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

[Anton Khirnov <anton@khirnov.net>]

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.

> If you feel strongly about this, please refer this to the TC.

The TC is supposed to be invoked as a last resort.

-- 
Anton Khirnov
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