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

[Lynne]

Mar 5, 2020, 11:57 by onemda at gmail.com:

> On 3/5/20, Paul B Mahol <onemda at gmail.com> wrote:
>
>> On 3/5/20, Andreas Rheinhardt <andreas.rheinhardt at gmail.com> wrote:
>>
>>> 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.
>>>>
>>> You seem to intend for there to be two forms of AC-4 packets (at least
>>> your decoder tries to support both): The one with syncwords, size
>>> field and CRC stripped away and another one where these elements are
>>> still present. This immediately brings a few questions:
>>>
>>> 1. Can a stripped packet be mistaken for an unstripped one, i.e. is it
>>> possible for the data after the header to begin with a syncword?
>>> 2. How reversible is stripping away the header and the CRC? Stripping
>>> away the CRC is obviously irreversible unless we presume that the CRC
>>> matches. But is stripping the size field away reversible (it is not
>>> reversible if one is allowed to use the three byte size field if the
>>> two byte size field would suffice)?
>>> 3. Are there any container that use the stripped version?
>>>
>>> If the answer to the third question is no, then I don't think we
>>> should create a new packetization of AC-4 and potentially incur the
>>> first problem as well as the inability to retain CRCs for codec copy.
>>>
>>
>> Decoder should support both, but I do not remember of stripped files.
>>
>
> Actually mp4 dash files are stripped one.
>

I think the raw demuxer should strip the CRCs, that's a container-level thing IMO.
The decoder should only support one version of packets.