def build_jpg_object(data, jpg) 
      color_space = case jpg.channels
      when 1
        :DeviceGray
      when 3
        :DeviceRGB
      when 4
        :DeviceCMYK
      else
        raise ArgumentError, 'JPG uses an unsupported number of channels'
      end
      obj = ref(
          :Type             => :XObject,
          :Subtype          => :Image,
          :Filter           => :DCTDecode,
          :ColorSpace       => color_space,
          :BitsPerComponent => jpg.bits,
          :Width            => jpg.width,
          :Height           => jpg.height,
          :Length           => data.size ) 
 
      # add extra decode params for CMYK images. By swapping the
      # min and max values from the default, we invert the colours. See
      # section 4.8.4 of the spec.
      if color_space == :DeviceCMYK
        obj.data[:Decode] = [ 1.0, 0.0, 1.0, 0.0, 1.0, 0.0, 1.0, 0.0 ]
      end
 
      obj << data
      return obj
    end