module ChunkyPNG::Canvas::PNGEncoding

def encode_png_scanline_up(original_bytes, previous_bytes, pixelsize = 3)
  encoded_bytes = []
  for index in 0...original_bytes.length do
    b = previous_bytes[index]
    encoded_bytes[index] = (original_bytes[index] - b) % 256
  end
  [ChunkyPNG::FILTER_UP] + encoded_bytes
end