title: ffmpeg keyframe extraction
body: <p>I have been trying to extract keyframes from video using ffmpeg 0.11.1 . So far all the commands I have tried do not extract keyframes but return all the frames ie 25fps*total time number of frames in the output. 
  I tried setting the keyint_min as 25 to make sure there is a amximum of 1 keyframe per second.</p>
  
  <pre><code>ffmpeg -vf select="eq(pict_type\,PICT_TYPE_I)" -g 250 -keyint_min 25 -i C:\test.mp4 -vsync 2 -f image2 C:\testTemp\thumbnails-%02d.jpeg
  </code></pre>
  
  <p>But still all the frames are returned.</p>
  
  <p>Then i tried, to separate the keyframes by 20 seconds.</p>
  
  <pre><code>ffmpeg -i C:\test.mp4 -vf select='eq(pict_type\,I)*(isnan(prev_selected_t)+gte(t-prev_selected_t\,20))' -vsync 0 -f image2 C:\testTemp\%09d.jpg
  </code></pre>
  
  <p>Again same result, all the frames are returned.</p>
  
  <p>What should I do?</p>