On Tue, 18 Feb 2020, Anton Khirnov wrote:
Quoting Marton Balint (2020-02-05 19:55:24)
On Tue, 7 Jan 2020, Anton Khirnov wrote:
> Quoting Nicolas George (2019-12-31 16:17:49)
>> Anton Khirnov (12019-12-29):
>> > I do not agree. Duplicated channels in a layout are expected to be a
>> > fringe thing and how you handle them highly depends on the specific use
>> > case. I expect a typical caller will want to disregard that possibility
>> > and just take the first channel of each semantics.
>> > So I do not believe a dedicated function for this makes sense. We could
>> > always add something later though, if it turns out to be necessary.
>>
>> I think you are making a mistake. I think that as soon as it will be
>> technically possible, we will see cases with duplicated channels. And I
>> know that some filters will do exactly that as soon as they are ported
>> to this new API.
Quicktime also allows duplicated channels in a single audio track, this is
unfortunately a commonly used feature. So if a new API is introduced to
overcome the limitations of the existing one, supporting this should be
seriously considered.
Can you provide a link to more information about this? I'd like to know
the specifics about how it's handled by quicktime. And do note that this
API does have some ammount of support for duplicated channels (though I
still consider streams containing them to be broken).
The 'chan' atom can hold this information:
https://developer.apple.com/library/archive/documentation/QuickTime/QTFF/QTFFChap3/qtff3.html#//apple_ref/doc/uid/TP40000939-CH205-SW53
It holds an AudioChannelLayout structure as defined in CoreAudioTypes.h:
https://developer.apple.com/documentation/coreaudiotypes/audiochannellayout
https://github.com/phracker/MacOSX-SDKs/blob/master/MacOSX10.15.sdk/System/Library/Frameworks/CoreAudioTypes.framework/Versions/A/Headers/CoreAudioBaseTypes.h#L1353
Which can hold an AudioChannelDescription array of channel descriptions
for each channel:
https://developer.apple.com/documentation/coreaudiotypes/audiochannellayout
https://github.com/phracker/MacOSX-SDKs/blob/master/MacOSX10.15.sdk/System/Library/Frameworks/CoreAudioTypes.framework/Versions/A/Headers/CoreAudioBaseTypes.h#L1335
Which can signal the same AudioChannelLabel for every channel in a track
if needed.
There is some support for it in libavformat/mov_chan.c.
Regards,
Marton
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