title: Opencv How to detect 4 corners of a rectangle after applied cv::findContours and cv::drawContour
body: <p>My problems is I want to find 4 corners of rectangles after using cvfindcontours ?
  Here is my result:</p>
  
  <p>image:<br>
  <a href="https://i.stack.imgur.com/vfk3X.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/vfk3X.png" alt="enter image description here"></a></p>
  
  <p>How can I find 4 corners of each rectangle? 
  I think the building function Cv find and draw contours have their information of the corners, so how can I read it?</p>