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

[Baptiste Coudurier]

> On Jun 5, 2025, at 2:49 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 | 31 +++++++++++++++----------------
> 1 file changed, 15 insertions(+), 16 deletions(-)
> 
> diff --git a/libavformat/movenc.c b/libavformat/movenc.c
> index 25360b004d..b546c8b545 100644
> --- a/libavformat/movenc.c
> +++ b/libavformat/movenc.c
> @@ -393,6 +393,10 @@ 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 enhanced AC-3 extension, 1 bit */
> +    uint8_t eac3_extension_type_a;
> +    /* indicates the decoding complexity, 8 bits */
> +    uint8_t complexity_index_type_a;
> };

I think we only need complexity_index_type_a, if it’s 0 it’s not necessary

> 
> static int mov_write_ac3_tag(AVFormatContext *s, AVIOContext *pb, MOVTrack *track)
> @@ -474,6 +478,9 @@ 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->eac3_extension_type_a   = hdr->eac3_extension_type_a;
> +    info->complexity_index_type_a = hdr->complexity_index_type_a;
> +
>     num_blocks = hdr->num_blocks;
> 
>     if (!info->ec3_done) {
> @@ -532,8 +539,6 @@ static int handle_eac3(MOVMuxContext *mov, AVPacket *pkt, MOVTrack *track)
>             int parent = hdr->substreamid;
> 
>             while (cumul_size != pkt->size) {
> -                GetBitContext gbc;
> -                int i;
>                 ret = avpriv_ac3_parse_header(&hdr, pkt->data + cumul_size, pkt->size - cumul_size);
>                 if (ret < 0)
>                     goto end;
> @@ -544,20 +549,9 @@ static int handle_eac3(MOVMuxContext *mov, AVPacket *pkt, MOVTrack *track)
>                 info->substream[parent].num_dep_sub++;
>                 ret /= 8;
> 
> -                /* header is parsed up to lfeon, but custom channel map may be needed */
> -                init_get_bits8(&gbc, pkt->data + cumul_size + ret, pkt->size - cumul_size - ret);
> -                /* skip bsid */
> -                skip_bits(&gbc, 5);
> -                /* skip volume control params */
> -                for (i = 0; i < (hdr->channel_mode ? 1 : 2); i++) {
> -                    skip_bits(&gbc, 5); // skip dialog normalization
> -                    if (get_bits1(&gbc)) {
> -                        skip_bits(&gbc, 8); // skip compression gain word
> -                    }
> -                }
>                 /* get the dependent stream channel map, if exists */
> -                if (get_bits1(&gbc))
> -                    info->substream[parent].chan_loc |= (get_bits(&gbc, 16) >> 5) & 0x1f;
> +                if (hdr->channel_map_present)
> +                    info->substream[parent].chan_loc |= (hdr->channel_map >> 5) & 0x1f;

Beautiful cleanup

>                 else
>                     info->substream[parent].chan_loc |= hdr->channel_mode;
>                 cumul_size += hdr->frame_size;
> @@ -614,7 +608,7 @@ 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 + ((32 * (info->num_ind_sub + 1) + 7 + 1 + 8) >> 3);

Let’s just make it optional on complexity_type_a

— 
Baptiste