svoid test_imageDifferenceAsTransparentOverlay() {
  var img1 = newImage(1, 1, Color.green);
  var img2 = newImage(1, 1, Color.blue);
  var diff = imageDifferenceAsTransparentOverlay(img1, img2);
  checkPixelArray(diff, 0xFF0000FF);
  
  img2 = newImage(1, 1, Color.green);
  diff = imageDifferenceAsTransparentOverlay(img1, img2);
  checkPixelArray(diff, 0);
}