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

[Marton Balint]

On Sun, 9 Dec 2018, Nicolas George wrote:

> 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.

Seeking will only work in the special case I provided in the commit 
message, when we always seek back to the beginning and otherwise read the 
referenced files continously.

In theory, when we are in "seekable" state then if we detect a duration 
change we could indeed update all subsequent start times to provide proper 
timestamps for seeking, but I did not need this for my purpose, and I am 
not sure if it's worth the extra code needed.

Regards,
Marton