A different way to constrain the image irradiance equation: add another one.

* Photometric stereo:* use more images; same viewing direction,
different illumination conditions (e.g., time of day).

Varying the direction of illumination changes the reflectance
map **R**. Suppose we measure for two illumination
conditions. We get two equations for :

The intersection of the isophotes in **p,q**-space determines the surface
orientation at .

Wed Oct 18 09:18:18 EDT 1995