Correct normal transformations for articulated models

Merry, Bruce, Patrick Marais and James Gain (2006) Correct normal transformations for articulated models. Technical Report CS06-01-00, Department of Computer Science, University of Cape Town.

Abstract

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. However, this is only valid where the transformation matrix is locally constant. This is not the case for models animated with skeletal subspace deformation (SSD), where the transformation matrix is computed for each vertex. We derive a formula for correctly transforming normals on SSD models.