Re: [FFmpeg-devel] [PATCH 4/5] avutil/channel_layout: add av_channel_layout_retype()

[James Almer <jamrial@gmail.com>]

On 1/29/2024 8:27 PM, Marton Balint wrote:
> Signed-off-by: Marton Balint <cus@passwd.hu>
> ---
>   doc/APIchanges             |  3 +++
>   libavutil/channel_layout.c | 48 ++++++++++++++++++++++++++++++++++++++
>   libavutil/channel_layout.h | 11 +++++++++
>   libavutil/version.h        |  2 +-
>   4 files changed, 63 insertions(+), 1 deletion(-)
> 
> diff --git a/doc/APIchanges b/doc/APIchanges
> index 8e8498f803..ce1e816fa5 100644
> --- a/doc/APIchanges
> +++ b/doc/APIchanges
> @@ -2,6 +2,9 @@ The last version increases of all libraries were on 2023-02-09
>   
>   API changes, most recent first:
>   
> +2024-02-xx - xxxxxxxxxx - lavu 58.38.100 - channel_layout.h
> +  Add av_channel_layout_retype().
> +
>   2024-02-xx - xxxxxxxxxx - lavu 58.37.100 - channel_layout.h
>     Add av_channel_layout_from_custom().
>   
> diff --git a/libavutil/channel_layout.c b/libavutil/channel_layout.c
> index 0810d32bf6..cc8c208080 100644
> --- a/libavutil/channel_layout.c
> +++ b/libavutil/channel_layout.c
> @@ -1036,3 +1036,51 @@ uint64_t av_channel_layout_subset(const AVChannelLayout *channel_layout,
>   
>       return ret;
>   }
> +
> +int av_channel_layout_retype(AVChannelLayout *channel_layout, enum AVChannelOrder order)
> +{
> +    if (!av_channel_layout_check(channel_layout))
> +        return AVERROR(EINVAL);
> +
> +    if (channel_layout->order == order)
> +        return 0;
> +
> +    switch (order) {
> +    case AV_CHANNEL_ORDER_UNSPEC: {
> +        int nb_channels = channel_layout->nb_channels;
> +        av_channel_layout_uninit(channel_layout);
> +        channel_layout->order       = AV_CHANNEL_ORDER_UNSPEC;
> +        channel_layout->nb_channels = nb_channels;
> +        return 0;
> +        }
> +    case AV_CHANNEL_ORDER_CUSTOM: {
> +        AVChannelLayout custom = { 0 };
> +        int ret = av_channel_layout_from_custom(&custom, channel_layout->nb_channels);
> +        if (ret < 0)
> +            return ret;
> +        if (channel_layout->order != AV_CHANNEL_ORDER_UNSPEC)
> +            for (int i = 0; i < channel_layout->nb_channels; i++)
> +                custom.u.map[i].id = av_channel_layout_channel_from_index(channel_layout, i);
> +        av_channel_layout_uninit(channel_layout);
> +        *channel_layout = custom;
> +        return 0;
> +        }
> +    case AV_CHANNEL_ORDER_NATIVE:
> +        if (channel_layout->order == AV_CHANNEL_ORDER_CUSTOM) {
> +            uint64_t mask = 0;
> +            for (int i = 0; i < channel_layout->nb_channels; i++) {
> +                enum AVChannel ch = channel_layout->u.map[i].id;
> +                if (ch >= 0 && ch < 63 && mask < (1ULL << ch))
> +                    mask |= (1ULL << ch);
> +                else
> +                    return 1;
> +            }
> +            av_channel_layout_uninit(channel_layout);
> +            return av_channel_layout_from_mask(channel_layout, mask);
> +        } else {
> +            return 1;
> +        }
> +    default:
> +        return 1;
> +    }
> +}

I wrote a function like this some time ago, but i lost the patch by 
accident during a migration.

The way i approached it was making the return codes reflect if the 
conversion was lossless or not, as in, custom -> native is lossless only 
if channels have no custom names (and possible only if the ids are 
within UINT64_MAX and in order). Anything to Unspec is always lossy, etc.

Custom -> Ambi: Possible only if it contains ambi channels. Lossless.
Custom -> Native: Possible only if has no ambi channels and all ids < 64 
and in order. Lossy if it has custom names, otherwise lossless.
Ambi -> Custom: Lossless.
Ambi -> Native: Not possible.
Native -> Custom: Lossless.
Native -> Ambi: Not possible.
Any -> Unspec: Possible but lossy.

So 0 for lossless, 1 for lossy, ENOSYS for not possible.
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