// clip is in output coordinates
static BufferedImage scaledGrayImageFromBWIntegralImage_clip(BWIntegralImage img, double scalingFactor, Rect clip) {
  int w = clip.w, h = clip.h;
  int w1 = img.w, h1 = img.h;
  byte[] pixels = new[w*h];
  int i = 0;
  int x2 = clip.x2(), y2 = clip.y2();
  for (int y = clip.y; y < y2; y++)
    for (int x = clip.x; x < x2; x++) {
      // TODO: optimize
      int x1 = x/scalingFactor, x2 = max(x1+1, (x+1)/scalingFactor);
      int y1 = y/scalingFactor, y2 = max(y1+1, (y+1)/scalingFactor);
      int pixel = iround(img.getPixelAverage(x1, y1, x2, y2));
      pixels[i++] = clampToUByte(pixel);
    }
  ret newGrayBufferedImage(w, h, pixels);
}