static int bwContrastQuadtree_minOfArea(BWImage min, Rect rect) {
  ret bwContrastQuadtree_minOfArea(min, rect, imageRect(min));
}

static int bwContrastQuadtree_minOfArea(BWImage min, Rect rect, Rect block) {
  Rect r2 = intersectRects(rect, block);
  if (r2.empty()) ret 255; // simple case 1: no intersection
  //print(rect + " => " + r2);
  
  // simple case 2: whole block
  if (eq(r2, block)) ret min.getInt(block.x, block.y);
  
  // complex case: process subregions
  int w = block.w, hx = (w+1)/2;
  int h = block.h, hy = (h+1)/2;
  ret intMin(
    // top left area
    bwContrastQuadtree_minOfArea(min, r2, Rect(block.x, block.y, hx, hy)),
    // tall slice on the right
    bwContrastQuadtree_minOfArea(min, r2, Rect(block.x+hx, block.y, w-hx, h)),
    // wide slice on the bottom
    bwContrastQuadtree_minOfArea(min, r2, Rect(block.x, block.y+hy, w, h-hy)),
    // bottom right area
    bwContrastQuadtree_minOfArea(min, r2, Rect(block.x+hx, block.y+hy, w-hx, h-hy)));
}