// Limitation: img must be square with w/h a power of 2
svoid storeIntegralImageProgressively(IntegralImage img, IVF1<Pt, Int> store) {
  assertSquare(img);
  int w = img.getWidth();
  assertPowerOfTwo(w);
  
  int exponent = 0, size = 1;
  new Set<Pt> seen;
  while (size <= w) {
    for (int y = 0; y < w; y += size)
      for (int x = 0; x < w; x += size)
        if (seen.add(pt(x, y))) // TODO: can't you calculate this?!
          store(pt(x, y), img.get(x, y));
    exponent++;
    size <<= 1;
  }
}