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

[Stefano Sabatini]

Hi,

sorry if the topic is not really for ffmpeg-devel, but I'm having this
problem and I'm not sure if this is a bug or what.

I'm splitting a file using the segment muxer:
ffmpeg -i IN.ts -codec copy -f segment -segment_time 6 -map 0 -segment_format mpegts seg-%d.ts

The file is correctly split at a key packet, as I can see with:
$ ffprobe seg-1.ts -show_entries packet=pts_time,flags -select_streams v -of compact -read_intervals "%+#3"
packet|pts_time=6.286400|flags=K
packet|pts_time=6.328111|flags=_
packet|pts_time=6.369811|flags=_

But the first frame is not correctly decoded, so I get the following
$ ffprobe seg-1.ts -show_entries packet=pts_time,flags:frame=pkt_pts_time,pict_type -select_streams v -of compact -read_intervals "%+#3"
[h264 @ 0x3823400] non-existing PPS referenced
[h264 @ 0x3823400] non-existing PPS 0 referenced
[h264 @ 0x3823400] decode_slice_header error
[h264 @ 0x3823400] no frame!
[h264 @ 0x3823400] non-existing PPS referenced
[h264 @ 0x3823400] non-existing PPS 0 referenced
[h264 @ 0x3823400] decode_slice_header error
[h264 @ 0x3823400] no frame!
[repeated multiple times]
Input #0, mpegts, from 'seg-1.ts':
  Duration: 00:00:03.67, start: 6.286400, bitrate: 2627 kb/s
  Program 1 
    Metadata:
      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
packet|pts_time=6.286400|flags=K
packet|pts_time=6.328111|flags=_
packet|pts_time=6.369811|flags=_
frame|pkt_pts_time=6.328111|pict_type=P
frame|pkt_pts_time=6.369811|pict_type=P

I'd expect the first key packet to contain an I-frame, and assume that
this should be decodable with no reference to previous frames, but
maybe this is not the case.

So the question is: is this a decoder bug or is expected behavior?

How it is possible to generate a H.264 stream for which any key-packet
corresponds to a fully decodable I-frame (so that key-packet splitting
won't result in corrupted or missing frames)?

Is it possible to export the frame dependency information, and show it
with ffprobe?
-- 
FFmpeg = Friendly and Fabulous Mournful Philosophical Extroverse Game