title: "opaque" pointer in ffmpeg AVFrame
body: <p>In <code>ffmpeg</code> there is a structure <a href="https://libav.org/documentation/doxygen/master/structAVFrame.html" rel="nofollow">AVFrame</a> describing decoded video or audio data.</p>
  
  <p>It has a void pointer <code>opaque</code>. The documentation claims it is "for some private data of the user".</p>
  
  <p>What does this mean?
  Can it be used to transport any additional data as per-frame metadata?</p>