如何从opencv cv::Mat或以行为主的数组初始化特征矩阵?

I found that Eigen's Matrix is default column-major, which is like MATLAB, but how do I initialize an Eigen::MatrixXd from an cv::Mat? The following code is my test. But none of them could be compiled successfully. Could someone give me some advice, please? or some other links? Thanks.I found that Eigen's Matrix is default column-m