Myujth

recently I'm studying ORB_SLAM2.In the open source code,I find the code that put on this page. I have a question about that,why dose the matrix A only have 4*4 dimension,I think it should have 6*4 dimension.Because,it is from two matrix of multiply.[kp1.pt.x,kp1.pt.y]*P1?

Triangulate(const cv::KeyPoint &kp1, const cv::KeyPoint &kp2,const cv::Mat &P1, const cv::Mat &P2, cv::Mat &x3D) 

cv::Mat A(4,4,CV_32F);

A.row(0) = kp1.pt.x*P1.row(2)-P1.row(0);
A.row(1) = kp1.pt.y*P1.row(2)-P1.row(1);
A.row(2) = kp2.pt.x*P2.row(2)-P2.row(0);
A.row(3) = kp2.pt.y*P2.row(2)-P2.row(1);

cv::Mat u,w,vt;
cv::SVD::compute(A,w,u,vt,cv::SVD::MODIFY_A

//I think the matrix A from triangulation should do is in the beneth
A.row(0) = kp1.pt.y*P1.row(2)-P1.row(1);
A.row(1) = P1.row(0)-kp1.pt.x*P1.row(2);
A.row(2) = kp1.pt.x*P1.row(1)-kp1.pt.y*P1.row(0);
A.row(3) = kp2.pt.y*P2.row(2)-P2.row(1);
A.row(4) = P2.row(0)-kp2.pt.x*P2.row(2);
A.row(5) = kp2.pt.x*P2.row(1)-kp2.pt.y*P2.row(0);

I mainly doubt the dimension of matrix A how to initialize! 4*4 or 6*4 ??