[FFmpeg-devel] [PATCH 2/3] avformat: add raw AC-4 demuxer

[Paul B Mahol]

On 3/4/20, James Almer <jamrial at gmail.com> wrote:
> On 3/4/2020 7:26 PM, Paul B Mahol wrote:
>> Signed-off-by: Paul B Mahol <onemda at gmail.com>
>> ---
>>  libavformat/Makefile     |   1 +
>>  libavformat/ac4dec.c     | 104 +++++++++++++++++++++++++++++++++++++++
>>  libavformat/allformats.c |   1 +
>>  3 files changed, 106 insertions(+)
>>  create mode 100644 libavformat/ac4dec.c
>>
>> diff --git a/libavformat/Makefile b/libavformat/Makefile
>> index e0681058a2..b4e8d20e65 100644
>> --- a/libavformat/Makefile
>> +++ b/libavformat/Makefile
>> @@ -70,6 +70,7 @@ OBJS-$(CONFIG_AA_DEMUXER)                += aadec.o
>>  OBJS-$(CONFIG_AAC_DEMUXER)               += aacdec.o apetag.o img2.o
>> rawdec.o
>>  OBJS-$(CONFIG_AC3_DEMUXER)               += ac3dec.o rawdec.o
>>  OBJS-$(CONFIG_AC3_MUXER)                 += rawenc.o
>> +OBJS-$(CONFIG_AC4_DEMUXER)               += ac4dec.o
>>  OBJS-$(CONFIG_ACM_DEMUXER)               += acm.o rawdec.o
>>  OBJS-$(CONFIG_ACT_DEMUXER)               += act.o
>>  OBJS-$(CONFIG_ADF_DEMUXER)               += bintext.o sauce.o
>> diff --git a/libavformat/ac4dec.c b/libavformat/ac4dec.c
>> new file mode 100644
>> index 0000000000..8c6e539409
>> --- /dev/null
>> +++ b/libavformat/ac4dec.c
>> @@ -0,0 +1,104 @@
>> +/*
>> + * RAW AC-4 demuxer
>> + * Copyright (c) 2019 Paul B Mahol
>> + *
>> + * This file is part of FFmpeg.
>> + *
>> + * FFmpeg is free software; you can redistribute it and/or
>> + * modify it under the terms of the GNU Lesser General Public
>> + * License as published by the Free Software Foundation; either
>> + * version 2.1 of the License, or (at your option) any later version.
>> + *
>> + * FFmpeg is distributed in the hope that it will be useful,
>> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
>> + * Lesser General Public License for more details.
>> + *
>> + * You should have received a copy of the GNU Lesser General Public
>> + * License along with FFmpeg; if not, write to the Free Software
>> + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
>> 02110-1301 USA
>> + */
>> +
>> +#include "libavutil/avassert.h"
>> +#include "libavutil/crc.h"
>> +#include "avformat.h"
>> +#include "rawdec.h"
>> +
>> +static int ac4_probe(const AVProbeData *p)
>> +{
>> +    const uint8_t *buf = p->buf;
>> +    int left = p->buf_size;
>> +    int max_frames = 0;
>> +
>> +    while (left > 7) {
>> +        int size;
>> +
>> +        if (buf[0] == 0xAC &&
>> +            (buf[1] == 0x40 ||
>> +             buf[1] == 0x41)) {
>> +            size = (buf[2] << 8) | buf[3];
>> +            if (size == 0xFFFF)
>> +                size = 3 + (buf[4] << 16) | (buf[5] << 8) | buf[6];
>> +            size += 4;
>> +            if (buf[1] == 0x41)
>> +                size += 2;
>> +            max_frames++;
>> +            left -= size;
>> +            buf += size;
>> +        } else {
>> +            break;
>> +        }
>> +    }
>> +
>> +    return FFMIN(AVPROBE_SCORE_MAX, max_frames * 7);
>> +}
>> +
>> +static int ac4_read_header(AVFormatContext *s)
>> +{
>> +    AVStream *st;
>> +
>> +    st = avformat_new_stream(s, NULL);
>> +    if (!st)
>> +        return AVERROR(ENOMEM);
>> +
>> +    st->codecpar->codec_type = AVMEDIA_TYPE_AUDIO;
>> +    st->codecpar->codec_id   = AV_CODEC_ID_AC4;
>> +
>> +    return 0;
>> +}
>> +
>> +static int ac4_read_packet(AVFormatContext *s, AVPacket *pkt)
>> +{
>> +    AVIOContext *pb = s->pb;
>> +    int64_t pos;
>> +    uint16_t sync;
>> +    int ret, size;
>> +
>> +    if (avio_feof(s->pb))
>> +        return AVERROR_EOF;
>> +
>> +    pos   = avio_tell(s->pb);
>> +    sync = avio_rb16(pb);
>
> If there are sync codes then it sounds like the proper thing to do is,
> much like with AC3, writing a trivial parser to assemble frames and then
> use ff_raw_audio_read_header() and ff_raw_read_partial_packet() here
> instead of custom functions.

That is over complication for simple parsing like here.
Every raw packet have exact frame size set in bitstream.

>
>> +    size = avio_rb16(pb);
>> +    if (size == 0xffff)
>> +        size = avio_rb24(pb);
>> +
>> +    ret = av_get_packet(pb, pkt, size);
>> +    pkt->pos = pos;
>> +    pkt->stream_index = 0;
>> +
>> +    if (sync == 0xAC41)
>> +        avio_skip(pb, 2);
>> +
>> +    return ret;
>> +}
>> +
>> +AVInputFormat ff_ac4_demuxer = {
>> +    .name           = "ac4",
>> +    .long_name      = NULL_IF_CONFIG_SMALL("raw AC-4"),
>> +    .read_probe     = ac4_probe,
>> +    .read_header    = ac4_read_header,
>> +    .read_packet    = ac4_read_packet,
>> +    .flags          = AVFMT_GENERIC_INDEX,
>> +    .extensions     = "ac4",
>> +};
>> diff --git a/libavformat/allformats.c b/libavformat/allformats.c
>> index 0209bf0e30..d2afcb2bdd 100644
>> --- a/libavformat/allformats.c
>> +++ b/libavformat/allformats.c
>> @@ -33,6 +33,7 @@ extern AVInputFormat  ff_aa_demuxer;
>>  extern AVInputFormat  ff_aac_demuxer;
>>  extern AVInputFormat  ff_ac3_demuxer;
>>  extern AVOutputFormat ff_ac3_muxer;
>> +extern AVInputFormat  ff_ac4_demuxer;
>>  extern AVInputFormat  ff_acm_demuxer;
>>  extern AVInputFormat  ff_act_demuxer;
>>  extern AVInputFormat  ff_adf_demuxer;
>>
>
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