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// drawing the resulting image on img1 gives img2
static BufferedImage imageDifferenceAsTransparentOverlay(BufferedImage img1, BufferedImage img2) {
  int w = img1.getWidth(), h = img1.getHeight();
  assertSameSize(img1, img2);
  int[] pixels = new int[w*h];
  
  GrabbableIntPixels gp1 = grabbableIntPixels_fastOrSlow(img1);
  GrabbableIntPixels gp2 = grabbableIntPixels_fastOrSlow(img2);
  
  int iPixels1 = gp1.offset, iPixels2 = gp2.offset;
  int[] pixels1 = gp1.data, pixels2 = gp2.data;
  int iPixels = 0;
  
  for y to h: {
    for x to w: {
      int pix; 
      if (pixels1[iPixels1+x] != (pix = pixels2[iPixels2+x]))
        pixels[iPixels++] = pix;
    }
    iPixels1 += gp1.scanlineStride-w;
    iPixels2 += gp2.scanlineStride-w;
  }
  
  ret newBufferedImageWithAlpha(pixels, w, h);
}