static BWImage img_bw_vdiff(BWImage a) {
  int w = a.w(), h = a.h()-1;
  BWImage b = new BWImage(w, h);
  for y to h: for x to w:
    b.setPixel(x, y, Math.abs(a.getPixel(x, y)-a.getPixel(x, y+1)));
  ret b;
}