static void assertBWImagesIdentical(BWImage a, BWImage b) {
  int w = a.getWidth(), h = a.getHeight();
  assertEquals("Width", w, b.getWidth());
  assertEquals("Height", h, b.getHeight());
  for y to h: for x to w: {
    byte b1 = a.getByte(x, y), b2 = b.getByte(x, y);
    if (b1 != b2)
      fail("Pixel at  " + x + "/" + y + ": " + b1 + " vs " + b2);
  }
}