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// returns value from 0 to 1
static double 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 double ii_twoByTwoVariation(IntegralImage ii) {
  ret ii_twoByTwoVariation(ii, doubleRect(ii.getWidth(), ii.getHeight()));
}