Re: [FFmpeg-devel] [PATCH 1/3] avutil/channel_layout: rename 7.1(top) channel layout to 5.1.2

[Hendrik Leppkes <h.leppkes@gmail.com>]

On Tue, Oct 24, 2023 at 1:24 AM James Almer <jamrial@gmail.com> wrote:
>
> This layout maps to ITU-R BS.2051-3 "Sound System C" and ITU-R BS.1196-8 "Channel
> Configuration 14", and it being the first layout with top layer channels, it's
> best to use a different scheme to properly convey the presence and amount of said
> channels.
> The new name will also be a better fit for the additions in the following commits.
>
> Signed-off-by: James Almer <jamrial@gmail.com>
> ---
> diff --git a/libavutil/channel_layout.h b/libavutil/channel_layout.h
> index ac2ddfa022..8c40c81b44 100644
> --- a/libavutil/channel_layout.h
> +++ b/libavutil/channel_layout.h
> @@ -232,13 +232,15 @@ enum AVChannelOrder {
>  #define AV_CH_LAYOUT_7POINT1           (AV_CH_LAYOUT_5POINT1|AV_CH_BACK_LEFT|AV_CH_BACK_RIGHT)
>  #define AV_CH_LAYOUT_7POINT1_WIDE      (AV_CH_LAYOUT_5POINT1|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER)
>  #define AV_CH_LAYOUT_7POINT1_WIDE_BACK (AV_CH_LAYOUT_5POINT1_BACK|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER)
> -#define AV_CH_LAYOUT_7POINT1_TOP_BACK  (AV_CH_LAYOUT_5POINT1_BACK|AV_CH_TOP_FRONT_LEFT|AV_CH_TOP_FRONT_RIGHT)
> +#define AV_CH_LAYOUT_5POINT1POINT2     (AV_CH_LAYOUT_5POINT1_BACK|AV_CH_TOP_FRONT_LEFT|AV_CH_TOP_FRONT_RIGHT)

Looking at the specs and the naming of the thing, I don't think it
should be using AV_CH_LAYOUT_5POINT1_BACK as the base layer, but
rather AV_CH_LAYOUT_5POINT1. "5.1" is that, and 5.1.2 should extend
that, and not swap the base layer speakers.
Looking at the specifications, they don't map properly to our speaker
definitions, as they assign it a generic "surround" title (at 100-120
degrees azimuth), while a rear/back speaker should be at 135 degrees,
and a side speaker at 90 (as is the standard for 7.1 setups).

Yes, this doesn't match the one you are renaming, but I think
consistency within our own defined formats is important here. 5.1 ->
5.1.2 should just add two.

The renamed one could be 5.1.2(back) or whatever we want to name those.

Or we do it properly and actually include a proper "surround" speaker
definition and get rid of the confusion between side and back. But
that breaks compat with WAVEFORMATEXTENSIBLE that our channel
definitions are mostly sourced from, so might not be worth it.

- Hendrik


- Hendrik
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