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()); }

}