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

[Nicolas George]

Le quintidi 15 thermidor, an CCXX, Clément Bœsch a écrit :
> It still works with my test case (./ffmpeg -i in.mkv -ss 00:04:10 -t 30
> -c:s ass -y out.mkv, and it also works with -c:s copy).
> 
> Note: this version is assuming enc->time_base and ost->st->time_base have
> the same value.

I do not think you can rely on that (ffmpeg just happens to set the
time_base to 1/1000, which is the same as hardcoded in Matroska), but that
is just an additional rescale.

> From: =?UTF-8?q?Cl=C3=A9ment=20B=C5=93sch?= <ubitux at gmail.com>
> Date: Wed, 1 Aug 2012 18:39:46 +0200
> Subject: [PATCH] ffmpeg: honor -ss and -t parameters with muxed subtitles.
> 
> 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()).
> 
>  - It also adjusts the PTS sent 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 | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/ffmpeg.c b/ffmpeg.c
> index f85d8e0..620903e 100644
> --- a/ffmpeg.c
> +++ b/ffmpeg.c
> @@ -1683,12 +1683,16 @@ static void do_subtitle_out(AVFormatContext *s,
>      else
>          nb = 1;
>  
> +    /* shift timestamp to honor -ss and make check_recording_time() work with -t */
> +    pts = av_rescale_q(pts, ist->st->time_base, AV_TIME_BASE_Q)
> +        - output_files[ost->file_index]->start_time;
>      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, 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);
> +        sub->pts = pts;
>          // start_display_time is required to be 0
>          sub->pts               += av_rescale_q(sub->start_display_time, (AVRational){ 1, 1000 }, AV_TIME_BASE_Q);
>          sub->end_display_time  -= sub->start_display_time;
> @@ -1703,7 +1707,7 @@ static void do_subtitle_out(AVFormatContext *s,
>          av_init_packet(&pkt);
>          pkt.data = subtitle_out;
>          pkt.size = subtitle_out_size;

> -        pkt.pts  = av_rescale_q(sub->pts, AV_TIME_BASE_Q, ost->st->time_base);
> +        pkt.pts  = ost->sync_opts;

Why this change? It looked right before.

Also, I can imagine some obscure subtitle codec that have perforce
start_display_time = 200ms (i.e. the subtitle is always muxed 200ms ahead):
the decoder would then adjust sub->pts accordingly. Or something.

Regards,

-- 
  Nicolas George
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