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;

}