text: -- file: ch12/Barcode.hs
  luminance :: (Pixel, Pixel, Pixel) -> Pixel
  luminance (r,g,b) = round (r' * 0.30 + g' * 0.59 + b' * 0.11)
      where r' = fromIntegral r
            g' = fromIntegral g
            b' = fromIntegral b