title: Use Kalman filter to track the position of an object, but need to know the position of that object as an input of Kalman filter. What is going on?
body: <p>I am trying to study how to use Kalman filter in tracking an object (ball) moving in a video sequence by myself so please explain it to me as I am a child.</p>
  
  <ul>
  <li><p>By some algorithms (color analysis, optical flow...), I am able to get a binary image of each video frame in which there is the tracking object ( white pixels) and background (black pixels) -> I know the object size, object centroid, object position -> Just simple draw a bounding box around the object --> Finish. Why do I need to use Kalman filter here?</p></li>
  <li><p>Ok, somebody told me that because I can not detect the object in each video frame because of noise, I need to use Kalman filter to estimate the position of the object. Ok, fine. But as I know, I need to provide the input to Kalman filter. They are previous state and measurement.</p>
  
  <ul>
  <li>previous state ( so I think it is the position, the velocity, acceleration...of the object in the previous frame) -> Ok, this is fine to me.</li>
  <li><p><strong>measurement</strong> of current state: Here is what I can not understand. What can measurement be?
              - The position of the object in the current frame? It is funny because if I know the position of the object, all I need is just to draw a simple boundingbox (rectangular) around the object. Why I need Kalman filter here anymore? Therefore, it is impossible to take the position of the object in the current frame as measurement value.
             - "Kalman Filter Based Tracking in an Video Surveillance System" article says</p>
  
  <blockquote>
    <p>The main role of the Kalman filtering block is to assign a tracking
    filter to each of the measurements entering the system from the
    optical flow analysis block.</p>
  </blockquote>
  
  <p>If you read the full paper, you will see that the author takes the maximum number of blob and the minimum size of the blob as an input to the Kalman filter. How can those parameters be used as <strong>measurement</strong>?</p></li>
  </ul></li>
  </ul>
  
  <p>I think I am in a loop now. I want to use Kalman filter to track the position of an object, but I need to know the position of that object as an input of Kalman filter. What is going on?</p>
  
  <p>And 1 more question, I dont understand the term "number of Kalman filter". In a video sequence, if there are 2 objects need to track -> need to use 2 Kalman filter? Is that what it means?</p>