[FFmpeg-devel] [PATCH 4/4] avformat/concatdec: always re-calculate start time and duration

[Nicolas George]

Marton Balint (2018-11-22):
> This allows the underlying files to change their duration on subsequent
> avformat context opens.
> 
> An example use case where this matters:
> 
> ffconcat version 1.0
> file dummy.mxf
> file dummy.mxf
> 
> ffmpeg -re -stream_loop -1 -i dummy.ffconcat -f sdl2 none
> 
> The user can seamlessly change the input by atomically replacing dummy.mxf.
> 
> Signed-off-by: Marton Balint <cus at passwd.hu>
> ---
>  libavformat/concatdec.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/libavformat/concatdec.c b/libavformat/concatdec.c
> index ebc50324cc..2ebd2120c3 100644
> --- a/libavformat/concatdec.c
> +++ b/libavformat/concatdec.c
> @@ -355,14 +355,12 @@ static int open_file(AVFormatContext *avf, unsigned fileno)
>          return ret;
>      }
>      cat->cur_file = file;
> -    if (file->start_time == AV_NOPTS_VALUE)
> -        file->start_time = !fileno ? 0 :
> -                           cat->files[fileno - 1].start_time +
> -                           cat->files[fileno - 1].duration;
> +    file->start_time = !fileno ? 0 :
> +                       cat->files[fileno - 1].start_time +
> +                       cat->files[fileno - 1].duration;
>      file->file_start_time = (cat->avf->start_time == AV_NOPTS_VALUE) ? 0 : cat->avf->start_time;
>      file->file_inpoint = (file->inpoint == AV_NOPTS_VALUE) ? file->file_start_time : file->inpoint;
> -    if (file->duration == AV_NOPTS_VALUE)
> -        file->duration = get_best_effort_duration(file, cat->avf);
> +    file->duration = get_best_effort_duration(file, cat->avf);
>  
>      if (cat->segment_time_metadata) {
>          av_dict_set_int(&file->metadata, "lavf.concatdec.start_time", file->start_time, 0);
> @@ -529,8 +527,7 @@ static int open_next_file(AVFormatContext *avf)
>      ConcatContext *cat = avf->priv_data;
>      unsigned fileno = cat->cur_file - cat->files;
>  
> -    if (cat->cur_file->duration == AV_NOPTS_VALUE)
> -        cat->cur_file->duration = get_best_effort_duration(cat->cur_file, cat->avf);
> +    cat->cur_file->duration = get_best_effort_duration(cat->cur_file, cat->avf);
>  
>      if (++fileno >= cat->nb_files) {
>          cat->eof = 1;

I do not think it works. If the duration of the second file changes, for
example, then the start time of all subsequent files changes too, but
this patch does not update it. Seeking will show strange results.

Regards,

-- 
  Nicolas George
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