Re: [FFmpeg-devel] [PATCH] Set native order for wav channel layouts up until 8 channels.

[James Almer <jamrial@gmail.com>]

On 2/23/2024 7:32 PM, Romain Beauxis wrote:
> Le ven. 23 févr. 2024 à 15:11, Marton Balint <cus@passwd.hu> a écrit :
>>
>>
>>
>> On Fri, 23 Feb 2024, Romain Beauxis wrote:
>>
>>> The new default channel layout for the various RIFF/WAV decoders is not
>>> backward compatible.
>>>
>>> Historically, most decoders will expect the channel layouts to follow
>>> the native layout up-to a reasonable number of channels.
>>>
>>> Additionally, non-native layouts are causing troubles with filters
>>> chaining.
>>>
>>> This PR changes the default channel layout reported by RIFF/WAV decoders
>>> to default to the native layout when the number of channels is up-to 8.
>>>
>>> The logic for these changes is the same as the logic for the vorbis/opus
>>> decoders.
>>
>> For Vorbis the channel layout is in the actual Vorbis specification. So
>> you should follow the specification, simple guessing in the demuxer likely
>> won't be acceptable.
> 
> I would argue that even though there is no official specification on
> channel layout for wav/riff, the de-facto assumption that _most_ users
> of the library would expect is the native layout.
> 
> Typically, 1 and 2 channels would be assumed to be mono and stereo by
> most users.
> 
> It's great that the API does provide flexibility but the default
> should be set to satisfy most users and, in that regard, it seems that
> assuming a native layout is what the vast majority of library's users
> will expect.
> 
> This choice also implies at least two ABI breakage for applications
> using the deprecated API but running on a the new ABI:
> 
> 1: With the updated API, the library is not able to provide a backward
> compatible `channel_layout` for AVFrame when the new channel order is
> AV_CHANNEL_ORDER_UNSPEC which breaks ABI compatibility as the field is
> reported as `0`.
> 
> 2. AV_CHANNEL_ORDER_UNSPEC channel order also breaks filters chaining.
> The issue appears to be that the unspec channel order implicitly sets
> the the filter to accept all channel order, which breaks compatibility
> here:
> 
>          if (link->incfg.channel_layouts->all_layouts) {
>              av_log(link->src, AV_LOG_ERROR, "Cannot select channel layout for"
>                     " the link between filters %s and %s.\n", link->src->name,
>                     link->dst->name);
>              if (!link->incfg.channel_layouts->all_counts)
>                  av_log(link->src, AV_LOG_ERROR, "Unknown channel layouts not "
>                         "supported, try specifying a channel layout using "
>                         "'aformat=channel_layouts=something'.\n");
>              return AVERROR(EINVAL);
>          }

It's not the demuxer's job to guess what the caller would like or 
expect. It's meant to export what the input contains.
If you want to fix the CLI behavior, then you need to do it in the CLI. 
Remember that ffmpeg.c is not the only lavf user.
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