fillSizeDifference = (width, height) => (image) => {
  const inArea = (x, y) => y > height || x > width;
  for (let y = 0; y < image.height; y++) {
    for (let x = 0; x < image.width; x++) {
      if (inArea(x, y)) {
        const idx = ((image.width * y) + x) << 2;
        image.data[idx] = 0;
        image.data[idx + 1] = 0;
        image.data[idx + 2] = 0;
        image.data[idx + 3] = 64;
      }
    }
  }
  return image;
}
Fills diff area with black transparent color for meaningful diff