def encodeMask(M):
        """
        Encode binary mask M using run-length encoding.
        :param   M (bool 2D array)  : binary mask to encode
        :return: R (object RLE)     : run-length encoding of binary mask
        """
        [h, w] = M.shape
        M = M.flatten(order='F')
        N = len(M)
        counts_list = []
        pos = 0
        # counts
        counts_list.append(1)
        diffs = np.logical_xor(M[0:N-1], M[1:N])
        for diff in diffs:
            if diff:
                pos +=1
                counts_list.append(1)
            else:
                counts_list[pos] += 1
        # if array starts from 1. start with 0 counts for 0
        if M[0] == 1:
            counts_list = [0] + counts_list
        return {'size':      [h, w],
               'counts':    counts_list ,
               }

        Encode binary mask M using run-length encoding.
        :param   M (bool 2D array)  : binary mask to encode
        :return: R (object RLE)     : run-length encoding of binary mask
        
lib/dataset/pycocotools/coco.py