title: OpenCV access element from Mat
body: <p>I am attempting to learn OpenCV, and being a hard-head, I am trying to run the following algorithm:</p>
  
  <pre><code> cv::Mat cur_features;
   cv::goodFeaturesToTrack(current_image, cur_features, 400, 0.01, 0.01);
  </code></pre>
  
  <p>Now, being the hard-headed individual, I am interested to see what cur_features is holding... I expected a 400x2 cv::Mat but instead I got a 400x1 cv::Mat</p>
  
  <p>No biggy, I think maybe its a direct index.  However, for the LIFE of me I CANNOT extract a value from cur_features.at(0) and print it out.</p>
  
  <p>What am I doing wrong? I have seen the goodFeaturesToTrack_Demo.cpp.  Some things to note on that demo that differ for mine.  I tried the following calls given that example:</p>
  
  <pre><code> std::cout &lt;&lt; cur_features.size() &lt;&lt; std::endl; // This throws a compile time error even though its in the example
   std::cout &lt;&lt; cur_features.at&lt;Point2f&gt;(0).x &lt;&lt; std::endl; //This throws a run time error.
  </code></pre>
  
  <p>Could anyone direct me to some documentation that explains how to achieve my goal?  The goodFeaturesToTrack tells you it returns an OutputArray which is a vector of corners, but nowhere does it describe what the type of those corners are.  Where in the documentation would I look for this answer in case I get it with other methods?</p>
  
  <hr>
  
  <p><strong>Edit:</strong>  Also, whats the Point of Mat::type().  I cannot find where the returned value can be explained... I'm looking for an enumeration in the documentation but having trouble finding it.</p>
  
  <pre><code> std::cout &lt;&lt; current_image.type() &lt;&lt; std::endl; //This returns 0
   std::cout &lt;&lt; cur_features.type() &lt;&lt; std::endl; //This returns 13
  </code></pre>