title: MPEG-4 and alpha transparency - the ongoing saga
body: <p>I'd like to reach some sort of definitive answer for the following questions:</p>
  
  <ol>
  <li>Is alpha transparency supported in MPEG-4-based codecs?</li>
  <li>Is there any way to be reasonably certain that there is no alpha-channel from the output of <code>ffprobe</code>?</li>
  </ol>
  
  <p>Some links I've found on the subject:</p>
  
  <ul>
  <li><a href="https://stackoverflow.com/a/5057631/476716">https://stackoverflow.com/a/5057631/476716</a></li>
  </ul>
  
  <blockquote>
    <p>Also, transparent alpha layers, an optional part of the H.264 specification, are now supported in H.264-based QuickTime movies.</p>
  </blockquote>
  
  <ul>
  <li><a href="http://software.intel.com/en-us/articles/alpha-channel-support-in-the-h264-decoder-in-ipp/" rel="nofollow noreferrer">http://software.intel.com/en-us/articles/alpha-channel-support-in-the-h264-decoder-in-ipp/</a> </li>
  </ul>
  
  <blockquote>
    <p>The support for alpha channel in the H.264 decoder is now available in Intel® Integrated Performance Primitives 6.0 version.</p>
  </blockquote>
  
  <ul>
  <li><a href="http://forums.creativecow.net/readpost/335/30475" rel="nofollow noreferrer">http://forums.creativecow.net/readpost/335/30475</a> </li>
  </ul>
  
  <blockquote>
    <p>The MPEG4 Simple Studio Profile supports auxiliary channels, which could be used for alpha or depth -- and presumably even displacement, motion vectors, normals, etc.</p>
  </blockquote>
  
  <p>However, I've never seen a concrete example, or indeed any details on how you use these announced features.</p>