[FFmpeg-devel] [PATCH] ffmpeg: honor -ss and -t parameters with muxed subtitles.

[Nicolas George]

Le quintidi 15 thermidor, an CCXX, Clément Bœsch a écrit :
> This patch fixes two things:
> 
>  - in case of subtitles, check_recording_time() is comparing the current
>    PTS to the recording time (-t option, set to INT_MAX by default), so
>    the -ss option needs to be taken into account. It is not required in
>    do_{audio,video}_out() because this adjustment is set while polling
>    the filtergraph (see poll_filters()).
> 
>  - the second fix is also an adjustment of the PTS send to the encoder
>    (and later transmitted to the muxer) so the TS in the output make
>    sense and are not kept verbatim.
> 
> Note: this only works for muxers honoring the PTS, such as
> lavf/matroskaenc. But for other such as the ASS muxer which just does a
> verbatim copy, or the SubRip muxer which doesn't write the TS in some
> cases, it will not work yet.
> ---
>  ffmpeg.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/ffmpeg.c b/ffmpeg.c
> index f85d8e0..c6fafe4 100644
> --- a/ffmpeg.c
> +++ b/ffmpeg.c
> @@ -1684,13 +1684,22 @@ static void do_subtitle_out(AVFormatContext *s,
>          nb = 1;
>  
>      for (i = 0; i < nb; i++) {
> -        ost->sync_opts = av_rescale_q(pts, ist->st->time_base, enc->time_base);
> +        ost->sync_opts = av_rescale_q(pts, ist->st->time_base, enc->time_base)
> +                       /* start time adjustment so -ss and -t works */
> +                       - av_rescale_q(output_files[ost->file_index]->start_time, AV_TIME_BASE_Q, enc->time_base);
> +
>          if (!check_recording_time(ost))
>              return;
>  
>          sub->pts = av_rescale_q(pts, ist->st->time_base, AV_TIME_BASE_Q);
>          // start_display_time is required to be 0
>          sub->pts               += av_rescale_q(sub->start_display_time, (AVRational){ 1, 1000 }, AV_TIME_BASE_Q);
> +
> +        /* The following adjustment is only done for audio & video types when
> +         * dealing with lavfi. This should be dropped when subtitles are part
> +         * of the lavfi loop */
> +        sub->pts -= output_files[ost->file_index]->start_time;
> +
>          sub->end_display_time  -= sub->start_display_time;
>          sub->start_display_time = 0;
>          subtitle_out_size = avcodec_encode_subtitle(enc, subtitle_out,

I believe you could do simpler by adding at the beginning:

pts = av_rescale_q(pts, ist->st->time_base, AV_TIME_BASE_Q) - start_time;

and then working with it. But do not put the conversion in the nb loop, or
it will be done twice.

Regards,

-- 
  Nicolas George
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