As was seen in Section 3.3.1, even for an arbitrary geometric structure, the projections of points in two views contains some structure. Finding back this structure is not only very interesting since it is equivalent to the projective calibration of the camera for the two views, but also allows to simplify the search for more matches since these have to satisfy the epipolar constraint. As will be seen further it also allows to eliminate most of the outliers from the matches.