Cross ratio is invariant under perspective projections

In this paper it is proved that the cross ratio is invariant tinder perspective and stereographic projections. For a set of three dimensional points, stereo images can be obtained and the measurements of the disparity of the image points in the stereo pair can be used to compute the relative distances of tile points From the image plane. The focal length and base line of the imaging system are used in the above computations. It is true that invariant metric relationship between the object points can be derived from measured relationships among the image points, These relationships are a generalisation of the invariance of cross ratio of distances between points on a line to three dimensions. In three dimensions the invariants are tile cross ratios of areas and volumes which are defined by the object points.