[FFmpeg-devel] [PATCH] added parameter to dash encoder for start available time

[Gyan Doshi]

On 2021-03-19 03:22, Jan Ekström wrote:
> On Thu, Mar 18, 2021 at 5:07 PM Jan Ekström <jeebjp at gmail.com> wrote:
>> On Thu, Mar 18, 2021, 17:01 Gyan Doshi <ffmpeg at gyani.pro> wrote:
>>>
>>>
>>> On 2021-03-18 20:12, Jan Ekström wrote:
>>>> So right now we do have an option in ffmpeg.c called itsoffset. It
>>>> lets you configure the input offset in time (seconds - such as
>>>> "1337.123" or time format - such as "00:00:01.123"). The only problem
>>>> with it right now is that unless you set -copyts it will not get
>>>> applied for anything else than subtitles, as the offset gets applied
>>>> before and is not taken into account in the "start from PTS 0"
>>>> calculation for video/audio streams.
>>> Not the case.
>>>
>>>       ffmpeg -f lavfi -itsoffset 3 -i nullsrc=r=1:d=3 -vf showinfo -f null -
>>>
>>> gives
>>>
>>> [Parsed_showinfo_0 @ 0000019cc1b88540] n:   0 pts:      3 pts_time:3   ...
>>> [Parsed_showinfo_0 @ 0000019cc1b88540] n:   1 pts:      4 pts_time:4   ...
>>> [Parsed_showinfo_0 @ 0000019cc1b88540] n:   2 pts:      5 pts_time:5   ...
>>>
>>> I've used it many times to adjust sync among streams.
>>>
>>> Regards,
>>> Gyan
>>
>> Very interesting!
>>
>> I will have to retest, I just recall that the last time I tried it got applied when there were multiple streams in an input when the start_time based adjustment was done.
> Right, so taking in a Matroska file it works fine, but when taking in
> an MPEG-TS file with a start_time it not only transforms (start_time +
> input_ts_offset) to (input_ts_offset), but just to 0 :) .
>
> Example:
> ffmpeg -v verbose -debug_ts -itsoffset "1616090389" -i
> 'https://megumin.fushizen.eu/samples/switchingaudio.ts' -c copy -bsf:a
> aac_adtstoasc -movflags dash+frag_discont+delay_moov -vframes 5 -f mp4
> - > test.mp4
>
> Probed start_time:
>    Duration: 00:00:15.25, start: 19516.478644, bitrate: 23823 kb/s
>
> And how ffmpeg.c then adjusted it:
> demuxer -> ist_index:2 type:audio next_dts:NOPTS next_dts_time:NOPTS
> next_pts:NOPTS next_pts_time:NOPTS pkt_pts:1756483078
> pkt_pts_time:19516.5 pkt_dts:1756483078 pkt_dts_time:19516.5
> off:1616070872521356 off_time:1.61607e+09
> demuxer+ffmpeg -> ist_index:2 type:audio pkt_pts:0 pkt_pts_time:0
> pkt_dts:0 pkt_dts_time:0 off:-19516478644 off_time:-19516.5
>
> Thus, ffmpeg.c is not only adjusting the timestamp to start at zero
> with the start_time, but also takes the itsoffset off as well.

This affects formats with AVFMT_TS_DISCONT flag, ffmpeg checks for a 
discontinuity and readjusts the file offset to make output timestamps 
smooth.

Add -dts_delta_threshold 1616090389 alongside itsoffset to prevent this 
readjustment. Perhaps, both the itsoffset and ts_scale adjustments 
should be moved to after the discontinuity checks.

Regards,
Gyan