dC_dx problem

[Wang-Minxin]

Hi. In QuadricCamera.cpp， why the code "Eigen::Matrix<double, 16, 16> dXi_dX = -kron(Xi.transpose(), Xi); " has a negative sign.
In my opinion Xi = X.inverse(), so Xi = XiXXi , and then vec(Xi)=kron(Xi.transpose(),Xi)vec(X)