[FFmpeg-devel] [PATCH 2/3] avformat: add raw AC-4 demuxer
Paul B Mahol
onemda at gmail.com
Thu Mar 5 12:28:59 EET 2020
On 3/5/20, James Almer <jamrial at gmail.com> wrote:
> 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().
Disagree, it is over complication for zero gain.
>
>>
>>>
>>>> + 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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>
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