title: Measuring an object from a picture using a known object size
body: <p>So what I need to do is measuring a foot length from an image taken by an ordinary user. That image will  contain a foot with a black sock wearing, a coin (or other known size object), and a white paper (eg A4) where the other two objects will be upon. 
  What I already have?
  -I already worked with opencv but just simple projects;
  -I already started to read some articles about Camera Calibration (<a href="http://www.amazon.co.uk/Learning-OpenCV-Computer-Vision-Library/dp/0596516134/ref=sr_1_1?ie=UTF8&amp;qid=1341846705&amp;sr=8-1" rel="noreferrer">"Learn OpenCv"</a>) but still don't know if I have to go so far.</p>
  
  <p>What I am needing now is some orientation because I still don't understand if I'm following right way to solve this problem. I have some questions: Will I realy need to calibrate camera to get two or three measures of the foot? How can I find the points of interest to get the line to measure, each picture is a different picture or there are techniques to follow?  </p>
  
  <p>Ps: sorry about my english,  I really have to improve it :-/</p>