module ChunkyPNG::Canvas::PNGEncoding

def encode_png_scanline_sub(original_bytes, previous_bytes = nil, pixelsize = 3)
  encoded_bytes = []
  for index in 0...original_bytes.length do
    a = (index >= pixelsize) ? original_bytes[index - pixelsize] : 0
    encoded_bytes[index] = (original_bytes[index] - a) % 256
  end
  [ChunkyPNG::FILTER_SUB] + encoded_bytes
end