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.
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
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;
}
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