Since lines are projective invariants, the two reference plane method can easily be extended to deal with the projective case.
Consider the above figure. The left epipole is denoted as and the ray projects on to the line which is an epipolar line.
The points , , and are collinear
and projectively related to , , and ,
and therefore have the same cross-ratio. The projective structure
invariant can thus be defined as
Note that when the center of projections are at infinity the epipole also becomes a point at infinity and the structure invariant reduces to the affine structure invariant.