static BufferedImage img_getCenterPortion(BufferedImage img, int w, int h) {
  int iw = img.getWidth(), ih = img.getHeight();
  if (iw < w || ih < h) fail("Too small");
  int x = (iw-w)/2, y = (ih-h)/2;
  ret clipBufferedImage(img, x, y, w, h);
}