Re: [FFmpeg-devel] [PATCH] lavc: export flag for MPEG audio dual channel

[Anton Khirnov <anton@khirnov.net>]

Quoting Scott Theisen (2022-09-22 03:04:16)
> On 9/21/22 15:51, James Almer wrote:
> > On 9/21/2022 4:44 PM, Rémi Denis-Courmont wrote:
> >> Le keskiviikkona 21. syyskuuta 2022, 22.26.11 EEST Scott Theisen a 
> >> écrit :
> >>> diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
> >>> index 7db5d1b1c5..bcf3a845a8 100644
> >>> --- a/libavcodec/avcodec.h
> >>> +++ b/libavcodec/avcodec.h
> >>> @@ -2076,6 +2076,17 @@ typedef struct AVCodecContext {
> >>>        *             The decoder can then override during decoding 
> >>> as needed.
> >>> */
> >>>       AVChannelLayout ch_layout;
> >>> +
> >>> +    /**
> >>> +     * Audio only.  This flag is set when MPEG audio mode dual 
> >>> channel has
> >>> been detected. +     * This signals that the audio is two 
> >>> independent mono
> >>> channels. +     *
> >>> +     * 0 normally, 1 if dual channel flag is set.
> >>> +     *
> >>> +     * - encoding: currently unused (functionally equivalent to 
> >>> stereo,
> >>> patch welcome) +     * - decoding: set by lavc
> >>> +     */
> >>> +    int mpeg_audio_mode_dual_channel;
> >>>   } AVCodecContext;
> >>
> >> I agree that the dual mono flag should be exposed to the application 
> >> somehow,
> >> but isn't this a slient ABI break?
> >
> > It's not a break, but it's overkill for what's essentially a flag.
> 
> This is how MythTV customized FFmpeg (in 2006) for this and this way was 
> probably the easiest.  It /is/ a flag, so maybe adding an int as a 
> bitset instead of as a bool so other flags could be added if necessary?
> 
> >
> > The proper way to do this would be to signal such a layout as an 
> > actual channel layout, using a custom order one where both channels 
> > are set as Front Center. But i don't know if until the old channel 
> > layout API fields are gone we should have decoders setting something 
> > only new API users will understand. Old API field users would look at 
> > channels and see a 2, and channel_layout and see it's empty, but then 
> > old and new API values would technically conflict, so I'd like to hear 
> > some opinions.
> 
> I'm not very familiar with either channel layout API, but the audio is 
> encoded identically to stereo other than the flag, so could we add 
> another entry, e.g. AV_CHANNEL_ORDER_MONO, to the `enum AVChannelOrder` 
> to signify that each channel is independent, but is otherwise identical 
> to AV_CHANNEL_ORDER_NATIVE?

The whole point is that it's NOT identical to AV_CHANNEL_ORDER_NATIVE.
ORDER_CUSTOM with two FC channels is exactly the right way to handle
this IMO.

-- 
Anton Khirnov
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