static BWImage getAlphaChannel(BufferedImage src) {
  if (src.getType() != BufferedImage.TYPE_INT_ARGB)
      throw new RuntimeException("Wrong image type.");

  int w = src.getWidth();
  int h = src.getHeight();
  int[] srcBuffer = src.getData().getPixels(0, 0, w, h, null);
  
  BWImage alpha = new BWImage(w, h);
  for (int i=0; i<w*h; i++)
    alpha.setByte(i % w, i / w, (srcBuffer[i] >> 24) & 0xff);
  ret alpha;
}