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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