Precise determination of object planes in images is very important in applications of computer vision, such as pattern recognition and 3D reconstruction. The corners of a polygonal object plane, e.g. roof, wall, etc., can be determined in an image by detecting and intersecting edge straight lines bounding the plane. Any two non-parallel lines in an image intersect at an image point. If this intersection corresponds to a 3D point in the scene, it is called a real intersection, otherwise it is called a virtual intersection. An automatic system for locating image lines is likely to produce many virtual intersections. This paper presents a computational technique to discriminate between real and virtual intersections. The method is based on rectified images obtained from a pair of uncalibrated images and is illustrated with images of a real scene. The results obtained showed reliable decisions.