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how to initialize a Eigen Matrix from an opencv cv::Mat, or from an array which is row-major?

Tags:

c++

eigen

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.

    cv::Mat A_M=cv::Mat(rows, cols, CV_64FC1);
    double *A=(double *)A_M.data();
    typedef Map<MatrixXd> MapMat;
    MapMat A_eigen(A,m,n);

    Eigen::Matrix<double,Eigen::Dynamic,Eigen::Dynamic,Eigen::RowMajor> A_eigen;
    Eigen::Map<Matrix<double,Eigen::Dynamic,Eigen::Dynamic,Eigen::RowMajor> >(A,m,n) = A_eigen;

Updated:

    double *A=(double *)A_M.data();//m*n
    double *B=(double *)B_M.data();//n*p
    double *C=(double *)C_M.data();//m*p
    //regular Eigen Matrix
    Eigen::MatrixXd A_eigenMat;
    Eigen::MatrixXd B_eigenMat;
    Eigen::MatrixXd C_eigenMat;
    Eigen::Map<Eigen::Matrix<double, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor> > A_mappedMat (A, m, n);
    Eigen::Map<Eigen::Matrix<double, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor> > B_mappedMat (B, n, p);
    Eigen::Map<Eigen::Matrix<double, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor> > C_mappedMat (C, m, p);
    // Eigen handles the conversion from row major to column major
    A_eigenMat = A_mappedMat;
    B_eigenMat = B_mappedMat;
    C_eigenMat = C_mappedMat;

    // multiplication
    C_eigenMat=A_eigenMat*B_eigenMat;

Then, when I output the M_C, its result is wrong. It seems the C_eigenMat didn't copy the data into M_C.data.

like image 230
mining Avatar asked Oct 01 '13 11:10

mining


2 Answers

Sample from Conversion between OpenCV and Eigen:

cv::Mat_<float> a = Mat_<float>::ones(2,2);
Eigen::Matrix<float,Dynamic,Dynamic> b;
cv2eigen(a,b);

It is already answered on SO:

//allocate memory for a 4x4 float matrix
cv::Mat cvT(4,4,CV_32FC1); 

//directly use the buffer allocated by OpenCV
Eigen::Map<Matrix4f> eigenT( cvT.data() ); 

and in one more SO post

like image 140
MikroDel Avatar answered Sep 28 '22 06:09

MikroDel


MikroDel's answer uses OpenCV's functionality, but cv2eigen includes a copy of the data, which is not optimal if you just want to map the data pointer. Using Eigen::Map<> is normally a better option. I modified your code, this example works:

int main(int argc, char *argv[]) {
  int rows = 4, cols = 3;
  cv::Mat A_M=cv::Mat::eye(rows, cols, CV_64FC1);
  A_M.at<double>(0,2) = 10;
  double *A=(double *)A_M.data;
  //regular Eigen Matrix
  Eigen::MatrixXd eigenMat;

  Eigen::Map<Eigen::Matrix<double,Eigen::Dynamic,Eigen::Dynamic,Eigen::RowMajor> > mappedMat (A,rows, cols);
  // Eigen handles the conversion from row major to column major
  eigenMat = mappedMat;   

  std::cout << A_M << std::endl;
  std::cout << eigenMat << std::endl;
  return 0;
}
like image 42
user1906 Avatar answered Sep 28 '22 06:09

user1906