svoid test_imageDifferenceAsTransparentOverlay() {
  var img1 = newImage(1, 1, Color.green);
  var img2 = newImage(1, 1, Color.blue);
  var diff = imageDifferenceAsTransparentOverlay(img1, img2);
  int[] pixels = grabbableIntPixels_fastOrSlow(diff).asPackedArray();
  checkPixelArray(pixels, 0xFF0000FF);
}