title: Print numbers on a OpenCV image
body: <p>i have a vector that describes all the features in an Mat image in OpenCV.
  I want to print all the points with their respective numbers that are nothing more than the indices of the vector.</p>
  
  <p>What i want it's an image like this:</p>
  
  <p><img src="https://i.stack.imgur.com/Pr1mx.png" alt="enter image description here"></p>
  
  <p>what is the easiest way?
  I have to use the cv::putText?</p>