static BufferedImage centerImageOnBackground(Color bg, WidthAndHeight size, BufferedImage img) {
  if (img == null) null;
  BufferedImage canvas = newBufferedImage(size, bg);
  drawImageOnImage(img, canvas,
    (size.w()-img.getWidth())/2,
    (size.h()-img.getHeight())/2);
  ret canvas;
}