English: In order to define the distortion correction dr the radius r is related to the point of symmetry S (centre of radial distortion). The coordinates of distorted (measured) point are (x_d, y_d), and the coordinates of the corrected (ideal) point are (x_c, y_c). The (measured) radius r results from the coordinates of distorted point (x_d, y_d) and the point of symmetry of distortion S. In order to get the ideal radius that is free from distortion the distortion correction dr has to be subtracted from r. Likewise, the corrected coordinates x_c and y_c are calculated by subtract the amount of distortion dx and dy from their distorted point coordinates x_d and y_d.