static BWIntegralImage blendBWIntegralImages(Cl<? extends IIntegralImage> images) {
  if (empty(images)) null;
  var img1 = first(images);
  int nImages = l(images);
  int w = img1.getWidth(), h = img1.getHeight(), n = w*h;
  double[] pixels = new[w*h];
  for (var img : images) {
    int i = 0;
    for y to h:
      for x to w:
        pixels[i++] += img.getPixel(x, y, 0);
  }
  
  byte[] bytes = new[n];
  double inv = 1.0/nImages;
  for i to n: bytes[i] = clampToUByte(iround(pixels[i]*inv));
  
  ret new BWIntegralImage(BWImage(w, h, bytes));
}

Began life as a copy of #1019595