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

[Paul B Mahol]

On 3/5/20, Andreas Rheinhardt <andreas.rheinhardt at gmail.com> wrote:
> Am 05.03.2020 um 00:05 schrieb James Almer:
>> On 3/4/2020 7:51 PM, Paul B Mahol wrote:
>>> 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.
>>
>> So does AC3, judging by how its parser assembles frames.
>>
>> An AVParser will let you resync after a bad seek, read frames in non
>> seekable input like a pipe, read frames within badly muxed files,
>> simplify the demuxer, etc, and is a matter of just looking for that
>> 16bit sync code and assembling a frame. Essentially just re-implementing
>> what you already did in ac4_probe().
>>
>
> If it is intended that the actual AVPackets should not contain the
> sync code and the size field at all, then this should be dropped in
> the demuxer here (but then the demuxer actually needs to check for
> sync codes and needs to handle resyncing itself). This will also mean
> that memcpy of the data can be avoided; which is not so when relying
> on a parser as the packet size is not chosen adaptively for the content.
>
> The decoder seems to allow both variants: With header and without
> header. Is there a packetization that strips this header away? (And
> what is actually contained in the last two bytes in case the sync code
> is 0xAC41?)

CRC word.

>
> - Andreas
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