[FFmpeg-devel] [PATCH] avcodec/mlpdec: Add decoding of object audio data

[James Almer]

On 3/23/2025 4:33 PM, Massimo Eynard wrote:
> On 23/03/2025 20:01, James Almer wrote:
>> On 3/22/2025 2:49 PM, Massimo Eynard wrote:
>>> This patch adds support for decoding the fourth MLP substream
>>> which contains the 16-channel presentation used for Atmos
>>> audio objects.
>>>
>>> By default only the first three substreams are decoded
>>> unless the new extract_objects flag is enabled as the resulting
>>> presentation contains audio object feeds instead of classic
>>> loudspeaker feeds.
>>>
>>> As this introduces interpolation of primitive matrices, precision
>>> has been increased to 2.18 fixed point. Therefore this requires
>>> DSP code upgrade which has been done for C and x86 implementations
>>> but not the ARM implementation.
>>>
>>> Adds two FATE tests using existing atmos.thd sample to reflect
>>> changes.
>>>
>>> Signed-off-by: Massimo Eynard <eynard.massimo at gmail.com>
>>> ---
>>>    libavcodec/arm/mlpdsp_armv5te.S  |   2 +-
>>>    libavcodec/arm/mlpdsp_init_arm.c |   3 +-
>>>    libavcodec/mlp.h                 |  10 +-
>>>    libavcodec/mlp_parse.c           |  31 ++-
>>>    libavcodec/mlp_parse.h           |   1 +
>>>    libavcodec/mlp_parser.c          |  11 +-
>>>    libavcodec/mlpdec.c              | 389 +++++++++++++++++++++++++++----
>>>    libavcodec/mlpdsp.c              |  50 +++-
>>>    libavcodec/mlpdsp.h              |  25 ++
>>>    libavcodec/x86/mlpdsp.asm        |  19 +-
>>>    tests/fate/truehd.mak            |  10 +
>>>    11 files changed, 476 insertions(+), 75 deletions(-)
>>
>> With atmos.thd i get:
>>
>>> [aist#0:0/truehd @ 00000209caf3ee00] Guessed Channel Layout: 7.1.4
>>> Input #0, truehd, from '../samples/truehd/atmos.thd':
>>>    Duration: N/A, start: 0.000000, bitrate: N/A
>>>    Stream #0:0: Audio: truehd (Dolby TrueHD + Dolby Atmos), 48000 Hz, 7.1.4, s32 (24 bit)
>>
>> Which is unlikely to be correct. The file has 11 (or 12) objects, which is exported as 12 channels in an unspecified layout, and automatically assumed to be a 7.1.4 fixed layout.
>>
> 
> This is caused by `guess_input_channel_layout` (in `ffmpeg_demux.c`) which tries to assume a layout.
> Would using `AV_CHANNEL_ORDER_CUSTOM` with all channels set to `AV_CHAN_UNKNOWN` (for unknown position, except LFE if present) be a better solution?

Possibly, but it may make the stream undecodable unless you remap the 
channels (probably with a filter in the filterchain).

Is there no better representation for the output? What are these 12 
channels the sample exports? 16 channels (as you say the MLP substream 
contains) would match Ambisonics 3rd order, but i assume that doesn't 
apply here, unless you should also be outputting something else.

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