title: Get (inner edge) contour pixel with opencv
body: <p>I'm searching for a way to extract the "inner" contours of a binary image with opencv. I know that <code>findContours</code> extracts contours but I need the silhouette pixel which belong to the thresholded object in my binary image and not the outer contours.</p>
  
  <p>Here is a fictive image which describes better what I'm searching for. I am searching for a method to extract the red contour. </p>
  
  <p><img src="https://i.stack.imgur.com/Aqyvr.png" alt="findcontours"></p>
  
  <p>I already tried a naive approach in copying the original binary image and shrinking the copy by 2 pixels each side and filling up the edges with black pixels and used <code>findContours</code>but the outcome is not satisfying.</p>