FFmpeg封装格式处理1-简介
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FFmpeg 封装格式处理相关内容分为如下几篇文章：
[1]. FFmpeg 封装格式处理-简介
[2]. FFmpeg 封装格式处理-解复用例程
[3]. FFmpeg 封装格式处理-复用例程
[4]. FFmpeg 封装格式处理-转封装例程
这几篇文章内容联系紧密，但放在一篇文章里内容太长，遂作拆分。章节号不作调整。基于 FFmpeg 4.1 版本。
1. 概述
1.1 封装格式简介
封装格式(container format)可以看作是编码流(音频流、视频流等)数据的一层外壳，将编码后的数据存储于此封装格式的文件之内。封装又称容器，容器的称法更为形象，所谓容器，就是存放内容的器具，以一瓶饮料为例，饮料是内容，那么装饮料的瓶子就是容器。
不同封装格式适用于不同的场合，不同封装格式支持的编码格式不一样，几个常用的封装格式如下：
下表引用自“视音频编解码技术零基础学习方法”
| 封装格式(文件扩展名) | 推出机构 | 流媒体 | 支持的视频编码 | 支持的音频编码 | 目前使用领域 |
|---|---|---|---|---|---|
| AVI(.avi) | Microsoft 公司 | 不支持 | 几乎所有格式 | 几乎所有格式 | BT 下载影视 |
| Flash Video(.flv) | Adobe 公司 | 支持 | Sorenson/VP6/H.264 | MP3/ADPCM/Linear PCM/AAC 等 | 互联网视频网站 |
| MP4(.mp4) | MPEG 组织 | 支持 | MPEG-2/MPEG-4/H.264/H.263 等 | AAC/MPEG-1 Layers I,II,III/AC-3 等 | 互联网视频网站 |
| MPEGTS(.ts) | MPEG 组织 | 支持 | MPEG-1/MPEG-2/MPEG-4/H.264 | MPEG-1 Layers I,II,III/AAC | IPTV，数字电视 |
| Matroska(.mkv) | CoreCodec 公司 | 支持 | 几乎所有格式 | 几乎所有格式 | 互联网视频网站 |
| Real Video(.rmvb) | Real Networks 公司 | 支持 | RealVideo 8,9,10 | AAC/Cook Codec/RealAudio Lossless | BT 下载影视 |
1.2 FFmpeg 中的封装格式
FFmpeg 关于封装格式的处理涉及打开输入文件、打开输出文件、从输入文件读取编码帧、往输出文件写入编码帧这几个步骤，这些都不涉及编解码层。
在 FFmpeg 中，mux 指复用，是 multiplex 的缩写，表示将多路流(视频、音频、字幕等)混入一路输出中(普通文件、流等)。demux 指解复用，是 mux 的反操作，表示从一路输入中分离出多路流(视频、音频、字幕等)。mux 处理的是输出格式，demux 处理的输入格式。输入/输出媒体格式涉及文件格式和封装格式两个概念。文件格式由文件扩展名标识，主要起提示作用，通过扩展名提示文件类型(或封装格式)信息。封装格式则是存储媒体内容的实际容器格式，不同的封装格式对应不同的文件扩展名，很多时候也用文件格式代指封装格式，例如常用 ts 格式(文件格式)代指 mpegts 格式(封装格式)。
例如，我们把 test.ts 改名为 test.mkv，mkv 扩展名提示了此文件封装格式为 Matroska，但文件内容并无任何变化，使用 ffprobe 工具仍能正确探测出封装格式为 mpegts。
1.2.1 查看 FFmpeg 支持的封装格式
使用ffmpeg -formats命令可以查看 FFmpeg 支持的封装格式。FFmpeg 支持的封装格式非常多，下面仅列出最常用的几种：
think@opensuse> ffmpeg -formats
File formats:
D. = Demuxing supported
.E = Muxing supported
--
DE flv FLV (Flash Video)
D aac raw ADTS AAC (Advanced Audio Coding)
DE h264 raw H.264 video
DE hevc raw HEVC video
E mp2 MP2 (MPEG audio layer 2)
DE mp3 MP3 (MPEG audio layer 3)
E mpeg2video raw MPEG-2 video
DE mpegts MPEG-TS (MPEG-2 Transport Stream)
1.2.2 h264/aac 裸流封装格式
h264 裸流封装格式和 aac 裸流封装格式在后面的解复用和复用例程中会用到，这里先讨论一下。
FFmpeg 音视频处理中，编码格式是指音视频帧的压缩格式，编码器编码过程和解码器解码过程处理的是编码格式，复用器复用过程和解复用器解复用过程处理的则是封装格式。通常提到 h264 是指 H.264 编码格式，当用作封装格式时表示的是 H.264 裸流格式，所谓裸流就是不含封装信息的流，也就是没穿衣服的流。aac 等封装格式类似。
我们看一下 FFmpeg 工程源码中 h264 编码格式以及 h264 封装格式的定义：
h264 编码格式定义：
h264 解码器定义：(FFmpeg 工程包含 h264 解码器，而不包含 h264 编码器(一般使用第三方 libx264 编码器用作 h264 编码)，所以只有解码器定义)
AVCodec ff_h264_decoder = {
.name = "h264",
.long_name = NULL_IF_CONFIG_SMALL("H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
.type = AVMEDIA_TYPE_VIDEO,
.id = AV_CODEC_ID_H264,
......
};
h264 封装格式定义：
h264 复用器(输出封装格式)定义：
AVOutputFormat ff_h264_muxer = {
.name = "h264",
.long_name = NULL_IF_CONFIG_SMALL("raw H.264 video"),
.extensions = "h264,264",
.audio_codec = AV_CODEC_ID_NONE,
.video_codec = AV_CODEC_ID_H264,
.write_header = force_one_stream,
.write_packet = ff_raw_write_packet,
.check_bitstream = h264_check_bitstream,
.flags = AVFMT_NOTIMESTAMPS,
};
h264 解复用器(输入封装格式)定义：
FF_DEF_RAWVIDEO_DEMUXER(h264, "raw H.264 video", h264_probe, "h26l,h264,264,avc", AV_CODEC_ID_H264)
#define FF_DEF_RAWVIDEO_DEMUXER(shortname, longname, probe, ext, id)\
FF_DEF_RAWVIDEO_DEMUXER2(shortname, longname, probe, ext, id, AVFMT_GENERIC_INDEX)
#define FF_DEF_RAWVIDEO_DEMUXER2(shortname, longname, probe, ext, id, flag)\
FF_RAWVIDEO_DEMUXER_CLASS(shortname)\
AVInputFormat ff_ ## shortname ## _demuxer = {\
.name = #shortname,\
.long_name = NULL_IF_CONFIG_SMALL(longname),\
.read_probe = probe,\
.read_header = ff_raw_video_read_header,\
.read_packet = ff_raw_read_partial_packet,\
.extensions = ext,\
.flags = flag,\
.raw_codec_id = id,\
.priv_data_size = sizeof(FFRawVideoDemuxerContext),\
.priv_class = &shortname ## _demuxer_class,\
};
将上述三段代码宏展开，可得到 h264 解复用器(输入封装格式)定义如下：
AVInputFormat ff_h264_demuxer = {
.name = h264,
.long_name = "raw H.264 video",
.read_probe = h264_probe,
.read_header = ff_raw_video_read_header,
.read_packet = ff_raw_read_partial_packet,
.extensions = "h26l,h264,264,avc",
.flags = AVFMT_GENERIC_INDEX,
.raw_codec_id = AV_CODEC_ID_H264,
.priv_data_size = sizeof(FFRawVideoDemuxerContext),
.priv_class = &h264_demuxer_class,
};
1.2.3 mpegts 封装格式
再看一下 mpegts 封装格式定义，AVInputFormat 用于定义输入封装格式，AVOutputFormat 用于定义输出封装格式。mpegts 输入封装格式中并未指定文件扩展名，而 mpegts 输出封装格式中则指定了文件扩展名为"ts,m2t,m2ts,mts"。
AVInputFormat ff_mpegts_demuxer = {
.name = "mpegts",
.long_name = NULL_IF_CONFIG_SMALL("MPEG-TS (MPEG-2 Transport Stream)"),
.priv_data_size = sizeof(MpegTSContext),
.read_probe = mpegts_probe,
.read_header = mpegts_read_header,
.read_packet = mpegts_read_packet,
.read_close = mpegts_read_close,
.read_timestamp = mpegts_get_dts,
.flags = AVFMT_SHOW_IDS | AVFMT_TS_DISCONT,
.priv_class = &mpegts_class,
};
AVOutputFormat ff_mpegts_muxer = {
.name = "mpegts",
.long_name = NULL_IF_CONFIG_SMALL("MPEG-TS (MPEG-2 Transport Stream)"),
.mime_type = "video/MP2T",
.extensions = "ts,m2t,m2ts,mts",
.priv_data_size = sizeof(MpegTSWrite),
.audio_codec = AV_CODEC_ID_MP2,
.video_codec = AV_CODEC_ID_MPEG2VIDEO,
.init = mpegts_init,
.write_packet = mpegts_write_packet,
.write_trailer = mpegts_write_end,
.deinit = mpegts_deinit,
.check_bitstream = mpegts_check_bitstream,
.flags = AVFMT_ALLOW_FLUSH | AVFMT_VARIABLE_FPS | AVFMT_NODIMENSIONS,
.priv_class = &mpegts_muxer_class,
};
1.2.4 文件扩展名与封装格式
在 FFmpeg 命令行中，输入文件扩展名是错的也没有关系，因为 FFmpeg 会读取一小段文件来探测出真正的封装格式；但是如果未显式的指定输出封装格式，就只能通过输出文件扩展名来确定封装格式，就必须确保扩展名是正确的。
做几个实验，来研究一下 FFmpeg 中文件扩展名与封装格式的关系：
测试文件下载(右键另存为)：tnhaoxc.flv
文件信息如下：
think@opensuse> ffprobe tnhaoxc.flv
ffprobe version 4.1 Copyright (c) 2007-2018 the FFmpeg developers
Input #0, flv, from 'tnhaoxc.flv':
Metadata:
encoder : Lavf58.20.100
Duration: 00:02:13.68, start: 0.000000, bitrate: 838 kb/s
Stream #0:0: Video: h264 (High), yuv420p(progressive), 784x480, 25 fps, 25 tbr, 1k tbn, 50 tbc
Stream #0:1: Audio: aac (LC), 44100 Hz, stereo, fltp
实验 1：将 flv 封装格式转换为 mpegts 封装格式
使用转封装指令将 flv 封装格式转换为 mpegts 封装格式，依次运行如下两条命令：
ffmpeg -i tnhaoxc.flv -map 0 -c copy tnhaoxc.ts
ffmpeg -i tnhaoxc.flv -map 0 -c copy tnhaoxc.m2t
生成 tnhaoxc.ts 和 tnhaoxc.m2t 文件，比较一下两文件有无不同：
diff tnhaoxc.ts tnhaoxc.m2t
命令行无输出，表示两文件内容相同。即两文件仅是扩展名不同，封装格式都是 mpegts，文件内容并无任何不同。
实验 2：为输出文件指定错误的扩展名
指定一个错误的扩展名再试一下(误把封装格式名称当作文件扩展名)：
ffmpeg -i tnhaoxc.flv -map 0 -c copy tnhaoxc.mpegts
命令行输出如下错误信息：
ffmpeg version 4.1 Copyright (c) 2000-2018 the FFmpeg developers
Input #0, flv, from 'tnhaoxc.flv':
Metadata:
encoder : Lavf58.20.100
Duration: 00:02:13.68, start: 0.000000, bitrate: 838 kb/s
Stream #0:0: Video: h264 (High), yuv420p(progressive), 784x480, 25 fps, 25 tbr, 1k tbn, 50 tbc
Stream #0:1: Audio: aac (LC), 44100 Hz, stereo, fltp
[NULL @ 0x1d62e80] Unable to find a suitable output format for 'tnhaoxc.mpegts'
tnhaoxc.mpegts: Invalid argument
提示无法确定输出格式。FFmpeg 无法根据此扩展名确定输出文件的封装格式。
实验 3：为输出文件指定错误的扩展名但显式指定封装格式
通过-f mpegts选项显式指定封装格式为 mpegts：
ffmpeg -i tnhaoxc.flv -map 0 -c copy -f mpegts tnhaoxc.mpegts
命令执行成功，看一下文件内容是否正确：
diff tnhaoxc.mpegts tnhaoxc.ts
发现 tnhaoxc.mpegts 和 tnhaoxc.ts 文件内容完全一样，虽然 tnhaoxc.mpegts 有错误的文件扩展名，仍然得到了我们期望的封装格式。
不知道什么命令可以查到封装格式对应的扩展名。可以在 FFmpeg 工程源码中搜索封装格式名称，如搜索“mpegts”，可以看到其扩展名为“ts,m2t,m2ts,mts”。
2. API 介绍
最主要的 API 有如下几个。FFmpeg 中将编码帧及未编码帧均称作 frame，本文为方便，将编码帧称作 packet，未编码帧称作 frame。
2.1 avformat_open_input()
/**
* Open an input stream and read the header. The codecs are not opened.
* The stream must be closed with avformat_close_input().
*
* @param ps Pointer to user-supplied AVFormatContext (allocated by avformat_alloc_context).
* May be a pointer to NULL, in which case an AVFormatContext is allocated by this
* function and written into ps.
* Note that a user-supplied AVFormatContext will be freed on failure.
* @param url URL of the stream to open.
* @param fmt If non-NULL, this parameter forces a specific input format.
* Otherwise the format is autodetected.
* @param options A dictionary filled with AVFormatContext and demuxer-private options.
* On return this parameter will be destroyed and replaced with a dict containing
* options that were not found. May be NULL.
*
* @return 0 on success, a negative AVERROR on failure.
*
* @note If you want to use custom IO, preallocate the format context and set its pb field.
*/
int avformat_open_input(AVFormatContext **ps, const char *url, AVInputFormat *fmt, AVDictionary **options);
这个函数会打开输入媒体文件，读取文件头，将文件格式信息存储在第一个参数 AVFormatContext 中。
2.2 avformat_find_stream_info()
/**
* Read packets of a media file to get stream information. This
* is useful for file formats with no headers such as MPEG. This
* function also computes the real framerate in case of MPEG-2 repeat
* frame mode.
* The logical file position is not changed by this function;
* examined packets may be buffered for later processing.
*
* @param ic media file handle
* @param options If non-NULL, an ic.nb_streams long array of pointers to
* dictionaries, where i-th member contains options for
* codec corresponding to i-th stream.
* On return each dictionary will be filled with options that were not found.
* @return >=0 if OK, AVERROR_xxx on error
*
* @note this function isn't guaranteed to open all the codecs, so
* options being non-empty at return is a perfectly normal behavior.
*
* @todo Let the user decide somehow what information is needed so that
* we do not waste time getting stuff the user does not need.
*/
int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options);
这个函数会读取一段视频文件数据并尝试解码，将取到的流信息填入 AVFormatContext.streams 中。AVFormatContext.streams 是一个指针数组，数组大小是 AVFormatContext.nb_streams。
2.3 av_read_frame()
/**
* Return the next frame of a stream.
* This function returns what is stored in the file, and does not validate
* that what is there are valid frames for the decoder. It will split what is
* stored in the file into frames and return one for each call. It will not
* omit invalid data between valid frames so as to give the decoder the maximum
* information possible for decoding.
*
* If pkt->buf is NULL, then the packet is valid until the next
* av_read_frame() or until avformat_close_input(). Otherwise the packet
* is valid indefinitely. In both cases the packet must be freed with
* av_packet_unref when it is no longer needed. For video, the packet contains
* exactly one frame. For audio, it contains an integer number of frames if each
* frame has a known fixed size (e.g. PCM or ADPCM data). If the audio frames
* have a variable size (e.g. MPEG audio), then it contains one frame.
*
* pkt->pts, pkt->dts and pkt->duration are always set to correct
* values in AVStream.time_base units (and guessed if the format cannot
* provide them). pkt->pts can be AV_NOPTS_VALUE if the video format
* has B-frames, so it is better to rely on pkt->dts if you do not
* decompress the payload.
*
* @return 0 if OK, < 0 on error or end of file
*/
int av_read_frame(AVFormatContext *s, AVPacket *pkt);
本函数用于解复用过程。
本函数将存储在输入文件中的数据分割为多个 packet，每次调用将得到一个 packet。packet 可能是视频帧、音频帧或其他数据，解码器只会解码视频帧或音频帧，非音视频数据并不会被扔掉、从而能向解码器提供尽可能多的信息。
对于视频来说，一个 packet 只包含一个视频帧；对于音频来说，若是帧长固定的格式则一个 packet 可包含整数个音频帧，若是帧长可变的格式则一个 packet 只包含一个音频帧。
读取到的 packet 每次使用完之后应调用av_packet_unref(AVPacket *pkt)清空 packet。否则会造成内存泄露。
2.4 av_write_frame()
/**
* Write a packet to an output media file.
*
* This function passes the packet directly to the muxer, without any buffering
* or reordering. The caller is responsible for correctly interleaving the
* packets if the format requires it. Callers that want libavformat to handle
* the interleaving should call av_interleaved_write_frame() instead of this
* function.
*
* @param s media file handle
* @param pkt The packet containing the data to be written. Note that unlike
* av_interleaved_write_frame(), this function does not take
* ownership of the packet passed to it (though some muxers may make
* an internal reference to the input packet).
* <br>
* This parameter can be NULL (at any time, not just at the end), in
* order to immediately flush data buffered within the muxer, for
* muxers that buffer up data internally before writing it to the
* output.
* <br>
* Packet's @ref AVPacket.stream_index "stream_index" field must be
* set to the index of the corresponding stream in @ref
* AVFormatContext.streams "s->streams".
* <br>
* The timestamps (@ref AVPacket.pts "pts", @ref AVPacket.dts "dts")
* must be set to correct values in the stream's timebase (unless the
* output format is flagged with the AVFMT_NOTIMESTAMPS flag, then
* they can be set to AV_NOPTS_VALUE).
* The dts for subsequent packets passed to this function must be strictly
* increasing when compared in their respective timebases (unless the
* output format is flagged with the AVFMT_TS_NONSTRICT, then they
* merely have to be nondecreasing). @ref AVPacket.duration
* "duration") should also be set if known.
* @return < 0 on error, = 0 if OK, 1 if flushed and there is no more data to flush
*
* @see av_interleaved_write_frame()
*/
int av_write_frame(AVFormatContext *s, AVPacket *pkt);
本函数用于复用过程，将 packet 写入输出媒体。
packet 交织是指：音频 packet 和视频 packet 在输出媒体文件中应严格按照 packet 中 dts 递增的顺序交错存放。
本函数直接将 packet 写入复用器(muxer)，不会缓存或记录任何 packet。本函数不负责不同流的 packet 交织问题。由调用者负责。
如果调用者不愿处理 packet 交织问题，应调用 av_interleaved_write_frame()替代本函数。
2.5 av_interleaved_write_frame()
/**
* Write a packet to an output media file ensuring correct interleaving.
*
* This function will buffer the packets internally as needed to make sure the
* packets in the output file are properly interleaved in the order of
* increasing dts. Callers doing their own interleaving should call
* av_write_frame() instead of this function.
*
* Using this function instead of av_write_frame() can give muxers advance
* knowledge of future packets, improving e.g. the behaviour of the mp4
* muxer for VFR content in fragmenting mode.
*
* @param s media file handle
* @param pkt The packet containing the data to be written.
* <br>
* If the packet is reference-counted, this function will take
* ownership of this reference and unreference it later when it sees
* fit.
* The caller must not access the data through this reference after
* this function returns. If the packet is not reference-counted,
* libavformat will make a copy.
* <br>
* This parameter can be NULL (at any time, not just at the end), to
* flush the interleaving queues.
* <br>
* Packet's @ref AVPacket.stream_index "stream_index" field must be
* set to the index of the corresponding stream in @ref
* AVFormatContext.streams "s->streams".
* <br>
* The timestamps (@ref AVPacket.pts "pts", @ref AVPacket.dts "dts")
* must be set to correct values in the stream's timebase (unless the
* output format is flagged with the AVFMT_NOTIMESTAMPS flag, then
* they can be set to AV_NOPTS_VALUE).
* The dts for subsequent packets in one stream must be strictly
* increasing (unless the output format is flagged with the
* AVFMT_TS_NONSTRICT, then they merely have to be nondecreasing).
* @ref AVPacket.duration "duration") should also be set if known.
*
* @return 0 on success, a negative AVERROR on error. Libavformat will always
* take care of freeing the packet, even if this function fails.
*
* @see av_write_frame(), AVFormatContext.max_interleave_delta
*/
int av_interleaved_write_frame(AVFormatContext *s, AVPacket *pkt);
本函数用于复用过程，将 packet 写入输出媒体。
本函数将按需在内部缓存 packet，从而确保输出媒体中不同流的 packet 能按照 dts 增长的顺序正确交织。
2.6 avio_open()
/**
* Create and initialize a AVIOContext for accessing the
* resource indicated by url.
* @note When the resource indicated by url has been opened in
* read+write mode, the AVIOContext can be used only for writing.
*
* @param s Used to return the pointer to the created AVIOContext.
* In case of failure the pointed to value is set to NULL.
* @param url resource to access
* @param flags flags which control how the resource indicated by url
* is to be opened
* @return >= 0 in case of success, a negative value corresponding to an
* AVERROR code in case of failure
*/
int avio_open(AVIOContext **s, const char *url, int flags);
创建并初始化一个 AVIOContext，用于访问输出媒体文件。
2.7 avformat_write_header()
/**
* Allocate the stream private data and write the stream header to
* an output media file.
*
* @param s Media file handle, must be allocated with avformat_alloc_context().
* Its oformat field must be set to the desired output format;
* Its pb field must be set to an already opened AVIOContext.
* @param options An AVDictionary filled with AVFormatContext and muxer-private options.
* On return this parameter will be destroyed and replaced with a dict containing
* options that were not found. May be NULL.
*
* @return AVSTREAM_INIT_IN_WRITE_HEADER on success if the codec had not already been fully initialized in avformat_init,
* AVSTREAM_INIT_IN_INIT_OUTPUT on success if the codec had already been fully initialized in avformat_init,
* negative AVERROR on failure.
*
* @see av_opt_find, av_dict_set, avio_open, av_oformat_next, avformat_init_output.
*/
av_warn_unused_result
int avformat_write_header(AVFormatContext *s, AVDictionary **options);
向输出文件写入文件头信息。
2.8 av_write_trailer()
/**
* Write the stream trailer to an output media file and free the
* file private data.
*
* May only be called after a successful call to avformat_write_header.
*
* @param s media file handle
* @return 0 if OK, AVERROR_xxx on error
*/
int av_write_trailer(AVFormatContext *s);
向输出文件写入文件尾信息。
6. 参考资料
[1] WIKI，Digital_container_format
[2] WIKI，Comparison_of_container_formats
[3] 雷霄骅，使用 FFMPEG 类库分离出多媒体文件中的 H.264 码流，https://blog.csdn.net/leixiaohua1020/article/details/11800877
[4] 雷霄骅，最简单的基于 FFmpeg 的封装格式处理：视音频分离器简化版，https://blog.csdn.net/leixiaohua1020/article/details/39767055
7. 修改记录
2019-03-08 V1.0 解复用例程初稿
2019-03-09 V1.0 拆分笔记
2019-03-10 V1.0 增加复用例程和转封装例程
