static int bwContrastQuadtree_minOfArea(BWContrastQuadTree qt, int originalSize, Rect rect) {
  ret bwContrastQuadtree_minOfArea(qt, Rect(0, 0, originalSize, originalSize), rect);
}

static int bwContrastQuadtree_minOfArea(BWContrastQuadTree qt, Rect block, Rect rect) {
  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 qt.min;
  
  // simple case 3: block is one color
  if (!(qt instanceof BWContrastQuadTree.Composite))
    ret qt.min;
  
  // complex case: process subregions
  BWContrastQuadTree.Composite qt2 = cast qt;
  int w = block.w, r = w/2;
  ret intMin(
    // top left area
    bwContrastQuadtree_minOfArea(qt2.a, Rect(block.x, block.y, r, r), r2),
    // tall slice on the right
    bwContrastQuadtree_minOfArea(qt2.b, Rect(block.x+r, block.y, r, r), r2),
    // wide slice on the bottom
    bwContrastQuadtree_minOfArea(qt2.c, Rect(block.x, block.y+r, r, r), r2),
    // bottom right area
    bwContrastQuadtree_minOfArea(qt2.d, Rect(block.x+hx, block.y+r, r, r), r2));
}