title: Convert the cv::Mat of OpenCV to Eigen
body: <p>This <a href="https://stackoverflow.com/questions/14783329/opencv-cvmat-and-eigenmatrix">post</a> show us how to map the <code>cv::Mat</code> to Eigen matrix without copy the data, it works fine, but there are one thing I do not understand.</p>
  
  <pre><code>Mat A(20, 20, CV_32FC1);
  cv::randn(A, 0.0f, 1.0f); // random data
  
  // Map the OpenCV matrix with Eigen:
  Eigen::Map&lt;Eigen::Matrix&lt;float, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor&gt;&gt; A_Eigen(A.ptr&lt;float&gt;(), A.rows, A.cols);
  </code></pre>
  
  <p>The problem is, the <code>Mat A</code> do not tell the <code>A_Eigen</code> how many bytes per row it should take, as far as I know the <code>cv::Mat</code> of OpenCV may or may not do padding on each row, do  I need to tell the Eigen how many bytes per row (how?)?, or I can safely omit it?</p>
  
  <p>Ps: I am using Eigen 3</p>