A constraint on five points in two images

                         T. Werner

It is well-known that epipolar geometry between two uncalibrated
cameras is determined by at least seven pairs of matching points. If
there are more than seven matches, their positions cannot be arbitrary
if they are to be projections of any world points by any two
cameras. Less than seven matches have been thought not to be
constrained in any way. We show that there is a constraint even on
five matches, i.e., that there exist forbidden configurations of five
points in two images. For allowed configurations, we show that
epipoles must lie in domains with piecewise-conic boundary, and how to
compute these domains. The constraint is obtained by formulating the
situation not in projective geometry but rather oriented projective
geometry language -- in other words, points on the wrong side of rays
are not allowed. This assumption is correct for a pair of any
conventional or panoramic central cameras.