// drawing the resulting image on img1 gives img2

srecord noeq ImageDifferenceAsTransparentOverlay(BufferedImage img1, BufferedImage img2) {

  int w, h;

  int[] pixels;

  int x1, y1, x2, y2; // changed area (inclusive)

  GrabbableIntPixels gp1, gp2;

  run {

    if (pixels != null) ret;

    int w = this.w = img1.getWidth(), h = this.h = img1.getHeight();

    assertSameSize(img1, img2);

    int[] pixels = this.pixels = new int[w*h];

    gp1 = grabbableIntPixels_fastOrSlow(img1);

    gp2 = grabbableIntPixels_fastOrSlow(img2);

    int iPixels1 = gp1.offset, iPixels2 = gp2.offset;

    int[] pixels1 = gp1.data, pixels2 = gp2.data;

    int iPixels = 0;

    x1 = w; y1 = h; x2 = 0; y2 = 0;

    for y to h: {

      for x to w: {

        int pix; 

        if ((pixels1[iPixels1+x] & 0xFFFFFF)

          != (pix = pixels2[iPixels2+x] & 0xFFFFFF)) {

          pixels[iPixels] = pix | 0xFF000000;

          if (x < x1) x1 = x;

          if (x > x2) x2 = x;

          if (y < y1) y1 = y;

          if (y > y2) y2 = y;

        }

        iPixels++;

      }

      iPixels1 += gp1.scanlineStride;

      iPixels2 += gp2.scanlineStride;

    }

  }

  BufferedImage get() {

    run();

    ret newBufferedImageWithAlpha(w, h, pixels);

  }

  Rect changedBounds() {

    run();

    ret rectFromPointsInclusive(x1, y1, x2, y2);

  }

}