def to_png(options={})
      output_to_string_io do |io|
        Cairo::ImageSurface.new(options[:format],
                                full_width(options),
                                full_height(options)) do |surface|
          render(surface, options)
          surface.write_to_png(io)
        end
      end
    end
def to_png(options={})
      output_to_string_io do |io|
        Cairo::ImageSurface.new(options[:format],
                                full_width(options),
                                full_height(options)) do |surface|
          render(surface, options)
          surface.write_to_png(io)
        end
      end
    end
Render the barcode to a PNG image