title: Ffmpeg color opacity
body: <p>I tried to rotate a video and keep the unused place due to the rotation transparent, to be able to overlay it later and to do that, I suppose that if the color was transparent is will be right; and in the official page (<a href="https://www.ffmpeg.org/ffmpeg-filters.html#rotate" rel="nofollow">https://www.ffmpeg.org/ffmpeg-filters.html#rotate</a>) they said that you can give a color that ffmpeg would use in place of unused place due to the rotation of the video. So I tried, relatively to <a href="https://www.ffmpeg.org/ffmpeg-utils.html#Color" rel="nofollow">https://www.ffmpeg.org/ffmpeg-utils.html#Color</a> to put a color with these commands </p>
  
  <blockquote>
    <p>ffmpeg -i video.mp4 -vf "rotate=PI/6:fillcolor=red@0.0" -acodec copy output.mp4</p>
  </blockquote>
  
  <p>and</p>
  
  <blockquote>
    <p>ffmpeg -i video.mp4 -vf "rotate=PI/6:fillcolor=red@0x00" -acodec copy output.mp4</p>
  </blockquote>
  
  <p>And it doesn't work. The color remain opaque, and that whatever the value of the opacity (0.0 to 1.0)</p>
  
  <p>Any of you can know what is happened?</p>