static int bwContrastQuadtree_minOfArea(BWImage min, Rect rect) {
  ret bwContrastQuadtree_minOfArea(min, rect, 0, 0, img.getWidth());
}

static int bwContrastQuadtree_minOfArea(BWImage min, Rect rect, Rect block) {
  Rect r2 = intersectRects(rect, block);
  if (r2.empty()) ret 255;
  
  int r = block.w/2;
  // process 4 subsquares
  ret intMin(
    bwContrastQuadtree_minOfArea(min, r2, Rect(block.x, block.y, r, r)),
    bwContrastQuadtree_minOfArea(min, r2, Rect(block.x+r, block.y, r, r)),
    bwContrastQuadtree_minOfArea(min, r2, Rect(block.x, block.y+r, r, r)),
    bwContrastQuadtree_minOfArea(min, r2, Rect(block.x+r, block.y+r, r, r)));
}