static int ocr_walkInDirection(RGBImage img, int x, int y, int dx, int dy, float tolerance) {
  int length = 0;
  int w = img.w(), h = img.h();
  RGB rgb = img.getPixel(x, y);
  while (x >= 0 && x < w && y >= 0 && y < h && rgbDiff(x, y) <= tolerance) {
    ++length;
    x += dx;
    y += dy;
  }
  ret length;
}