def register(self, src, trg, trg_mask=None, src_mask=None):
        """ Implementation of pair-wise registration and warping using Enhanced Correlation Coefficient

        This function estimates an Euclidean transformation (x,y translation + rotation) using the intensities of the
        pair of images to be registered. The similarity metric is a modification of the cross-correlation metric, which
        is invariant to distortions in contrast and brightness.

        :param src: 2D single channel source moving image
        :param trg: 2D single channel target reference image
        :param trg_mask: Mask of target image. Not used in this method.
        :param src_mask: Mask of source image. Not used in this method.
        :return: Estimated 2D transformation matrix of shape 2x3
        """
        # Parameters of registration
        warp_mode = cv2.MOTION_EUCLIDEAN
        # Specify the threshold of the increment
        # in the correlation coefficient between two iterations
        termination_eps = 1e-10
        # Define termination criteria
        criteria = (cv2.TERM_CRITERIA_EPS | cv2.TERM_CRITERIA_COUNT,
                    self.params['MaxIters'], termination_eps)
        # Initialise warp matrix
        warp_matrix = np.eye(2, 3, dtype=np.float32)
        # Run the ECC algorithm. The results are stored in warp_matrix.
        _, warp_matrix = cv2.findTransformECC(src.astype(np.float32),
                                              trg.astype(np.float32),
                                              warp_matrix, warp_mode, criteria)
        return warp_matrix