// returns [<cell, average color>]
static LPair<DoubleRect, RGB> ii_twoByTwoVariation(IntegralImage ii, DoubleRect r) {
  RGB col = ii_averageColorOfArea(ii, r);
  double sum = 0;
  for (RGB col2 : pairsB(ii_scanGrid(ii, 2, 2))
    sum += rgbDiff(col, col2);
  ret sum/4;
}

static LPair<DoubleRect, RGB> ii_twoByTwoVariation(IntegralImage ii) {
  ret ii_twoByTwoVariation(ii, doubleRect(ii.getWidth(), ii.getHeight()));
}