title: How to calculate video frame number from VoTT exported image timestamp
body: <p>When I export tagged frames from a video via <a href="https://github.com/microsoft/VoTT" rel="nofollow noreferrer">Microsoft VoTT</a>, I get a list of all exported images in a CSV file. Each of these images contain their timestamp from within the video in their file names. For example:</p>
  <pre><code>video.mp4#t=260.433333.jpg
  </code></pre>
  <p>Now I want to calculate which frame this image is in the video. So in this example when this <code>video.mp4</code> has 60 FPS, I would assume that I can simply calculate that frame number via</p>
  <pre><code>timestmap * fps = int(260.433333 * 60) = 15626
  </code></pre>
  <p>Thus I would assume that timestamp 260.433333 corresponds to frame number 15626 in the video. As it turned out, however, this is close, but not exactly correct. After some manual tweaking I found out the correct frame number is 15610. The further I go in video time, the greater this difference becomes.</p>
  <p>Now I am wondering: Where does this difference of 16 frames come from, and how do I correctly calculate the frame number from these timestamps?</p>