title: about fmp4 encoding , how to fill the mdat box wit H264 frame?
body: <p>I used C language to encode H264 frames (just have I/P frame) into fmp4 files. When I fill the mdat box, I don't know how to fill the frame data.<br>
  Is (I/P)frame data filled directly?<br>
  Do the first 4 bytes need to be filled with frame length? </p>
  
  <p>I tried to use FFMPG to encode fmp4 file, and found that the data in mdat box was completely different from the original I/P frame data.<br>
  Is it coded again?<br>
  Can I directly fill the data of I/P frame into mdat box?</p>