title: ffmpeg: scale output cropped width/height doesn't work
body: <p>I'm trying to crop video frames with ffmpeg, and I would like to scale the cropped image automatically.
  I saw an option at av filter: <a href="http://ffmpeg.org/libavfilter.html#SEC41" rel="noreferrer">http://ffmpeg.org/libavfilter.html#SEC41</a> </p>
  
  <p>./ffmpeg -i video.mp4 -vf "crop=640:480,<strong>scale=ow:oh</strong>" -f mpegts udp://127.0.0.1:1234</p>
  
  <p>I receive an error: <em>Error when evaluating the expression 'oh'</em></p>