title: Non IDR Picture NAL Units - 0x21 and 0x61 meaning
body: <p>Does anyone know what does <code>0x21</code> and <code>0x61</code> means in h.264 encoded video stream?</p>
  
  <p>I know that <code>0x01</code> means it's a <code>b-frame</code> and <code>0x41</code> means it's a <code>p-frame</code>. My encoded video gives me two <code>0x21</code> frame followed by one <code>b-frame</code>.</p>
  
  <pre><code>I 21 21 B 21 21 B...... 
  </code></pre>
  
  <p>What is this <code>0x21</code>?</p>