title: how to access nth channel of a Mat in opencv?
body: <p>I know how to access three channels <code>cv::Mat</code> using <code>Vec3b</code>. But now I have an <code>n</code> channel <code>cv::Mat</code> and <code>n</code> is not constant (to use <code>cv::Vec&lt;uchar, n&gt;</code>). How I can access <code>cv::Mat</code> channels now?  </p>