static ImageSurface imageSurfaceCheckerBoardBackground(ImageSurface is) {
  if (is.drawBackground != null) ret is;
  double pixelsPerSecond = 8;
  is.drawBackground = (w, h, g) -> {
    var cb = new CheckerBoard2().w(w).h(h);
    double time = sysSeconds();
    int shift = iround(time*pixelsPerSecond);
    cb.shiftX(shift).shiftY(shift);
    cb.drawOn(g);
  };
  setDoubleBuffered(is);
  repaintEvery(1.0/pixelsPerSecond, is);
  ret is;
}