title: Error "Transparency encoding with auto_alt_ref does not work" when converting a .mov with Alpha to .webm with alpha with ffmpeg
body: <p>I am trying to convert a .mov file with alpha transparency into a .webm file and have been following this thread for help: <a href="https://stackoverflow.com/questions/34856236/convert-mov-with-alpha-to-vp9-webm-with-alpha-using-ffmpeg">Convert mov with Alpha to VP9 Webm with Alpha Using ffmpeg</a></p>
  
  <p>The command line I have been using is </p>
  
  <pre><code>ffmpeg -r 24/1 -i Desktop/Skel_Walk_1.mov -c:v libvpx -pix_fmt yuva420p Desktop/Skel_Walk_1.webm
  </code></pre>
  
  <p>However when I go to run the command it comes up with 2 errors</p>
  
  <pre><code>Transparency encoding with auto_alt_ref does not work
  </code></pre>
  
  <p>and</p>
  
  <pre><code>Error initializing output stream 0:0 -- Error while opening encoder for output stream #0:0 
  - maybe incorrect parameters such as bit_rate, rate, width or height
  </code></pre>
  
  <p>I am not too sure what the problem is here, any suggestions, I am very confused!</p>