A simplified method of endoscopic image distortion correction based on grey level registration

We present a new method of endoscopic camera calibration for non-linear radial distortion correction. The algorithm implemented computes both projective (camera) and polynomial (distortion) transformations. The optimization process registrates the corrected distorted pattern image with the non-distorted one. Mutual information was used as measure of similarity and stochastic gradient descent method for optimization. The algorithm was tested with two b/w (chessboard, concentric circles) and one grey level patterns, for 3 angular positions of the endoscope (0°, 5° and 10° to perpendicular). Convergence time increased with the angle. Maximal mean correction error was less than 0.45 % with optimized distortion parameters calculated for the grey level pattern. Tested inclinations did not have significant effects on errors. Results obtained show the interest of the method proposed that requires only approximative perpendicular positioning of the endoscope and simple grey level calibration patterns without precise geometrical characteristics.