On 3/4/2020 7:51 PM, Paul B Mahol wrote:
> On 3/4/20, James Almer <jamr...@gmail.com> wrote:
>> On 3/4/2020 7:26 PM, Paul B Mahol wrote:
>>> Signed-off-by: Paul B Mahol <one...@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().
>
>>
>>> + 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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