sclass G22MakeMaskCorrelationMatrix is MakesBufferedImage {
  settable long[][] imagesH;
  settable long[][] imagesW;
  
  // size w()*h(), every entry is a pixel diff count
  gettable int[] matrix;
  
  selfType images(long[][] images) {
    ret imagesW(images).imagesH(images);
  }
  
  selfType images(L<long[]> images) {
    ret images(toArray(long[].class, images));
  }
  
  public int w() { ret imagesH.length; }
  public int h() { ret imagesW.length; }
  
  int maxPixelDiff() { ret l(first(imagesW))*64; }
  
  bool symmetrical() { ret imagesW == imagesH; }
  
  run {
    int w = w(), h = h();
    var matrix = this.matrix = new int[w*h];
    int i = 0;
    if (symmetrical())
      for y to h: {
        long[] imageH = imagesH[y];
        // mirror along diagonal
        int x = 0, i2 = y;
        for (; x < y; x++, i2 += w)
          matrix[i++] = matrix[i2];
        for (; x < w; x++)
          matrix[i++] = countDifferingBits(imageH, imagesW[x]);
      }
    else
      for y to h: {
        long[] imageH = imagesH[y];
        for x to w:
          matrix[i++] = countDifferingBits(imageH, imagesW[x]);
      }
  }
  
  FloatBWImage get() {
    if (matrix == null) run();
    int w = w(), h = h();
    float max = maxPixelDiff();
    ret floatBWImageFromFunction(w, h,
      (x, y) -> 1f-matrix[y*w+x]/max);
  }
  
  public BufferedImage getBufferedImage() { ret toBufferedImage (get()); }
}