A new approach for nonlinear distortion correction in endoscopic images based on least squares estimation

Images captured with a typical endoscope show spatial distortion, which nec essitates distortion correction for subsequent analysis. In this paper, a n ew methodology based on Least squares estimation is proposed to correct the nonlinear distortion in the endoscopic images. A mathematical model based on polynomial mapping is used to map the images from distorted image space onto the corrected image space. The model parameters include the polynomial coefficients, distortion center, and corrected center. The proposed method utilizes a line search approach of global convergence for the iterative pr ocedure to obtain the optimum expansion coefficients. A new technique to fi nd the distortion center of the image based on curvature criterion is prese nted. A dual-step approach comprising token matching and integrated neighbo rhood search is also proposed for accurate extraction of the centers of the dots contained in a rectangular grid, used for the model parameter estimat ion. The model parameters were verified with different grid patterns, The d istortion-correction model is applied to several gastrointestinal images an d the results are presented. The proposed technique provides high-speed res ponse and forms a key step toward online camera calibration, which is requi red for accurate quantitative analysis of the images.