title: Accessing elements of a cv::Mat with at<float>(i, j). Is it (x,y) or (row,col)?
body: <p>When we access specific elements of a cv::Mat structure, we can use mat.at(i,j) to access the element at position i,j.  What is not immediately clear, however, whether (i,j) refers to the x,y coordinate in the matrix, or the ith row and the jth column.</p>