[FFmpeg-devel] [PATCH 1/5] avutil/channel_layout: add 22.2 layout

[James Almer]

On 8/1/2020 9:17 AM, Jan Ekström wrote:
> On Sat, Aug 1, 2020 at 3:08 PM Carl Eugen Hoyos <ceffmpeg at gmail.com> wrote:
>>
>> Am Sa., 1. Aug. 2020 um 14:06 Uhr schrieb Jan Ekström <jeebjp at gmail.com>:
>>
>>> Additionally, the spec only mentions stereo/5.1 for direct mapping, so
>>> I've kept to 5.1 since we have the capability for that one.
>>
>> And you still believe it would be a disadvantage if the decoder outputs
>> this 5.1 mapping by default if nothing else was requested?
>>
> 
> Yes. API users so far have received the audio frames according to
> encoded layout by default, so doing something else breaks that rule of
> least surprises.
> 
>> Is there an option to request the 5.1 mapping?
>>
> 
> As noted in the 0/0 cover letter, you can do it just fine with
> `-channel_layout "5.1"` with ffmpeg.c. I have verified that it returns
> the 5.1 channels as-is.

I assume that does a "conversion" using swr from 22.2 to 5.1? (even if
it just outputs the first six channels without extra resampling). What
Carl wants is the decoder outputting only the first six channels,
without the need for external resampling. This should be achievable with
the -request_channel_layout input option (Try it with the DTS decoder on
DTS-MA 7.1 samples to get 5.1, or in some DTS core samples to get
stereo). No external conversion takes place in this scenario, since the
decoder takes the hint to only output the requested channels, assuming
it's doable (In the DTS case, the stream may not have a defined way
within the bitstream to achieve this, and so the decoder just outputs
the original channel layout silently).

This would of course be separate work, but IMO very welcome and worth a
look, and not a blocker. In any case, the decoder should by default
output what the stream reports it contains, and not some arbitrary
layout we think a CLI or player user will prefer. The user can and is
expected to use a resampling library for that.

> 
> At this point I would not like to attempt to figure out how to skip
> channel coding units in the AAC decoder, as swresample seems to handle
> this nicely. So far the only usage for requested channel layout in
> aacdec has been to skip reordering with AV_CH_LAYOUT_NATIVE , to
> output the coded units as-is.
> 
> Best regards,
> Jan
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