title: ffmpeg thumbnails: Every 2 seconds, generated at 0, 1, 3, 5 seconds
body: <p>I'm using ffmpeg to generate thumbnails from a piece of .mov video.  I want to get thumbnails every two seconds, and so, copying almost directly from <a href="https://trac.ffmpeg.org/wiki/Create%20a%20thumbnail%20image%20every%20X%20seconds%20of%20the%20video" rel="nofollow">https://trac.ffmpeg.org/wiki/Create%20a%20thumbnail%20image%20every%20X%20seconds%20of%20the%20video</a>, I'm doing:</p>
  
  <pre><code>ffmpeg -i test.mov -f image2 -vf fps=fps=1/2 thumb%04d.jpg
  </code></pre>
  
  <p>This works, and gives me thumbnails, but they correspond to the images from the video at 0, 1, 3, 5, ... seconds.  I was expecting 0, 2, 4, 6, ... seconds; I'm not sure why there's an initial one second gap between thumbnails before it settles into making them every two seconds.</p>
  
  <p>Is there a way to get them to come out at 0, 2, 4, 6... seconds, or is this just "how it works"?  Thanks!</p>