PS封装流使用ffmpeg转码使用java语言

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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。

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