Define matrix
Matrix<typename Scalar, int RowsAtCompileTime, int ColsAtCompileTime>
Matrix<float, 4, 4>
is a defination of matrix with float elements with size 4 x 4
Matrix<double, Dynamic, Dynamic>
is matrix with initially unknown size which is similar to this MatrixXd;
Resources
https://cppsecrets.com/users/141731151079711010097114105641121044610510511611446979946105110/C00-EigenIntroduction.php