Re: [FFmpeg-devel] [PATCH 2/4] avutil/channel_layout: factorize ambisonic order detection

[James Almer <jamrial@gmail.com>]

On 3/15/2022 5:30 PM, Marton Balint wrote:
> Signed-off-by: Marton Balint <cus@passwd.hu>
> ---
>   libavutil/channel_layout.c | 40 ++++++++++++++++++++++++++------------
>   1 file changed, 28 insertions(+), 12 deletions(-)
> 
> diff --git a/libavutil/channel_layout.c b/libavutil/channel_layout.c
> index c60ccf368f..fb2335a334 100644
> --- a/libavutil/channel_layout.c
> +++ b/libavutil/channel_layout.c
> @@ -644,29 +644,29 @@ int av_channel_layout_copy(AVChannelLayout *dst, const AVChannelLayout *src)
>   }
>   
>   /**
> - * If the custom layout is n-th order standard-order ambisonic, with optional
> - * extra non-diegetic channels at the end, write its string description in bp.
> - * Return a negative error code on error.
> + * If the layout is n-th order standard-order ambisonic, with optional
> + * extra non-diegetic channels at the end, return the order.
> + * Return a negative error code otherwise.
>    */
> -static int try_describe_ambisonic(AVBPrint *bp, const AVChannelLayout *channel_layout)
> +static int ambisonic_order(const AVChannelLayout *channel_layout)
>   {
>       int i, highest_ambi, order;
>   
>       highest_ambi = -1;
> -    if (channel_layout->order == AV_CHANNEL_ORDER_AMBISONIC)
> +    if (channel_layout->order == AV_CHANNEL_ORDER_AMBISONIC) {
>           highest_ambi = channel_layout->nb_channels - av_popcount64(channel_layout->u.mask) - 1;
> -    else {
> +    } else if (channel_layout->order == AV_CHANNEL_ORDER_CUSTOM) {

It's not called for any other order, so this is not needed.

Maybe add an av_assert0() instead.

>           const AVChannelCustom *map = channel_layout->u.map;
>           for (i = 0; i < channel_layout->nb_channels; i++) {
>               int is_ambi = CHAN_IS_AMBI(map[i].id);
>   
>               /* ambisonic following non-ambisonic */
>               if (i > 0 && is_ambi && !CHAN_IS_AMBI(map[i - 1].id))
> -                return 0;
> +                return AVERROR(EINVAL);
>   
>               /* non-default ordering */
>               if (is_ambi && map[i].id - AV_CHAN_AMBISONIC_BASE != i)
> -                return 0;
> +                return AVERROR(EINVAL);
>   
>               if (CHAN_IS_AMBI(map[i].id))
>                   highest_ambi = i;
> @@ -674,17 +674,33 @@ static int try_describe_ambisonic(AVBPrint *bp, const AVChannelLayout *channel_l
>       }
>       /* no ambisonic channels*/
>       if (highest_ambi < 0)
> -        return 0;
> +        return AVERROR(EINVAL);
>   
>       order = floor(sqrt(highest_ambi));
>       /* incomplete order - some harmonics are missing */
>       if ((order + 1) * (order + 1) != highest_ambi + 1)
> +        return AVERROR(EINVAL);
> +
> +    return order;
> +}
> +
> +/**
> + * If the custom layout is n-th order standard-order ambisonic, with optional
> + * extra non-diegetic channels at the end, write its string description in bp.
> + * Return a negative error code on error.
> + */
> +static int try_describe_ambisonic(AVBPrint *bp, const AVChannelLayout *channel_layout)
> +{
> +    int nb_ambi_channels;
> +    int order = ambisonic_order(channel_layout);
> +    if (order < 0)
>           return 0;
>   
>       av_bprintf(bp, "ambisonic %d", order);
>   
>       /* extra channels present */
> -    if (highest_ambi < channel_layout->nb_channels - 1) {
> +    nb_ambi_channels = (order + 1) * (order + 1);
> +    if (nb_ambi_channels < channel_layout->nb_channels) {
>           AVChannelLayout extra = { 0 };
>           char buf[128];
>   
> @@ -696,12 +712,12 @@ static int try_describe_ambisonic(AVBPrint *bp, const AVChannelLayout *channel_l
>               const AVChannelCustom *map = channel_layout->u.map;
>   
>               extra.order       = AV_CHANNEL_ORDER_CUSTOM;
> -            extra.nb_channels = channel_layout->nb_channels - highest_ambi - 1;
> +            extra.nb_channels = channel_layout->nb_channels - nb_ambi_channels;
>               extra.u.map       = av_calloc(extra.nb_channels, sizeof(*extra.u.map));
>               if (!extra.u.map)
>                   return AVERROR(ENOMEM);
>   
> -            memcpy(extra.u.map, &map[highest_ambi + 1],
> +            memcpy(extra.u.map, &map[nb_ambi_channels],
>                      sizeof(*extra.u.map) * extra.nb_channels);
>           }

Should be ok.
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