// img must be square with width/height being a power of 2
static Pair<BWImage> bwImageContrastQuadtree(BWImage img) {
  int w = bw.getWidth(), h = bw.getHeight();
  assertEquals("width/height", w, h);
  if (!isPowerOf2(w)) fail("image size not a power of 2: " + w);
  BWImage min = new(w, h);
  BWImage max = new(w, h);
  bwImageContrastQuadtree_make(img, min, max, 0, 0, w, h);
}

bwImageContrastQuadtree_make