[FFmpeg-devel] [PATCH 1/2] avcodec/s302m: enable non-PCM decoding

[Gyan Doshi]

On 2024-02-15 09:40 pm, Anton Khirnov wrote:
> Quoting Gyan Doshi (2024-02-15 13:31:59)
>> On 2024-02-15 04:17 pm, Anton Khirnov wrote:
>>> Hi,
>>> sorry for the delay, I've been busy fixing things for the release
>>> Quoting Gyan Doshi via ffmpeg-devel (2024-01-29 05:00:33)
>>>> On 2024-01-28 04:24 pm, Anton Khirnov wrote:
>>>>>> a) it would mean essentially inlining this decoder in the demuxer.
>>>>> Why is that a problem? This decoder seems like it shouldn't be a
>>>>> decoder.
>>>>>
>>>>> I agree with Andreas that this seems like it's a demuxer pretending to
>>>>> be a decoder.
>>>> This module transforms the entire raw payload data to generate its
>>>> output, even if the syntax is simple which
>>>> essentially makes it a de-coder. The de-multiplexer aspect of multiple
>>>> streams is an academic possibility allowed
>>>> by the standard but not seen in any sample which makes me suspect it's
>>>> used for carriage between broadcast
>>>> facilities rather than something ever sent to an OTT provider, let alone
>>>> an end user.
>>> If it dynamically generates nested decoders, then it's not a proper
>>> codec in our model. It should be either a part of the demuxer, or a
>>> bitstream filter (possibly inserted automatically by the demuxer).
>> s302m is a hybrid creature and does not slot cleanly into any role. So
>> there is no theoretically proper place for this component - any choice
>> is a least-out-of-place accommodation.
>>
>> But it is much more out of place inside a demuxer. Analyzing packet
>> payload and then manipulating that entire payload is much closer to a
>> decoding role than data chunk extraction for packetization.
> I don't see why specifically this property should be the one
> distinguishing demuxers from decoders, it sounds pretty arbitrary to me.
> Many demuxers apply transformations of far higher complexity to the
> bytestream before exporting it, e.g. in matroska the packet data may be
> compressed, laced, etc.
>
>> And the stream extracted from the container is meant to be SMPTE ST
>> 302 not PCM* or Dolby-E/AC-3..etc,
> "meant to be"? By whom?
>
> The point of libavformat is to abstract away the differences between
> containers as much as is reasonably feasible, and export the data in the
> format most useful to the caller for decoding or other processing.
>
>> which will both misrepresent what the container carries
> Why should the caller care?
>
>> and possibly discard S-ADM metadata, if present, in the packet.
> Why could that not be exported as side data?
>
>> A bsf in principle would work but in practice, can't as Andreas
>> clarified that bsfs can't set or alter codec_id after init. And
>> resetting the codec id requires packet inspection.
> There are two possibilities then - either extend the BSF API to support
> multiple output streams, or implement it inside libavformat as a
> post-demuxer hook called in the same place as parsing.
>
>> Nested decoders are used without issue in components like imm5 or ftr
>> (upto 64 nested decoders!) among others. There's no breaking of new
>> ground here.
> Nested decoders are certainly not "without issue" - they are a constant
> source of issues, since implementing nesting properly is very tricky. I
> am fairly sure most nested decoders we have are subtly broken in various
> ways.

This patch facilitates a certain productive use of ffmpeg with respect 
to processing of live inputs that wasn't possible earlier,
and which currently is being used successfully by multiple people over 
the past few weeks.
It applies a processing model already implemented in multiple other 
decoders for a number of years. I haven't seen many reports
of issues with them. And surely something being 'a constant source of 
issues' would be a lot more than 'subtly broken' as you describe 
them.You're the only one who has objected on architectural grounds and 
this looks to be a fundamental disagreement.
If you are blocking this patch, do acknowledge here within 24 hours and 
we can send this to the TC else I'll push it after that period.

Regards,
Gyan