def field_widths
            widths = self.W

            unless widths.is_a?(Array) and widths.length == 3 and widths.all? {|w| w.is_a?(Integer) and w >= 0 }
                raise InvalidXRefStreamObjectError, "Invalid W field: #{widths}"
            end

            widths
        end
def field_widths
            widths = self.W

            unless widths.is_a?(Array) and widths.length == 3 and widths.all? {|w| w.is_a?(Integer) and w >= 0 }
                raise InvalidXRefStreamObjectError, "Invalid W field: #{widths}"
            end

            widths
        end
Check and return the internal field widths.