[FFmpeg-trac] #3442(avcodec:new): h264 decoder decodes complete GOP wrong / skips one GOP if stream does not start with SPS/PPS

FFmpeg trac at avcodec.org
Fri Mar 7 10:40:38 CET 2014

#3442: h264 decoder decodes complete GOP wrong / skips one GOP if stream does not
start with SPS/PPS
             Reporter:  munderl      |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:  avcodec      |                  Version:  git-
             Keywords:  H.264        |  master
  broken frames PAFF MBAFF           |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
 Summary of the bug:

 I recorded a DVB TS stream. It contains an H264 video stream which first
 NALU is a B slice, followed by the SPS, PPS  NALUs and an IDR frame.

 If this stream is played with ffplay (either the TS or the demuxed
 elementary stream), the decoder seems to get so much confused by the
 leading B slice that the following valid GOP is completely decoded
 erroneous until the next GOP arrives. ffplay skips the broken frames and
 starts playing at the second GOP by default. One has to specify -flags2
 +showall to see the GOP of broken frames.

 If the B slice is removed from the stream, playing starts fine from the
 first frame/GOP on.

 I see that it is intended that ffmpeg tries to decode and conceal every
 frame coming in, but that should effect only one frame (the B slice) and
 not the following GOP.

 However, I'm not so sure why the B slice is concealed as there is no valid
 SPS/PPS yet which is mandatory to decode the slice. But that might be a
 different problem.

 Take a h264 stream starting with a B slice, followed by the SPS/PPS/IDR. I
 uploaded a sample TS file to ftp://upload.ffmpeg.org/MPlayer/incoming/
 called DVB_testvideo.ts. One can either play that file or extract the H264
 elementary stream and use it directly.

 The stream is using PAFF/MBAFF, but I don't know if this is related.

 How to reproduce:
 % ffplay DVB_testvideo.ts
 skips the first valid GOP completely and starts playing from frame 16

 % ffplay DVB_testvideo.ts -flags2 +showall
 starts playing 16 broken frames, then recovers and plays fine.

 Now remove the first B slice (skip 17756 from the H264 bitstream or 20304
 bytes from the whole TS file).

 % ffplay DVB_testvideo.ts
 plays fine from the first frame on (15 frames valid frames more then

 % ffplay DVB_testvideo.ts -flags2 +showall
 shows no broken frames anymore

 ffmpeg version N-42015-g64e4bd7
 built on openSUSE 13.1 (gcc 4.8.1)

Ticket URL: <https://trac.ffmpeg.org/ticket/3442>
FFmpeg <https://ffmpeg.org>
FFmpeg issue tracker

More information about the FFmpeg-trac mailing list