An algorithm for the execution of arbitrary geometric trans formations, which can be applied to an extensive class ofproblam, is described. A computerized procedure to reduce the radiometric distorsion caused by vignetting is also illustrated. Pictures taken by cameras or mul ti-spectral sensors (MSS)from aerial platforms are affected by various types of distorsion. Such distorsions may be either geometric (represented mathematically using a coordinate transformation) or.radiometric (alteration of the luminous intensity of parts of the image) or a combination of the two. A typical example of geometric distorsion in pictures taken from aerial platforms is image deformation which may be caused by unwanted changes in platform attitude (e.g. rolling, pitching or yawing, alterations in height, etc.) or otherwise because the area being taken is not a plane surface. Another geometric deformation, found this time in multi-spectral data, is the panoramic effect. This consists of a compression of the image at the edges, in the scanning direction. If a simple geometric transformation is to be made, e.g. a rotation or change in scale, which affects the whole picture, it can be done in analog mode. This type of correction can, however, only be made during acquisition, or in any case before the image has been digitized, using optical or mechanical devices. If the image has already been digitized and is thus stored in an archive, the correction can only be implemented digitally. A digital correction is in fact preferible or often obligatory for complicated geometric trans- formations or for geometric transformations applied only to a part of the picture.
