Figure 1:Shape from shading and texture
In this section we shall consider the case of shape from shading as this is probably the most studied and well understood area. In order to achieve this, we need to make a number of simplifications about the nature of the surfaces and the lighting in particular. Robust shape from shading from real pictures remains a difficult problem.
As an illustration of the method, Figure 2 shows needle diagrams of simple
spheres, computed from the shading pattern and the extremal contour which
surrounds the sphere's surface. In fact this result was obtained by a
minimisation method, but it serevs as an illustration. The difference between
the two pictures is in the raytracer; the one on the left uses a more crudely
factetted approximation and the needle map is further for a true sphere. Another
illustration is given in Figure 3 from [Horn, Robot Vision, 1986]. This uses the
tip of the nose, or more accurately a quasi-circular strip surrounding the nose
as the set of starting points for the growth of the characteristic strips. The
result is a relief map of part of the face. 
Figure 2: Building the surface normal map of a
sphere 
Figure 3:
Building the shape of the face from characteristic strips