Flexible models or deformable templates are now widely used to aid image interpretation. Such models usually have a number of parameters to control the shape and pose of all or part of the model. For instance Yuille et al  and Lipson et al  use `hand-crafted' models of faces and transaxial slices through vertebrae. Scott  and Staib and Duncan  use closed contour shape models, based on expansions of trigonometric functions, for the interpretation of medical images. Pentland and Sclaroff  and Nastar and Ayache  work with finite element models of flexible objects. Grenander, Chow and Keenan  and Mardia, Kent and Walder  describe statistical models of shape. Bookstein  has applied statistical techniques to learn relationships between shape and other variables for morphometric analysis.