static BufferedImage renderBackgroundPlusBoxes(int w, int h, BackgroundPlusBoxes s) {
  BufferedImage bi = newBufferedImage(w, h, toColor(s.background));
  Graphics2D g = imageGraphics(bi);
  for (BoxWithColor e : s.boxes) {
    int x1 = min(e.a.x, e.b.x), y1 = min(e.a.y, e.b.y);
    int x2 = max(e.a.x, e.b.x), y2 = max(e.a.y, e.b.y);
    fillRect(g, x1, y1, x2-x1+1, y2-y1+1, toColor(e.color));
  }
  g.dispose();
  ret bi;
}

static BufferedImage renderBackgroundPlusBoxes(int w, int h, Color background, Color boxColor, L<Rect> rects) {
  ret renderBackgroundPlusBoxes(w, h, BackgroundPlusBoxes(background,
    map(rects, r -> BoxWithColor(r, boxColor))));
}

// default to black on white
static BufferedImage renderBackgroundPlusBoxes(int w, int h, L<Rect> rects) {
  ret renderBackgroundPlusBoxes(w, h, Color.white, Color.black, rects);
}