Mat ROI coordinate

I remember a joke in Friends, when Chandler tells Joey it is a J in joincidence, not a C in coincidence.

In opencv, rect is often used when setting the roi of a mat. However, the coordinate could be confusing. Take an example in here, what should be the lower left of an image? It should be Rect( 0, image.rows/2, image.cols/2, image.rows/2 ), instead of Rect( image.rows/2, 0, image.cols/2, image.rows/2 ).

Unlike accessing elements in a mat directly, for instance image.at<float>(row, col), the rect takes in (x, y), corresponding to (col, row). Therefore, the rect is always set by Rect(int x, int y, int width, int height), where width = no. of columns while height = no. of rows.

A good illustration of opencv image coordinate system can be found here, 

Bear in mind this coordinate system when converting opencv image coordinates to camera coordinates, like this