2013-03-14 02:39:00 -0500 asked a question Mat Image data access problem Hello everyone, First please forgive my english, i'm not native so i may write in a strange manner :S I recently had to work with openCV for a research project, so i'm quite new and have quite strange problems with it. My current problem is when I'm trying to use the at function of the Mat class: Mat cameraTransform(4,4,CV_32F); // I filled this matrix Mat homogeneousVector1 = (Mat_(4,1)<< projection1->x, projection1->y, projection1->z, 1.0); Mat coord4DInCurrentFrame1(4,1,CV_32FC1); double sum = 0; for(int i = 0; i < cameraTransform.rows; i++) { for(int k = 0; k(i,j)*homogeneousVector1.at(j,k); } coord4DInCurrentFrame1.at(i,k) = sum; cout << coord4DInCurrentFrame1.at(i,k) << endl; //(2) sum = 0; } } cout << coord4DInCurrentFrame1.at(0,0) << ", " << coord4DInCurrentFrame1.at(1,0) << ", " << coord4DInCurrentFrame1.at(2,0) << ")"; //(1)  The values of coord4dInCurrentFrame1 at the point (0,0), (1,0), (2,0), evaluated outside of the for loop(1) are not the same as it is when I evaluated them in the for loop(2). I've made this little program for test purpose, I know there is more simple way to do matrix multiplication but the error I got here seems very strange to me and I really want to understand where I am wrong in my program. Thanks for your help