class HexaPDF::ImageLoader::PNG

def alpha_mask_for_indexed_image(offset, decode_parms, trns)
  width = decode_parms[:Columns]
  bpc = decode_parms[:BitsPerComponent]
  bytes_per_row = (width * bpc + 7) / 8 + 1
  flate_decode = @document.config.constantize('filter.map', :FlateDecode)
  source = flate_decode.decoder(Fiber.new(&image_data_proc(offset)))
  mask_data = ''.b
  stream = HexaPDF::Utils::BitStreamReader.new
  while source.alive? && (data = source.resume)
    stream.append_data(data)
    while stream.remaining_bits / 8 >= bytes_per_row
      stream.read(8) # read filter byte
      i = 0
      while i < width
        index = stream.read(bpc)
        mask_data << (trns[index] || 255)
        i += 1
      end
      stream.read(8 - ((width * bpc) % 8)) if bpc != 8 # read remaining fill bits
    end
  end
  mask_data
end