title: Measure size of object using reference object in photo
body: <p>I'd like to calculate size of object in a photo which includes both a target object and <em>a reference one</em>.</p>
  
  <p>I think what I'd like to do is what this software achieves (I don't know how precise this software is)
  <a href="https://itunes.apple.com/us/app/photo-meter-picture-measuring/id579961082?mt=8" rel="noreferrer">https://itunes.apple.com/us/app/photo-meter-picture-measuring/id579961082?mt=8</a></p>
  
  <p>I've already found, in general, it's called <em>photogrammetry</em> and seems active research field.</p>
  
  <p><a href="https://stackoverflow.com/questions/2860325/how-would-you-find-the-height-of-objects-given-an-image">How would you find the height of objects given an image?</a>
  <a href="https://physics.stackexchange.com/questions/151121/can-i-calculate-the-size-of-a-real-object-by-just-looking-at-the-picture-taken-b">https://physics.stackexchange.com/questions/151121/can-i-calculate-the-size-of-a-real-object-by-just-looking-at-the-picture-taken-b</a></p>
  
  <p>But, I can not find</p>
  
  <ul>
  <li>what is basic way to measure a object in a photo <em>with a reference object</em>.</li>
  <li>a way to implement it or standard open source for it.</li>
  </ul>
  
  <h3>Update</h3>
  
  <ul>
  <li>I can not utilize the distance of the object and reference from the camera.</li>
  <li>The reference and target are on the (approximately) same plane.</li>
  </ul>