The above construction cannot be directly extended to the projective case because parallelism is not a projective invariant and we cannot compute ratios along parallel directions.
However, as a first step, we can extend the single reference plane method to a two reference plane method.
Let , be four non-coplanar points in space and be their corresponding projections in the two views. The points and lie on twodifferent planes. Therefore, we can account for the motion of all points coplanar with either of these two planes.
Let be a points of interest not coplanar with either of the
reference planes and let and be its
projections on to the two reference planes along the viewing
direction in the first image. The points , and
project on to , and
respectively, and an alternative affine structure invariant for
can be computed as
The main advantage in using the two reference plane construction is that , and lie along one line in the second image.