How to control the order of points in OpenCV findContours?

Question

I am working on an OpenCV code in C++ that basically tries to detect the boundary points of a page in an image. I use the findCountours() function to find the contours and after that, I try to find quads using the approxPolyDP() function and consider only quads with only 4 points. My code is based mostly on the squares.cpp code in the OpenCV samples.

It worked and returned 4 Points. And I was happy...but this happiness was not to last. :( I soon realised that the order in which the Points are returned appears to be random (different for different images). I want the algorithm to return the 4 Points in only a particular order (say, Top-left, then Top-right, then Bottom-right and then Bottom-left.

Getting the points in the same fixed order is critical for my application and hence, my question is "Is there any way to fix the order in which these points are detected and returned by the algorithm" (the code is almost same as the squares.cpp example of OpenCV). Any help will be really appreciated. And goes without saying, TIA. :D

Answer 1

If pointsVector is of type vector<cv::Point> and it has exactly the four coordinates of the rectangle or square that you need, use the following code.

struct sortY {
    bool operator() (cv::Point pt1, cv::Point pt2) { return (pt1.y < pt2.y);}
} mySortY;
struct sortX {
    bool operator() (cv::Point pt1, cv::Point pt2) { return (pt1.x < pt2.x);}
} mySortX;
std::sort(pointsVector.begin(),pointsVector.end(),mySortY);
std::sort(pointsVector.begin(),pointsVector.begin()+2,mySortX);
std::sort(pointsVector.begin()+2,pointsVector.end(),mySortX);

The above code will sort the pointsVector in the following order: topLeft, topRight, bottomLeft, bottomRight