title: How does OpenCV make use of Eigen?
body: <p>When compiling OpenCV from source, there's the CMake option WITH_EIGEN, which says "Include Eigen3 support". However, nowhere in the documentation (or with google, for that matter) I can find out what exactly this does and how to use it. I can imagine a few options:</p>
  
  <p>Can I just continue using cv::Mat, and certain functions (which ones?) like cv::Mat::inv() will start using the algorithms from Eigen?</p>
  
  <p>Or does the WITH_EIGEN flag basically do nothing and I need to convert the cv::Mat's to Eigen (or use Eigen::Map) and then use Eigen's algorithms manually?</p>