static BufferedImage transparentBufferedImage(int w, int h) {
  BufferedImage img = new BufferedImage(w, h, BufferedImage.TYPE_INT_ARGB);
  Graphics2D g = getGraphics2D(img);
  g.setComposite(AlphaComposite.getInstance(AlphaComposite.CLEAR));
  g.fillRect(0, 0, w, h);
  ret img;
}