title: Specify background colour when generating movie from images
body: <p>I generate a video from png images using</p>
  
  <pre><code>ffmpeg -i visualization/%d.png -c:v libx264 -vf "scale=500:trunc(ow/a/2)*2"  -pix_fmt yuv420p z.mov
  </code></pre>
  
  <p>and if images have transparencies, they become black. Can I somehow make them white?</p>