int getDifference(Image image, Colour q, int x1, int y1, int x2, int y2) throws ImageTooSmallException
	{
		if (image.getWidth() <= x1 || image.getHeight() <= y1 || image.getWidth() <= x2 || image.getHeight() <= y2 || x1 < 0 || x2 < 0 || y1 < 0 || y2 < 0)
		{
			throw new ImageTooSmallException();
		}
		return image.getPixel(y2, x2, q) - image.getPixel(y1,  x1, q);
	}