title: ffmpeg - how does moving overlay / text command work?
body: <p>In Ffmpeg you can create moving text:</p>
  
  <pre><code>ffmpeg -y -t 10 -s qcif -f rawvideo -pix_fmt rgb24 -s 1280x720 -i /dev/zero -g 1 -r 24 -vf drawtext="fontfile=~/fonts/Trebuchet_MS.ttf:text='thing crawls':fontsize=155:fontcolor=red:y=h-20*t" wow.mpg
  </code></pre>
  
  <p>So this will give me a black frame with "thing crawls" slowly going from bottom up.. </p>
  
  <p>If I know the length of the video (20 seconds) and want to, for example create "thing falls" that starts at the top of the screen at time 0 and goes to the bottom of the screen until 00:00:20, how do I do that?</p>
  
  <p>Also can I create the situation where the text will start going from top to bottom, but stop at the middle of the screen?</p>