title: How does FFMPEG change fps without dropping frames?
body: <p>Here I got a video which has the FPS 30, duration 10s, and has 300 frames. How could I turn the video to 25FPS without dropping frames.</p>
  
  <p>I suppose the <code>-r</code> or <code>fps=fps=25</code> is kind of resampling method or not working.</p>
  
  <p>My commands are like:</p>
  
  <pre><code>ffmpeg -i input.flv -vf "scale=800:450, fps=25" output1.flv
  </code></pre>
  
  <p>or</p>
  
  <pre><code>ffmpeg -i intput.flv -filter:v fps=fps=25 -c:v libx264 -c:a copy -pix_fmt yuv420p -profile:v high -f mp4 -vf scale=800:450 output2.mp4
  </code></pre>
  
  <p>The result is that <code>output1.flv</code> dropped frames, and <code>output2.mp4</code> didn't work.</p>