// see https://nanonets.com/blog/optical-flow/

srecord noeq GoodThingsToTrack(BWImage image) {
  int windowSize = 3;
  
  run {
    int w = image.getWidth(), h = image.getHeight();
    int wn = windowSize*windowSize;
    for (int y = 0; y < h-windowSize-1; y++)
      for (int x = 0; x < w-windowSize-1; x++) {
        double[] ix = new[wn];
        double[] iy = new[wn];
        int iw = 0;
        for yy to windowSize:
          for xx to windowSize: {
            double value = image.getInt(x+xx, y+yy);
            double right = image.getInt(x+xx+1, y+yy);
            double below = image.getInt(x+xx, y+yy+1);
            ix[iw] = right-value;
            iy[iw] = below-value;
            iw++;
          }
        
      }
  }
}