In my code, I have to convert cv::Mat to Eigen::Matrix so that I can use some of the functionalities of the Eigen library. I believe a straightforward option would be to manually copy paste each element of cv::Mat into Eigen::Matrix. However, I stumbled upon this question on SO and found out about the cv::cv2eigen function. It's located in opencv2/core/eigen.hpp file.

Here is the sample code:

#include <iostream>
#include <opencv2/core.hpp>
#include <opencv2/core/eigen.hpp>
#include <Eigen/Core>

int main(int argc, char const *argv[]) {
    cv::Mat cv_mat = cv::Mat::eye(3, 3, CV_64FC1);
    std::cout << cv_mat << std::endl;
    
    Eigen::Matrix3d eigen_mat;
    cv::cv2eigen(cv_mat, eigen_mat);
    std::cout << eigen_mat << std::endl;
    
    return 0;
}

However, weirdly enough, upon trying to build, I get way too many compilation errors from opencv2/core/eigen.hpp file:

/usr/local/<blah_blah>/include/opencv4/opencv2/core/eigen.hpp:63:22: error: ‘Eigen’ does not name a type; did you mean ‘eigen’?
 void eigen2cv( const Eigen::Matrix<_Tp, _rows, _cols, _options, _maxRows, _maxCols>& src, OutputArray dst )
                      ^~~~~
                      eigen
/usr/local/<blah_blah>/include/opencv4/opencv2/core/eigen.hpp:63:35: error: expected unqualified-id before ‘<’ token
 void eigen2cv( const Eigen::Matrix<_Tp, _rows, _cols, _options, _maxRows, _maxCols>& src, OutputArray dst )
                                   ^
/usr/local/<blah_blah>/include/opencv4/opencv2/core/eigen.hpp:63:35: error: expected ‘)’ before ‘<’ token
/usr/local/<blah_blah>/include/opencv4/opencv2/core/eigen.hpp:63:35: error: expected initializer before ‘<’ token
/usr/local/<blah_blah>/include/opencv4/opencv2/core/eigen.hpp:81:22: error: ‘Eigen’ does not name a type; did you mean ‘eigen’?
 void eigen2cv( const Eigen::Matrix<_Tp, _rows, _cols, _options, _maxRows, _maxCols>& src,
                      ^~~~~
                      eigen
.
.
.
/usr/local/<blah_blah>/include/opencv4/opencv2/core/eigen.hpp:234:6: error: redefinition of ‘template<class _Tp> void cv::cv2eigen(const cv::Mat&, int)’
 void cv2eigen( const Mat& src,
      ^~~~~~~~
/usr/local/<blah_blah>/include/opencv4/opencv2/core/eigen.hpp:141:6: note: ‘template<class _Tp> void cv::cv2eigen(const cv::Mat&, int)’ previously declared here
 void cv2eigen( const Mat& src,
      ^~~~~~~~
/usr/local/<blah_blah>/include/opencv4/opencv2/core/eigen.hpp:259:16: error: ‘Eigen’ has not been declared
                Eigen::Matrix<_Tp, 1, Eigen::Dynamic>& dst )
                ^~~~~
/usr/local/<blah_blah>/include/opencv4/opencv2/core/eigen.hpp:259:29: error: expected ‘,’ or ‘...’ before ‘<’ token
                Eigen::Matrix<_Tp, 1, Eigen::Dynamic>& dst )
                             ^
/usr/local/<blah_blah>/include/opencv4/opencv2/core/eigen.hpp: In function ‘void cv::cv2eigen(const cv::Matx<_Tp, 1, _cols>&, int)’:
.
.
.
/usr/local/<blah_blah>/include/opencv4/opencv2/core/eigen.hpp:262:23: error: ‘Eigen’ has not been declared
     if( !(dst.Flags & Eigen::RowMajorBit) )
                       ^~~~~

This is quite strange, right? Because I'm getting errors for the OpenCV functions that I'm not even touching.

So, how can I fix this weird issue?

P.S.

1. I'm using OpenCV 4.2.0
2. I've modified some explicit paths with <string>

CodePudding user response：

Actually, I found the root cause:

As per opencv/modules/core/include/opencv2/core/eigen.hpp on 4.x branch, line 78 & 79:

@note Using these functions requires the Eigen/Dense or similar header to be included before this header.

So, surprisingly enough, all I had to do was change the #include order from

#include <opencv2/core.hpp>
#include <opencv2/core/eigen.hpp>
#include <Eigen/Core>

to

#include <opencv2/core.hpp>
#include <Eigen/Core>
#include <opencv2/core/eigen.hpp>

On the other hand, I checked the existing /usr/local/<blah_blah>/include/opencv4/opencv2/core/eigen.hpp file (OpenCV 4.2.0) on my machine that, sadly, does not say anything about the #include order!

¯\_(ツ)_/¯