertex positions, display it for users, and let them place the vertex anywhere in this space. Our algorithm then computes the correct weights automatically. This method lets us provide a skin editing interface that gives users as much direct control as possible and makes explicit what deformations are possible.