public void background(PImage image) {
    if ((image.width != width) || (image.height != height)) {
      throw new RuntimeException(ERROR_BACKGROUND_IMAGE_SIZE);
    }
    if ((image.format != RGB) && (image.format != ARGB)) {
      throw new RuntimeException(ERROR_BACKGROUND_IMAGE_FORMAT);
    }
    backgroundColor = 0;  // just zero it out for images
    backgroundImpl(image);
  }