[FFmpeg-devel] [PATCH 000/279] New channel layout API

[James Almer]

On 12/12/2021 5:00 PM, Nicolas George wrote:
> Anton Khirnov (12021-12-12):
>> So what are you proposing? In my view, such higher level information
>> should live at a higher level - e.g. in the side data. You can then
>> have a filter that reads this side data and gets you the group you want.
> 
> So, what is the point of this new API if anything of value needs to be
> done with side data and yet another API?
> 
> It seems to me you are indulging in a sunken-costs fallacy: you wrote
> this API and all the code but you neglected to poll your fellow
> developers for needs that it should cover, and as a result got something
> much too limited. But now, you are trying to force it anyway.
> 
> What I propose is:
> 
> (1) define the needs properly;
> 
> (2) redesign the API;
> 
> (3) see how much code can still be used.
> 
> The needs as far as I can see, please add to the list:
> 
> A. Allow the same channel to appear several time in the layout. Hendrik
> agreed that it was useful for some kind of USER_SPECIFIED or UNKNOWN
> channel specification, but allowing for any channel specification is
> actually simpler.
> 
> It is not limited to just having the same channel in the list, it
> requires API and user interface support: the API must be able to tell
> the user "this USER_SPECIFIED channel is the oboe, this USER_SPECIFIED
> is the piano", and the user must be able to tell the API "the second
> USER_SPECIFIED channel" or "the USER_SPECIFIED channel relating to the
> piano".

To achieve this you don't need the same AVChannel value to appear 
several times in the same layout. You have INT_MAX values available, so 
just assign one to each of these you mentioned. No need for an abstract 
value "user defined" that would then show up several times in a layout. 
Oboe can be 65, piano can be 66.
Also, each channel is meant to map to a speaker in a different physical 
location. If your idea is to have oboe and piano play through the same 
speaker, then you're thinking filtering, and that sounds beyond the 
scope of a channel layout API.

The user interface part to query and tell non standard AVChannel values 
apart in a human readable way is a different thing. That would probably 
require giving each channel a user defined name in some form.

> 
> B. Possibly, I do not personally insist on it like A: groups of
> channels.
> 
> 
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