y :: RGB8 -> Double
y rgb = 0.299 * r' + 0.587 * g' + 0.114 * b' 
  where (r', g', b') = fromIntegral' rgb
y :: RGB8 -> Double
y rgb = 0.299 * r' + 0.587 * g' + 0.114 * b' 
  where (r', g', b') = fromIntegral' rgb
y rgb = 0.299 * r' + 0.587 * g' + 0.114 * b' 
  where (r', g', b') = fromIntegral' rgb