static BWImage getAlphaChannel(BufferedImage src) {
  //assertEquals("Wrong image type", BufferedImage.TYPE_INT_ARGB, src.getType());

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