Normal transformations for articulated models
It is well-established that when a matrix is used to transform a rigid object, the normals should be transformed by the inverse transpose of that matrix. For skeletally animated models, it is common to apply this approach to the blended matrix that animates each vertex. This is only an approximation, as it assumes that the blended matrix is locally constant. We derive a correct method for normal transformation in skeletally animated models, and examine the errors introduced by two approximations.