Re: [FFmpeg-devel] [PATCH 4/6] doc/muxers: add mkvtimestamp_v2

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Stefano Sabatini:
> On date Tuesday 2024-04-16 12:50:19 +0200, Andreas Rheinhardt wrote:
>> Stefano Sabatini:
>>> ---
>>>  doc/muxers.texi | 8 ++++++++
>>>  1 file changed, 8 insertions(+)
>>>
>>> diff --git a/doc/muxers.texi b/doc/muxers.texi
>>> index f94513527d..490d5557bf 100644
>>> --- a/doc/muxers.texi
>>> +++ b/doc/muxers.texi
>>> @@ -2933,6 +2933,14 @@ MicroDVD subtitle format muxer.
>>>  
>>>  This muxer accepts a single @samp{microdvd} subtitles stream.
>>>  
>>> +@section mkvtimestamp_v2
>>> +mkvtoolnix v2 timecode format muxer.
>>> +
>>> +Write the PTS rawvideo frame to the output, as supported by the
>>> +@command{mkvextact} tool from the @command{mkvtoolnix} suite.
>>> +
>>> +This muxer accepts a single @samp{rawvideo} stream.
>>> +
>>>  @section mp3
>>>  
>>>  The MP3 muxer writes a raw MP3 stream with the following optional features:
>>
> 
>> This is wrong: MKVToolNix switched to "# timestamp format v2" a long
>> time ago (we still write the old "# timecode format v2" header);
>> furthermore, MKVToolNix actually uses pts (which it reorders to be
>> ascending), not dts like our muxer. Furthermore MKVToolNix does not
>> force a 1ms precision on timestamps.
> 
> Correct.
> 
> I compared the output of the muxer and of mkvtoolnix extract
> timestamp_v2 and I'm not yet clear about the timestamp differences I'm
> observing (the muxer output maps with the timestamps, the mkvtoolnix
> timestamps differ by a few ms). But I think also mkvtoolnix use a 1ms
> timebase.

The accuracy of the timestamps output by mkvextract is determined by the
TimestampScale of the file in question; it is most often 1ms when the
file has video.
You need to provide more details if you want these discrepancies to be
analyzed.

> 
> Also, IIRC there is no generic way to reorder PTSs, so this might
> account for another difference which might be difficult to implement
> generically.

Write them into a buffer and reorder them at the end?
(No, I have no intention to actually implement this. I am rather leaning
to "this muxer should not exist".)

- Andreas

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