In this paper, an original and simplified formulation of the Kruppa?s equations is presented. This formulation is particularly of interest when only two images are available. Indeed, given two images taken with a camera with zero skew and known principal point, we show that there are only three possible solutions for the horizontal and vertical scale factors instead of four as widely believed so far. In addition, when the aspect ratio is unity, the horizontal and vertical scale factors are equal and the solution is unique and straightforward. No additional knowledge on the scene is required.

The issues of refining the solution and the influence of the principal point are addressed and discussed in our experiments.