// 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);
  }
}