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)));

}