How does OpenCV make use of Eigen?
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:
Can I just continue using cv::Mat, and certain functions (which ones?) like cv::Mat::inv() will start using the algorithms from Eigen?
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?
Answer
After working with it for a bit, I can provide the answer:
The WITH_EIGEN flag does nothing except making the eigen-opencv interopability functions available.
Can I just continue using cv::Mat, and certain functions (which ones?) like cv::Mat::inv() will start using the algorithms from Eigen?
No, cv::Mat::inv() has no smart logic and will use the OpenCV algorithms.
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?
Exactly, that's the way to go. I wouldn't necessarily recommend to use cv2eigen() and eigen2cv() though. I used Eigen::Map to just map the memory (no cost in copying anything) and cv::Mat(void*, ...) to map the data back. Be careful though with row/col-major flags and stuff.