Faugeras [2] approaches the problem from a slightly
different direction by using the fact that the point
must lie on the epipolar line corresponding to
:

That line contains two points, the epipole (the projection of the first camera's optical center into the second camera) and the point at infinity associated with :

In [2, pp. 40-41] it is shown that the epipole is given by

and the point at infinity by

Therefore, the epipolar line is:

where we have used the substitutions in (5). Combining with (11), we get the desired result: