[FFmpeg-devel] Question: about "igndts" flag, and how to handle DTS output problems.

[Daniel Cantarín]

I'm having trouble with DTS values in a single dvbsub stream while 
receiving LIVE MPEGTS with muxed video, audio, subs, and other data 
(like scte35) over multicast UDP. I'm doing transcoding tasks, and then 
output that to another MPEGTS stream that feeds other services. This are 
the problems I'm facing:

```
(...)
[mpegts @ 0x5591147cf280] Non-monotonous DTS in output stream 0:3; 
previous: 5729374, current: 5292077; changing to 5729375. This may 
result in incorrect timestamps in the output file.
[mpegts @ 0x5591147cf280] Non-monotonous DTS in output stream 0:3; 
previous: 5729375, current: 5293810; changing to 5729376. This may 
result in incorrect timestamps in the output file.
[mpegts @ 0x5591147cf280] Non-monotonous DTS in output stream 0:3; 
previous: 5729376, current: 5300564; changing to 5729377. This may 
result in incorrect timestamps in the output file.
[mpegts @ 0x5591147cf280] Non-monotonous DTS in output stream 0:3; 
previous: 5729377, current: 5302255; changing to 5729378. This may 
result in incorrect timestamps in the output file.
(...)
```

This has the effect that, later in my work's pipeline, subtitles give 
trouble with players.
I would like to avoid this ffmpeg DTS correction ("with the error", if 
you prefer that interpretation), and thus I tested lots of options 
without success, including the `+igndts` flag. It actually doesn't seem 
to do anything. So I went to check what it actually does, and found 
this.

```
user at computer:/path/to/ffmpeg/repo$ grep -rn "AVFMT_FLAG_IGNDTS" . 
2>/dev/null
./libavformat/options_table.h:48:{"igndts", "ignore dts", 0, 
AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_IGNDTS }, INT_MIN, INT_MAX, D, 
"fflags"},
./libavformat/avformat.h:1228:#define AVFMT_FLAG_IGNDTS       0x0008 
///< Ignore DTS on frames that contain both DTS & PTS
./libavformat/demux.c:963:    if ((s->flags & AVFMT_FLAG_IGNDTS) && 
pkt->pts != AV_NOPTS_VALUE)
user at computer:/path/to/ffmpeg/repo$
```

It seems it's only used on demux (input), and not at mux time (output). 
So I went to check what's going on with that "Non-monotonous" thing, and 
found that `fftools/ffmpeg_mux.c` has a code in line 108 where it checks 
for DTS values without any consideration for skipping such check.


With this in mind, I would like to ask a some questions here about my 
situation, which I'm unsure if it's worthy of a ticket.

1. Is there a way to ignore this DTS issue at mux time and just output 
whatever DTS is on the packet? Sounds like "igndts" would be useful at 
output too, by just adding it to that line 108 check. But perhaps there 
are other way to achieve it.

2. Do you know of some way I could fix this my own way without this 
ffmpeg mux auto-fix? I would do some setpts filter magic if I could, but 
as this is a subtitle stream I have no way to use filters on it. And 
being live streams is really troublesome to apply fixes: I'm trying 
several other tools as proxies right now.

3. I see another code before that 108 line. It involves 
`s->oformat->flags & AVFMT_TS_NONSTRICT`. But I'm not sure of how to 
toggle AVFMT_TS_NONSTRICT from command line to see its effects. Is it 
perhaps "-strict experimental"?


Thanks,
Daniel.