static double[][] integralImage(BufferedImage img) {
  ret integralImage(new BWImage(img));
}

ifclass RGBImage
static double[][] integralImage(RGBImage rgb) {
  ret integralImage(toBW(rgb));
}
endif

static double[][] integralImage(BWImage bw) {
  int w = bw.getWidth(), h = bw.getHeight();
  double[][] ii = new double[h][w];
  
  for y to h: {
    double rowSum = 0;
    for x to w: {
      rowSum += bw.getPixel(x, y);
      ii[y][x] = y > 0 ? rowSum + ii[y-1][x] : rowSum;
    }
  }

  ret ii;
}