def as_contours(self):
        """A dictionary of lists of contours keyed by byte_value"""
        contours = dict()
        for byte_value in self.__byte_values:
            if byte_value == 0:
                continue
            mask = (self.__array == byte_value) * 255
            found_contours = find_contours(mask, 254, fully_connected='high') # a list of array
            contours[byte_value] = ContourSet(found_contours)
        return contours
A dictionary of lists of contours keyed by byte_value