static IIntegralImage compressBWIntegralImage(IIntegralImage img, int recursionLevels default 5) {
  if (recursionLevels <= 0) ret BWIntegralImage(img);
  
  int w = img.getWidth(), h = img.getHeight();
  
  if (w >= 2) {
    IIHorizontalSplit split = new(img);
    var a = split.a();
    var b = split.b();
    var a2 = toIISingleColorIfPossible(a);
    var b2 = toIISingleColorIfPossible(b);
    if (a != a2 || b == b2) {
      split.setA(a != a2 ? a2 : compressBWIntegralImage(a, recursionLevels-1));
      split.setB(b != b2 ? b2 : compressBWIntegralImage(b, recursionLevels-1));
      img = split;
    }
  }
  
  // TODO: vertical split
  
  ret img;
}