[FFmpeg-devel] [PATCH v1] fftools/ffmpeg: set AV_PKT_FLAG_KEY for the subtitle packet

[Andreas Rheinhardt]

Jan Ekström:
> On Fri, Mar 20, 2020 at 5:45 PM <lance.lmwang at gmail.com> wrote:
>>
>> From: Limin Wang <lance.lmwang at gmail.com>
>>
>> This fixes webvtt segment output.
>>
>> Please testing with the following command and check the output:
>> ./ffmpeg -i ../fate-suite/sub/MicroDVD_capability_tester.srt -f segment -segment_time 10 \
>>     -segment_list_size 0  -segment_list sub.m3u8 -segment_format webvtt -scodec webvtt sub-%d.vtt
>>
>>
>> Signed-off-by: Limin Wang <lance.lmwang at gmail.com>
>> ---
>>  fftools/ffmpeg.c                 |  1 +
>>  tests/ref/fate/binsub-movtextenc |  2 +-
>>  tests/ref/fate/sub2video         | 86 ++++++++++++++++----------------
>>  3 files changed, 45 insertions(+), 44 deletions(-)
>>
>> diff --git a/fftools/ffmpeg.c b/fftools/ffmpeg.c
>> index aaaf241314..c5a2d0731d 100644
>> --- a/fftools/ffmpeg.c
>> +++ b/fftools/ffmpeg.c
>> @@ -1054,6 +1054,7 @@ static void do_subtitle_out(OutputFile *of,
>>              else
>>                  pkt.pts += av_rescale_q(sub->end_display_time, (AVRational){ 1, 1000 }, ost->mux_timebase);
>>          }
>> +        pkt.flags |= AV_PKT_FLAG_KEY;
>>          pkt.dts = pkt.pts;
>>          output_packet(of, &pkt, ost, 0);
>>      }
> 
> I do wonder if this is just a case of people forgetting to set the
> flag for the relevant packets in the relevant modules?
> 
> I'm not sure if all API users should be forced to handle this
> separately. If the packets are decode'able by themselves, they should
> be marked as such.
> 
> (Unfortunately, this probably means that all subtitle encoders and
> text-based subtitle format demuxers would have to be updated where
> this flag is not set)
> 
av_read_frame() already sets the AV_PKT_FLAG_KEY-flag for all subtitle
packets (see is_intra_only() in libavformat/utils.c; the subtitle
demuxer based around FFDemuxSubtitlesQueues actually have the flag set
even before it reaches is_intra_only()). One could do something similar
in libavformat/mux.c.
But are we actually sure that all subtitle packets are decodable by
themselves? IIRC this is not true for all PGS subtitles.

- Andreas

PS: The semantics of the AV_CODEC_PROP_INTRA_ONLY-flag seem to be based
around the assumption that subtitle packets are always intra-only: It is
only for video and audio-codecs only.