Re: [FFmpeg-devel] [PATCH v2] ogg/vorbis: implement header packet skip in chained ogg bitstreams.

[Romain Beauxis <romain.beauxis@gmail.com>]

Le mar. 12 août 2025 à 14:49, Romain Beauxis
<romain.beauxis@gmail.com> a écrit :
>
> Hi!
>
> Le mar. 12 août 2025 à 11:33, Yalda <marth64@proxyid.net> a écrit :
> >
> > Romain Beauxis:
> >
> > Thank you, Romain, for the clarity.
> >
> > I have some follow up questions just to solidify my understanding.
> > I think this is a good match since this sounds like a segment joining
> > problem which is ironically what I have been doing in principle
> > with my other contributions.
>
> Nice!
>
> > 1) Can core stream parameters (channels, sample rate) change mid-flight?
>
> In theory, yes. The two streams do not have to have anything in common.
>
> In practice, most of the situations where this happens are because the encoder wants to insert an in-band metadata so it's pretty reasonable to assume that encoding parameters are unlikely to change between streams, at least as a first approach.
>
> > 2) Why are the header packets emitted to begin with?
> > Are they necessary for the audible bitstream or preamble metadata?
> > Alternatively, a link to external reading is fine by me!
>
> You got the link I see :-)
>
> In ogg, there's usually at least 2 to 3 packets:
> 1. "hello" packet to detect the logical stream content. All first packets of all multiplexed streams are placed inside an initial page.
> 2. One metadata packet
> 3. Optionally: one or more codec specific packets
>
> (Similarly to considering theora as deprecated, I would also ignore the multiplexing aspect of the problem, at least in a first approach. Ogg streams with audio/video content are also pretty rare these days.)
>
> The codec specific packets can contain data required for the decoder.
>
> In practice, it seems that in ffmpeg, with ogg/flac and ogg/opus, the decoders are pretty happy continuing their decoding without having to process any new header packet.
>
> For opus, there does not seem to be any codec-specific header: https://wiki.xiph.org/OggOpus
>
> For flac, the spec says one or more metadata packets and no codec-specific packet: https://xiph.org/flac/ogg_mapping.html
>
> For those two codecs, the current libavcodec decoders are pretty happy without those mid-stream headers.
>
> With vorbis, the stream has one metadata packet and one codec specific packet that seems required to continue decoding.
>
> Thus, the current libavcodec vorbis decoder has to receive and process mid-stream headers, which is why suppressing those from the demuxer output was a trickier task and why this current patch is a hold-out.
>
> > 3) Is it possible that doing the stream copy is a front-line goal in actuality?
> > In other words, by solving 1/2/3, we are actually wanting to solve 4?
> > (If this thought makes sense)
>
> The most pressing user-facing features are: supporting in-band metadata and copy streams.
>
> In-band metadata is just a few commits behind the current pending one. I was looking at them yesterday, they are really super simple.
>
> These changes are blocked by the completion of the proper handling of header packets since metadata are passed through them.
>
> Supporting copy streams is more tricky as it will require fixing DTS and handling new ogg headers when generating the output streams.
>
> I do have most of this sketched out in my local FFmpeg repo.
>
> > 4) Is there a sample command to spawn such a source stream, or is setting
> > up Icecast with defaults enough and play segments to simulate the conditions?

Sorry I'm realizing you meant a live stream here.

You can use liquidsoap, which should be easily installable via the
binary packages here:
https://github.com/savonet/liquidsoap/releases/tag/v2.3.3

Or using `opam`:
https://www.liquidsoap.info/doc-2.3.3/install.html#install-using-opam
(make sure to install the vorbis package and also ffmpeg for decoding!)

A simple script could be:
```shell
% cat icecast-stream.liq
s = playlist("/path/to/directory")

output.icecast(
  fallible=true,
  host="...", # defaults to localhost
  port=..., # default to 8000
  mount="...", # mandatory
  password="...", # defaults to hackme
  %ffmpeg(%codec("libmp3lame")),
  s
)

% liquidsoap ./icecast-stream.liq
```

Alternatively you could pick any of the ogg/{opus, flac, vorbis}
stream in the xiph directory: https://dir.xiph.org/codecs/Vorbis

Thanks,
-- Romain
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