title: How do I measure distances in an image?
body: <p>Let's say we have an image like so </p>
  
  <p><a href="https://i.stack.imgur.com/P5T6n.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/P5T6n.jpg" alt="alt text"></a><br>
  <sub>(source: <a href="http://www.curiousexpeditions.org/North%20Reading%20Room,%20UC%20Berkeley,%20California,%20USA.jpg" rel="nofollow noreferrer">curiousexpeditions.org</a>)</sub>  </p>
  
  <p>Let's say we've already used filters and an edge detection algorithm in this pic. Now my goal is to measure distances (NOT actual distances, distance can be in any arbitrary unit) . 
  eg: How do I find the length of the hall? (until the window)</p>
  
  <p>Or the height of the bookshelves? How exactly do you place the "scale" and measure. I'm looking for ideas. However it would help if the answers were in terms of OpenCV.</p>