title: How to make ffmpeg preserve transparency?
body: <p>We know <code>ffmpeg</code> can help us with overlaying image on video. My target is to wrap a video with an image whose background is <code>transparent</code>.</p>
  
  <pre><code>ffmpeg -i device/nexus5_portrait.png -i device/input.mp4\
  -filter_complex "overlay=(W-w)/2:(H-h)/2:enable='between(t,0,5)'"\
  -b:v 16M -bufsize 16M device/output.mp4;
  </code></pre>
  
  <p><strong>Output.mp4 has a black background. So is there any way to preserve the <code>alpha</code> channel?</strong></p>
  
  <p>Thanks in advance!</p>