title: Point detection and circle area selection
body: <p>I'm working with a particular type of images. Those, after obtaining the spectrum (aply the fft), I obtain the following picture:</p>
  
  <p><img src="https://i.stack.imgur.com/Skp8k.png" alt="enter image description here"></p>
  
  <p>So I want to select one of those "points" (called orders of the spectrum), at the following way:</p>
  
  <p><img src="https://i.stack.imgur.com/CGpEn.png" alt="enter image description here"></p>
  
  <p>I mean "draw" a circle aroud it, select the pixels inside and then center those pixels(without the "border circle"):</p>
  
  <p><img src="https://i.stack.imgur.com/ee7wg.png" alt="enter image description here"></p>
  
  <p>How can I perform it using OpenCV? Does exist any function?</p>