The fundamental difficulty of recovering structure from apparent contours lies in the fact that the apparent contours are not fixed features. That is the contour generator slips over the surface under viewer motion, and the apparent contours observed from the different viewpoints do not have any correspondence in general. Thus, the structure of curved surfaces can not be recovered from two views.
However, if we have three sequential views, the local structure (i.e. surface curvature) can be computed just from apparent contours. That is if the camera is calibrated and its motion is known, we can define a straight line in the 3D space from a point on an image. If we have three views, we can therefore define three straight lines in the space, and the local structure of a curved surface can be recovered as a curvature of an osculate circle to these three lines (see Fig. 6).
To do this, we must identify the correspondences between the successive apparent contours and 3D surfaces. This can be achieved by using the epipolar parameterisation.
Figure 6: Recovery of local structures. The image points
(shown by 
)
viewed from three viewpoints, 
, 
and 
,
provide three straight lines, 
, 
and
, in the 3D space. The structure of the
curved surface can be recovered as an osculate circle to
the three lines locally.