Producing wrinkles and other effects for a computer-generated character based on surface stress

Abstract

Wrinkles are produced by computing directional stress, whether compression or stretching, for each pixel within each face of the mesh representing the skin, and then perturbing a surface normal based on the computed stress at each pixel in that face of the mesh. Directional stress at a given frame in an animation is determined, in general, by comparing the current state of the mesh at that frame (called a 'current pose') to the original state of the mesh (called a 'rest pose'). An artist specifies a wrinkle pattern by defining a texture that is mapped to the surface, using conventional techniques. A gradient texture is created from this wrinkle texture by computing the gradient at each pixel in the wrinkle texture. For each location in a face of the surface, the vector from the gradient texture is mapped to the corresponding face of the rest pose skin model and the current pose skin model, to produce two surface vectors. These two vectors are compared to provide an estimate of the surface stress at this location in the face. A wrinkle effect may be implemented using bump mapping, but the surface normal is perturbed differently for each location in the face of the mesh based on the skin stress estimated at that location. Other effects also may be created using the estimated stresses.