[FFmpeg-devel] [PATCH 2/3] avformat: add raw AC-4 demuxer
Paul B Mahol
onemda at gmail.com
Thu Mar 5 13:53:00 EET 2020
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.
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