static BWImage bwLocalMaximaFilter_autoContrast(BWImage img) {
  int w = img.getWidth(), h = img.getHeight();
  float[] f = new[w*h];
  int i = 0;
  for y to h:
    for x to w:
      f[i++] = 
        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 iBWAutoContrast(iBWImageFromFloats(w, f));
}