static bool rgbAllOneColor(RGBImage img, int x1, int y1, int w, int h) {
  if (w == 0 || h == 0) true;
  int color = img.getInt(x1, y1);
  for y to h: for x to w:
    if (img.getInt(x1+x, y1+y) != color)
      false;
  true;
}