Opencv's experience of confusing X and Y coordinates

Conclusion

All about cv::Mat Class method , The index order is First, then , namely First y after x.
All about other classes （ Such as cv::Rectcv::Point） Methods , The index order is Row after column , That's what we're familiar with First x after y.

First y after x Example

cv::Mat img(240, 320, CV_8U, 100); // Instantiate a 240 That's ok ,320 Column Mat Class image 

img.at<uchar>(j, i) = (...);  // Use at Method to access the j That's ok , The first i The pixel of the column 

First x after y Example

cv::Mat img(cv::Size(320, 240), CV_8U, 100); // Instantiate a 320 Column ,240 Yes Mat Class image 

cv::Rect rect(x, y, length, height); // Instantiate a Rect Rectangle like object   The coordinates of the upper left corner are (x, y), Width is length  The height is height
cv::Point p(x, y) // Instantiate a Point Class point variable , Coordinate for (x, y)