[FFmpeg-devel] Splitting H.264 stream at key packet, first frame is not an I-frame

Stefano Sabatini stefasab at gmail.com
Tue Dec 17 23:29:52 CET 2013

On date Tuesday 2013-12-17 20:55:57 +0100, Reimar Döffinger encoded:
> On Tue, Dec 17, 2013 at 08:14:38PM +0100, Stefano Sabatini wrote:
> > On date Wednesday 2013-12-18 02:56:34 +0900, Yusuke Nakamura encoded:
> > > 2013/12/17 Stefano Sabatini <stefasab at gmail.com>
> > [...]
> > > For AVC and HEVC stream, IDR-picture or I-picture with
> > > recovery_frame_cnt==0 themselves are not a Random Access Point (RAP).
> > > The specification doesn't forbid that these pictures have no corresponding
> > > parameter sets (VPS, SPS and PPS), but libavcodec/libavformat marks as a
> > > keyframe even if the corresponding parameter sets are missing.
> > 
> > What I don't understand is how the VPS/SPS/PPS are processed in the
> > FFmpeg framework, if they correspond to packets or if are stored
> > somewhere else when processing the bitstream (e.g. as extradata
> > used to decode the next frames).

> The first encountered is extracted into extradata, which means that
> for the common case seeking and everything will work well even with
> However if there are additional, different ones they will only be
> parsed as they come.

> The parser will ensure that they will be combined with the following
> packet, which ideally should be an I-frame, thus splitting is supposed
> to work just fine, too, both if either there is only one set of them
> (and thus extradata contains the proper information) or if all I-frames
> have them before.

In practice if there is no SPS/PPS before an I-frame, it is not merged
with it, and in case the frame is chosen as the first frame of a new
segment it will not contain the required information, and thus will
not be decoded.

Curiously, if I use the dump_extra bitstream filter I'm able to remove
the playback error, but the first key frame is still missing:

$ ffmpeg -i IN.ts -bsf:v dump_extra -codec copy -f segment -map 0 -segment_time 6 seg-%d.ts

$ ffprobe seg-1.ts -show_entries packet=pts_time,flags:frame=pkt_pts_time,pict_type -select_streams v -of compact -read_intervals "%+#3"
Input #0, mpegts, from 'seg-1.ts':
  Duration: 00:00:03.67, start: 6.286400, bitrate: 2628 kb/s
  Program 1 
      service_name    : Service01
      service_provider: FFmpeg
    Stream #0:0[0x100]: Video: h264 (Constrained Baseline) ([27][0][0][0] / 0x001B), yuv420p, 1024x576 [SAR 1:1 DAR 16:9], 23.98 fps, 23.98 tbr, 90k tbn, 47.95 tbc
    Stream #0:1[0x101]: Audio: aac ([15][0][0][0] / 0x000F), 22050 Hz, stereo, fltp, 57 kb/s
FFmpeg = Fanciful & Fierce Murdering Perennial Epic Gem

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