module ChunkyPNG::Canvas::PNGEncoding

def encode_png_str_scanline_up(stream, pos, prev_pos, line_width, pixel_size)
  line_width.downto(1) do |i|
    b = prev_pos ? stream.getbyte(prev_pos + i) : 0
    stream.setbyte(pos + i, (stream.getbyte(pos + i) - b) & 0xff)
  end
  stream.setbyte(pos, ChunkyPNG::FILTER_UP)
end