[FFmpeg-devel] [PATCH] movenc: calculate track_duration without packet duration

[Alfred E. Heggestad]

On 17/06/2019 10:23, Gyan wrote:
> 
> 
> On 17-06-2019 01:37 PM, Alfred E. Heggestad wrote:
>> From c69b63a7af5531257753754e64ac33b7ef530e75 Mon Sep 17 00:00:00 2001
>> From: "Alfred E. Heggestad" <alfred.heggestad at gmail.com>
>> Date: Mon, 17 Jun 2019 10:04:08 +0200
>> Subject: [PATCH] movenc: calculate track_duration without packet duration
>>
>> ---
>>  libavformat/movenc.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/libavformat/movenc.c b/libavformat/movenc.c
>> index 46d314ff17..fa5833962b 100644
>> --- a/libavformat/movenc.c
>> +++ b/libavformat/movenc.c
>> @@ -5486,7 +5486,7 @@ int ff_mov_write_packet(AVFormatContext *s, 
>> AVPacket *pkt)
>>                     "this case.\n",
>>                     pkt->stream_index, pkt->dts);
>>      }
>> -    trk->track_duration = pkt->dts - trk->start_dts + pkt->duration;
>> +    trk->track_duration = pkt->dts - trk->start_dts;
> 
> Why?
> 
>> trk->last_sample_is_subtitle_end = 0;
>>
>>      if (pkt->pts == AV_NOPTS_VALUE) {
> 

the background for this change is the check done in check_pkt():

    if (trk->entry) {
         ref = trk->cluster[trk->entry - 1].dts;
     } else if (   trk->start_dts != AV_NOPTS_VALUE
                && !trk->frag_discont) {
         ref = trk->start_dts + trk->track_duration;
     } else
         ref = pkt->dts; // Skip tests for the first packet

     if (trk->dts_shift != AV_NOPTS_VALUE) {
         /* With negative CTS offsets we have set an offset to the DTS,
          * reverse this for the check. */
         ref -= trk->dts_shift;
     }

     duration = pkt->dts - ref;
     if (pkt->dts < ref || duration >= INT_MAX) {
         av_log(s, AV_LOG_ERROR, "Application provided duration: 
%"PRId64" / timestamp: %"PRId64" is out of range for mov/mp4 format\n",
             duration, pkt->dts
         );

         pkt->dts = ref + 1;
         pkt->pts = AV_NOPTS_VALUE;
     }



it requires that the DTS of the packet is larger than
"ref" which in some cases is equal to track_duration.


line 5015:

      track->track_duration = pkt.dts - track->start_dts;

line 5489:

      trk->track_duration = pkt->dts - trk->start_dts + pkt->duration;


line 5615:

      trk->track_duration = pkt->dts - trk->start_dts;



setting track_duration is inconsistent; some times it includes
duration and some times not.


here is an example with a stream of AVPacket's that increment
the DTS by approx. 10 (and duration is set to 0):

pkt:  dts:
  1     0
  2    10
  3    20
  4    30
  5    40
  6    50


after packet 6 has arrived, the "ref" value in check_pkt will now
be calculated to 50 + 10 = 60. the code assumes a duration of 10.

when packet 7 arrives it has a DTS of 59 (which is valid).
but this packet will fail in check_pkt and print a warning,
because 59 < 60.






/alfred