The approach just described is * global*: the equations involve **p,q**
at all image points at the same time.

An alternative (Pentland): derive * local* shape from derivatives of
intensity ---

- Assume surface Lambertian, locally spherical; point light at infinity.
- Recover surface orientation by matching derivatives of measured intensity to those for image of sphere.

Wed Oct 18 09:18:18 EDT 1995