[FFmpeg-devel] [PATCH v2 4/4] avformat/movenc: handle EAC-3 extension bits for Atmos

[Baptiste Coudurier]

> On Jun 5, 2025, at 5:14 PM, James Almer <jamrial at gmail.com> wrote:
> 
> From: nyanmisaka <nst799610810 at gmail.com>
> 
> Fixes commit #9996.
> 
> Signed-off-by: James Almer <jamrial at gmail.com>
> ---
> libavformat/movenc.c | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/libavformat/movenc.c b/libavformat/movenc.c
> index 7bcdc2463d..cd5b45f6fe 100644
> --- a/libavformat/movenc.c
> +++ b/libavformat/movenc.c
> @@ -393,6 +393,8 @@ struct eac3_info {
>         uint16_t chan_loc;
>         /* if there is no dependent substream, then one bit reserved instead */
>     } substream[1]; /* TODO: support 8 independent substreams */
> +    /* indicates the decoding complexity, 8 bits */
> +    uint8_t complexity_index_type_a;
> };
> 
> static int mov_write_ac3_tag(AVFormatContext *s, AVIOContext *pb, MOVTrack *track)
> @@ -474,6 +476,8 @@ static int handle_eac3(MOVMuxContext *mov, AVPacket *pkt, MOVTrack *track)
>     info->data_rate = FFMAX(info->data_rate, hdr->bit_rate / 1000);
>     info->ac3_bit_rate_code = FFMAX(info->ac3_bit_rate_code,
>                                     hdr->ac3_bit_rate_code);
> +    info->complexity_index_type_a = hdr->complexity_index_type_a;
> +
>     num_blocks = hdr->num_blocks;
> 
>     if (!info->ec3_done) {
> @@ -601,7 +605,7 @@ static int mov_write_eac3_tag(AVFormatContext *s, AVIOContext *pb, MOVTrack *tra
>     }
> 
>     info = track->eac3_priv;
> -    size = 2 + ((34 * (info->num_ind_sub + 1) + 7) >> 3);
> +    size = 2 + (4 * (info->num_ind_sub + 1)) + (2 * !!info->complexity_index_type_a);
>     buf = av_malloc(size);
>     if (!buf) {
>         return AVERROR(ENOMEM);
> @@ -626,6 +630,11 @@ static int mov_write_eac3_tag(AVFormatContext *s, AVIOContext *pb, MOVTrack *tra
>             put_bits(&pbc, 9, info->substream[i].chan_loc);
>         }
>     }
> +    if (info->complexity_index_type_a) {
> +        put_bits(&pbc, 7, 0); /* reserved */
> +        put_bits(&pbc, 1, 1); // flag_eac3_extension_type_a
> +        put_bits(&pbc, 8, info->complexity_index_type_a);
> +    }
>     flush_put_bits(&pbc);
>     size = put_bytes_output(&pbc);

Looks good, confirmed that Quicktime recognizes Atmos after the changes

— 
Baptiste