title: ffmpeg: Low framerate when capturing with -vcodec mjpeg but not with -vcodec copy
body: <p>I'm trying to capture video from a webcam, and I find that when I use the <code>-vcodec copy</code> option, it works really well (far better than any other software I've tried).  However, I'd like my files to be a bit smaller, and it seems that every attempt I make to compress the video leads to extremely jumpy video.  If, for example, I switch the output vcodec to <code>mjpeg</code>, it changes from reporting 15 fps to reporting between 3 and 4 fps.  Am I doing something wrong??  Here is the call with <code>-vcodec copy</code>:</p>
  
  <pre><code>ffmpeg -y -f dshow -vcodec mjpeg -s 1184x656 -framerate 25 -i video="HD 720P Webcam" -vcodec copy test.avi
  </code></pre>
  
  <p>-- which gets me 15 fps.  But if I change to <code>mjpeg</code>, I get only 3-4 fps:</p>
  
  <pre><code>ffmpeg -y -f dshow -vcodec mjpeg -s 1184x656 -framerate 25 -i video="HD 720P Webcam" -vcodec mjpeg test.avi
  </code></pre>
  
  <p>Experimental attempts to put <code>-framerate 25</code> or <code>-r 25</code> before <code>test.avi</code> also does nothing to help the situation.  I'm not getting any smoother video when experimenting with <code>mpeg4</code> or <code>libx264</code> either.  Only the <code>copy</code> option gives me smooth video (btw I'm filming my hands playing a piano, so there is a lot of fast motion in the videos).</p>
  
  <p>Help!!!!  And thank you...</p>