[FFmpeg-devel] [PATCH v4 4/8] avformat/mux: add proper support for full N:M bitstream filtering

[Marton Balint]

On Tue, 5 May 2020, Andreas Rheinhardt wrote:

> Marton Balint:
>> 
>> 
>> On Tue, 5 May 2020, Andreas Rheinhardt wrote:
>> 
>>> Marton Balint:
>>>> Previously only 1:1 bitstream filters were supported, the end of the
>>>> stream was
>>>> not signalled to the bitstream filters and time base changes were
>>>> ignored.
>>>>
>>>> This change also allows muxers to set up bitstream filters regardless
>>>> of the
>>>> autobsf flag during write_header instead of during check_bitstream
>>>> and those
>>>> bitstream filters will always be executed.
>>>
>>> Ignoring the autobsf flag is not ok IMO. The muxer should not add a bsf
>>> when the user explicitly didn't want this.
>>>
>> 
>> The user should not be allowed to create broken files, and the only way
>> to achieve that is to force the BSF.
>
> No, one could also check in the muxer that the packets are ok (these
> checks could be disabled if the bsf is enabled, of course) and error out
> if it is not.

And that would duplicate some functionaltiy for negigable performance 
gain of not always doing a BSF.

>
>> I don't see a use case for
>> disabling BSF either for MXFenc of GXFenc. (in fact, from the top of my
>> head I can't see a use case for disabling automatically inserted BSF-s
>> in other muxers).
>> 
> A user who already knows that his packets are ok can do so to avoid the
> (admittedly small) overhead of these bsfs. (And even if you think that
> the option should be removed/should be made non-public (for internal
> testing only), then this does not change that this option exists and
> removing it would require the usual deprecation period (and consensus
> from other devs, of course; I personally don't have formed an opinion
> about this yet).)

With my patch the autobsf option is still honored for all existing 
bitstream filters which are inserted mid-stream (in check_bitstream). Only 
the ones which are inserted at write_header(), so in mxfenc and gxfenc are 
forced.

> One use-case is if a user runs the pcm_rechunk bsf filter manually and
> then sends the output to both a framecrc as well as the mxf muxers in
> order to be able to test more thoroughly whether any write errors
> happened. Or where you want to write the same audio to multiple mxf
> muxers, all with the same audio settings.

And for them the additional BSF which is auto inserted will only pass on 
their packets, it will only *check* for correct packets, exactly what you 
suggested. I cannot accept the performance gain as argument.

>
>> So if you feel strongly about this and we still want to avoid creating
>> corrupt files then the BSFs should be processed in mxf_interleave and
>> gxf_interleave separately from the auto inserted bitstream filters. Do
>> you think that is better? I don't think so.
>> 
> That would just be a worse form of automatically inserted bsf (and it
> would not work for av_write_frame()).

Please suggest something reasonable to follow up on this.

Thanks,
Marton