[FFmpeg-devel] [PATCH v2 0/3] Properly decode ogg metadata in ogg/flac chained bitstreams

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

This is a series of 3 patches to allow proper decoding of ogg metadata
in chained ogg/flac streams.

ogg/flac streams are pretty important because there are perhaps the only
combination of lossless audio codec and open-source container that
allows for proper transmittion of lossless audio data accross systems
such as Icecast, browser media tags and more.

In the context of long-running audio streams, the ogg bitstream specs[1]
have historically been very badly implemented. For each new track and
each new metadata, the specs require the logical bitstream to come to a
full EOF and then start with a full new logical stream.

These specs have often been confused with a gobal EOF by most
implementations.

Furtunately, FFmpeg is a little better at that in that it is capable to
parsing chained logical ogg bitstreams and properly output either
encoded ogg packets or decoded audio.

Chained bitstreams with more than one underlying type of content
(audio+video, etc) is not yet supported though this is a much less needed
feature.

The purpose of these changes is to also allow proper decoding of
metadata associated with subsequent streams in chained ogg/flac
bitstream.

This is done by simply intercepting ogg packets with comments in the
flac decoded, parsing the comment block and retaining it to be attached
with the next decoded audio frame.

Along with the changes is a new FATE test validating the implementation.

This solution keeps a proper separation of concerns: ogg packets are
sill output by the demuxer (as shown in the test) but consumer of decoded
data see decoded metadata in the decoded frames.

Only drawback is that this adds a dependency on libavformat to
libavcodec.

I have looked at moving the vorbis metadata parsing to libavutil but a
lot of definitions and utilities related to metadata are in fact located
in libavformat so perhaps this makes sense.

Follow-up work not addressed in this series of patch:
* Ensure valid PTS in decoded frames of subsequent streams?
* Generalize this approach to other chained ogg codec.

Thanks,
-- Romain

Romain Beauxis (3):
  libavformat/oggdec: Allow first parameter in ff_vorbis_comment to be a
    generic AVClass struct
  libavcodec/flacdec: parse vorbis metadata from ogg packets, add them
    to the next decoded frame.
  Add stream dump test with test for ogg/flac.

 libavcodec/flacdec.c                  |  12 +-
 libavformat/oggdec.h                  |   5 +-
 libavformat/oggparsevorbis.c          |   4 +-
 tests/Makefile                        |   2 +
 tests/api/Makefile                    |   2 +-
 tests/api/api-dump-stream-meta-test.c | 169 ++++++++++++++++++++++++++
 tests/fate/api.mak                    |   5 +
 tests/fate/ogg-flac.mak               |  11 ++
 8 files changed, 205 insertions(+), 5 deletions(-)
 create mode 100644 tests/api/api-dump-stream-meta-test.c
 create mode 100644 tests/fate/ogg-flac.mak

-- 
2.39.5 (Apple Git-154)

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