title: OpenCV PCA not initializable
body: <p>Given <a href="http://docs.opencv.org/modules/core/doc/operations_on_arrays.html" rel="nofollow">http://docs.opencv.org/modules/core/doc/operations_on_arrays.html</a></p>
  
  <p>PCA should be initializable by just passing it a matrix. </p>
  
  <pre><code>cv::Mat matrix;
  ...
  //If I do
  cv::PCA pca;
  pca(matrix);
  </code></pre>
  
  <p>I get the error, that function <code>cv::PCA::operator() cannot be called with the given arguments</code></p>
  
  <p>And if I do:</p>
  
  <pre><code>cv::PCA pca(matrix);
  </code></pre>
  
  <p>it says "no instance of constructor matches the argument list"</p>
  
  <p>And if I do:</p>
  
  <pre><code>pca.operator(matrix);
  </code></pre>
  
  <p>It says "Error: expected an operator". What is going wrong?</p>