docs: public static void process( GrayI orig,
  								GrayI derivX,
  								GrayI derivY) {
  		final int width = orig.getWidth();
  		final int height = orig.getHeight();
  
  		for (int y = 1; y < height - 1; y++) {
  
  			for (int x = 1; x < width - 1; x++) {
  
  				int dy = -(orig.get(x - 1, y - 1) + 2 * orig.get(x, y - 1) + orig.get(x + 1, y - 1));
  				dy += (orig.get(x - 1, y + 1) + 2 * orig.get(x, y + 1) + orig.get(x + 1, y + 1));
  
  
  				int dx = -(orig.get(x - 1, y - 1) + 2 * orig.get(x - 1, y) + orig.get(x - 1, y + 1));
  				dx += (orig.get(x + 1, y - 1) + 2 * orig.get(x + 1, y) + orig.get(x + 1, y + 1));
  
  				derivX.set(x, y, dx);
  				derivY.set(x, y, dy);
  			}
  		}
  	}