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

[Andreas Rheinhardt]

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.

> 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).)
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.

> 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()).

- Andreas