static BWImage bwLocalMaximaFilter(BWImage img) {
  int w = img.getWidth(), h = img.getHeight();
  BWImage img2 = new(w, h);
  for y to h:
    for x to w:
      img2.setPixel(x, y,
        img.getPixel(x, y)-
        (img.getPixel(x, max(0, y-1)) +
        img.getPixel(max(0, x-1), y) +
        img.getPixel(min(w-1, x+1), y) +
        img.getPixel(x, min(h-1, y+1)))/4);
  ret img2;
}