title: What are keypoints in image processing?
body: <p>When using OpenCV for example, algorithms like SIFT or SURF are often used to detect keypoints. My question is what actually are these keypoints? </p>
  
  <p>I understand that they are some kind of "points of interest" in an image.  I also know that they are scale invariant and are circular. </p>
  
  <p>Also, I found out that they have orientation but I couldn't understand what this actually is. Is it an angle but between the radius and something? Can you give some explanation? I think I need what I need first is something simpler and after that it will be easier to understand the papers.</p>