For smooth curved surfaces, their profiles are dominant in images. They are called apparent contours and are rich sources of geometric information about the surfaces and motions. These apparent contours are the projection of the so called contour generators, which separate the visible and occluded parts on the surface (see Fig. 1).
The contour generators slip over the surface under viewer motions, and the apparent contours observed from the different viewpoints do not have any physical correspondence in general. Thus, the computation of the epipolar geometry is very difficult.
Figure 1: Apparent contours and contour generators.