title: Access to each separate channel in OpenCV
body: <p>I have an image with 3 channels (img) and another one with a single channel (ch1).</p>
  
  <pre><code>    Mat img(5,5,CV_64FC3);
      Mat ch1 (5,5,CV_64FC1);
  </code></pre>
  
  <p>Is there any efficient way (not using for loop) to copy the first channel of <strong>img</strong> to <strong>ch1</strong>?</p>