title: opencv2/core/eigen.hpp Eigen: does not name a type
body: <p>I want to convert Eigen's matrix to opencv's mat. so, I used opencv function 'eigen2cv' included in 'opencv2/core/eigen.hpp'. But,this file seems to not include 'Eigen/Core', so that I got a lot of errors. it told me that 'Eigen' dose not name a type. There are more details:</p>
  
  <pre><code>/opt/ros/kinetic/include/opencv-3.2.0-dev/opencv2/core/eigen.hpp:63:22: error: ‘Eigen’ does not name a type
  void eigen2cv( const Eigen::Matrix&lt;_Tp, _rows, _cols, _options, _maxRows, _maxC
                    ^
  /opt/ros/kinetic/include/opencv-3.2.0-dev/opencv2/core/eigen.hpp:63:35: error: expected unqualified-id before ‘&lt;’ token
  void eigen2cv( const Eigen::Matrix&lt;_Tp, _rows, _cols, _options, _maxRows, _maxC
                                 ^
  /opt/ros/kinetic/include/opencv-3.2.0-dev/opencv2/core/eigen.hpp:63:35: error: expected ‘)’ before ‘&lt;’ token
  /opt/ros/kinetic/include/opencv-3.2.0-dev/opencv2/core/eigen.hpp:63:35: error: expected initializer before ‘&lt;’ token
  /opt/ros/kinetic/include/opencv-3.2.0-dev/opencv2/core/eigen.hpp:81:22: error: ‘Eigen’ does not name a type
  void eigen2cv( const Eigen::Matrix&lt;_Tp, _rows, _cols, _options, _maxRows, _maxC
                    ^
  /opt/ros/kinetic/include/opencv-3.2.0-dev/opencv2/core/eigen.hpp:81:35: error: expected unqualified-id before ‘&lt;’ token
  void eigen2cv( const Eigen::Matrix&lt;_Tp, _rows, _cols, _options, _maxRows, _maxC
                                 ^
  /opt/ros/kinetic/include/opencv-3.2.0-dev/opencv2/core/eigen.hpp:81:35: error: expected ‘)’ before ‘&lt;’ token
  </code></pre>
  
  <p>I have already installed Eigen3,Opencv-2.4.13 and ROS-Kinetic with OpenCV-3.2.0. My CmakeLists looks like this:</p>
  
  <pre><code>find_package(OpenCV REQUIRED)
  find_package(Eigen3 REQUIRED) 
  
  include_directories(
  include
  ${OpenCV_INCLUDE_DIRS}
  ${EIGEN3_INCLUDE_DIR}
  )
  </code></pre>
  
  <p>Thank you in advance!</p>