Perception of a quadrilateral

In this report, we show that an arbitrarily given quadrilateral can always be interpreted as an image of a parallelogram and that the interpretation is unique aside from a multiplicative constant. Several applications of this theorem are discussed. It could be used to prove an old, geometrical theorem; it could facilitate the matching process when a sequence of images are available; it could be used as a simple technique for passive ranging in industrial environment or autonomous landing of an aircraft on a moving platform.