FFmpeg封装格式处理

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,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,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

名称(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

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

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

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,这些都不涉及编码解码层面。

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处理的是输入格式,demux处理的输出格式。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。

h264本来是编码格式,当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也流,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: 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封装格式定义如下:

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,
};
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,
};

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: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,在SHELL中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:不同流的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

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