^-^
音视频开发_H264 和 AAC 合成 FLV
最近更新：2024-09-23   |   字数总计：2.3k   |   阅读估时：12分钟   |   阅读量：次
  1. 流程
  2. FFmpeg Mux主要步骤
  3. FFmpeg Mux源码分析
    1. avformat_write_header
    2. avformat_alloc_output_context2
    3. AVOutputFormat结构体
    4. avformat_new_stream
    5. av_interleaved_write_frame
    6. av_compare_ts

流程

  flowchart TB
      A[start] --> B["avformat_alloc_output_context2"]
      B --> C["fmt->video_codec = AV_CODEC_ID_H264"]
      B --> D["fmt->audio_codec = AV_CODEC_ID_AAC"]
      C --> E[add_stream]
      D --> E
      E --> F[open_audio]
      E --> G[open_video]
      F --> H[avio_open]
      G --> H
      H --> I[avformat_write_header]
      I --> J[write_audio_frame]
      I --> K[write_video_frame]
      J --> L[av_write_trailler]
      K --> L
      L --> M["close stream(oc, &video_st)"]
      L --> N["close stream(oc, &audio_st)"]
      M --> O[avio_closep]
      N --> O
      O --> P[avformat_free_context]

FFmpeg Mux主要步骤

  1. avformat_write_header
  2. av_write_frame/av_interleaved_write_frame
  3. av_write_trailer

FFmpeg Mux源码分析

avformat_write_header

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int avformat_write_header(AVFormatContext *s, AVDictionary **options)
{
    int ret = 0;
    int already_initialized = s->internal->initialized;
    int streams_already_initialized = s->internal->streams_initialized;
    if (!already_initialized)
    if ((ret = avformat_init_output(s, options)) < 0)
        return ret;
    if (!(s->oformat->flags & AVFMT_NOFILE) && s->pb)
        avio_write_marker(s->pb, AV_NOPTS_VALUE, AVIO_DATA_MARKER_HEADER);
    if (s->oformat->write_header) {
        ret = s->oformat->write_header(s); // 最终在该例中会调用到ff_flv_muxuer.write_header
        if (ret >= 0 && s->pb && s->pb->error < 0)
            ret = s->pb->error;
        if (ret < 0)
            goto fail;
        flush_if_needed(s);
    }
    if (!(s->oformat->flags & AVFMT_NOFILE) && s->pb)
        avio_write_marker(s->pb, AV_NOPTS_VALUE, AVIO_DATA_MARKER_UNKNOWN);
    if (!s->internal->streams_initialized) {
        if ((ret = init_pts(s)) < 0)
            goto fail;
    }
    return streams_already_initialized;
    fail:
        if (s->oformat->deinit)
            s->oformat->deinit(s);
        return ret;
}

avformat_alloc_output_context2

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int avformat_alloc_output_context2(AVFormatContext **ctx, ff_const59 AVOutputFormat *oformat, const char *format_name, const char *filename);
  • ctx:需要创建的context，返回NULL表示失败。
  • oformat:指定对应的AVOutputFormat，如果不指定，可以通过后⾯format_name、filename两个参数进⾏指定，让ffmpeg⾃⼰推断。
  • format_name: 指定⾳视频的格式，⽐如“flv”，“mpeg”等，如果设置为NULL，则由filename进⾏指定，让ffmpeg⾃⼰推断。
  • filename: 指定⾳视频⽂件的路径，如果oformat、format_name为NULL，则ffmpeg内部根据filename后缀名选择合适的复⽤器，⽐如xxx.flv则使⽤flv复⽤器。
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int avformat_alloc_output_context2(AVFormatContext **avctx, ff_const59 AVOutputFormat *oformat, const char *format, const char *filename)
{
    AVFormatContext *s = avformat_alloc_context();
    int ret = 0;
    *avctx = NULL;
    if (!s)
        goto nomem;
    if (!oformat) { // oformat为NULL
        if (format) {
            oformat = av_guess_format(format, NULL, NULL); //根据提供的格式进⾏查找 format
            if (!oformat) {
                av_log(s, AV_LOG_ERROR, "Requested output format '%s' is not a suitable output format\n", format);
                ret = AVERROR(EINVAL);
                goto error;
            }
        } else { // oformat和format都为NULL
            oformat = av_guess_format(NULL, filename, NULL); // 根据⽂件名后缀进⾏查找
            if (!oformat) {
                ret = AVERROR(EINVAL);
                av_log(s, AV_LOG_ERROR, "Unable to find a suitable output format for '%s'\n", filename);
                goto error;
            }
        }   
    }
    s->oformat = oformat;
    if (s->oformat->priv_data_size > 0) {
        s->priv_data = av_mallocz(s->oformat->priv_data_size);
        if (!s->priv_data)
            goto nomem;
        if (s->oformat->priv_class) {
            *(const AVClass**)s->priv_data= s->oformat->priv_class;
            av_opt_set_defaults(s->priv_data);
        }
    } else
        s->priv_data = NULL;
    if (filename) {
        #if FF_API_FORMAT_FILENAME
            FF_DISABLE_DEPRECATION_WARNINGS
            av_strlcpy(s->filename, filename, sizeof(s->filename));
            FF_ENABLE_DEPRECATION_WARNINGS
        #endif
        if (!(s->url = av_strdup(filename)))
            goto nomem;
    }
    *avctx = s;
    return 0;
    nomem:
        av_log(s, AV_LOG_ERROR, "Out of memory\n");
        ret = AVERROR(ENOMEM);
    error:
        avformat_free_context(s);
        return ret;
}
  • 重点
    1. avformat_alloc_context申请分配上下文
    1. av_guess_format通过filename和short_name来和所有的编码器进行比较，找出最接近的编码器然后返回
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ff_const59 AVOutputFormat *av_guess_format(const char *short_name, const char *filename, const char *mime_type)
{
    const AVOutputFormat *fmt = NULL;
    AVOutputFormat *fmt_found = NULL;
    void *i = 0;
    int score_max, score;
    /* specific test for image sequences */
    #if CONFIG_IMAGE2_MUXER
        if (!short_name && filename &&
            av_filename_number_test(filename) &&
            ff_guess_image2_codec(filename) != AV_CODEC_ID_NONE) {
                return av_guess_format("image2", NULL, NULL);
        }
    #endif
    /* Find the proper file type. */
    score_max = 0;
    while ((fmt = av_muxer_iterate(&i))) {
        score = 0;
        if (fmt->name && short_name && av_match_name(short_name, fmt->name)) // fmt->name⽐如ff_flv_muxer的为"flv"
            score += 100; // 匹配了name 最⾼规格
        if (fmt->mime_type && mime_type && !strcmp(fmt->mime_type, mime_type)) // ff_flv_muxer的为 "video/x-flv"
            score += 10; // 匹配mime_type
        if (filename && fmt->extensions && av_match_ext(filename, fmt->extensions)) { //ff_flv_muxer的为"flv"
            score += 5; // 匹配
        }
        if (score > score_max) {
            score_max = score; // 更新最匹配的分值
            fmt_found = (AVOutputFormat*)fmt;
        }
    }
    return fmt_found;
}

AVOutputFormat结构体

  • AVOutpufFormat表示输出⽂件容器格式，AVOutputFormat 结构主要包含的信息有：封装名称描述，编码格式信息(video/audio 默认编码格式，⽀持的编码格式列表)，⼀些对封装的操作函数(write_header,write_packet,write_tailer等)。
  • ffmpeg⽀持各种各样的输出⽂件格式，MP4，FLV，3GP等等。⽽ AVOutputFormat 结构体则保存了这些格式的信息和⼀些常规设置。
  • 每⼀种封装对应⼀个 AVOutputFormat 结构，ffmpeg将AVOutputFormat 按照链表存储；
    flowchart LR
  A["AVOutputFormat1{mp4(movenc.c)}"] --> B["AVOutputFormat2{flv(flvenc.c)}"]
  B --> C["AVOutputFormat3{mpegts(mpegtsenc.c)}"]
  C --> D["AVOutputFormat4{...}"]
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    typedef struct AVOutputFormat {
        const char *name; // 复用器名称
        /**
        * Descriptive name for the format, meant to be more human-readable
        * than name. You should use the NULL_IF_CONFIG_SMALL() macro
        * to define it.
        */
        const char *long_name; // 格式的描述性名称，易于阅读
        const char *mime_type; 
        const char *extensions; /**< comma-separated filename extensions */
        /* output support */
        enum AVCodecID audio_codec; /**< default audio codec */ // 默认的音频编解码器
        enum AVCodecID video_codec; /**< default video codec */ // 默认的视频编解码器
        enum AVCodecID subtitle_codec; /**< default subtitle codec */ // 默认的字幕编解码器
        /**
        * can use flags: AVFMT_NOFILE, AVFMT_NEEDNUMBER,
        * AVFMT_GLOBALHEADER, AVFMT_NOTIMESTAMPS, AVFMT_VARIABLE_FPS,
        * AVFMT_NODIMENSIONS, AVFMT_NOSTREAMS, AVFMT_ALLOW_FLUSH,
        * AVFMT_TS_NONSTRICT, AVFMT_TS_NEGATIVE
        */
        // ⼤部分复⽤器都有默认的编码器，所以⼤家如果要调整编码器类型则需要⾃⼰⼿动指定。
        int flags;
        /**
        * List of supported codec_id-codec_tag pairs, ordered by "better
        * choice first". The arrays are all terminated by AV_CODEC_ID_NONE.
        */
        const struct AVCodecTag * const *codec_tag;
        const AVClass *priv_class; ///< AVClass for the private context
        /*****************************************************************
        * No fields below this line are part of the public API. They
        * may not be used outside of libavformat and can be changed and
        * removed at will.
        * New public fields should be added right above.
        *****************************************************************
        */
        /**
        * The ff_const59 define is not part of the public API and will
        * be removed without further warning.
        */
    #if FF_API_AVIOFORMAT
    #define ff_const59
    #else
    #define ff_const59 const
    #endif
        ff_const59 struct AVOutputFormat *next;
        /**
        * size of private data so that it can be allocated in the wrapper
        */
        int priv_data_size;
        int (*write_header)(struct AVFormatContext *);
        /**
        * Write a packet. If AVFMT_ALLOW_FLUSH is set in flags,
        * pkt can be NULL in order to flush data buffered in the muxer.
        * When flushing, return 0 if there still is more data to flush,
        * or 1 if everything was flushed and there is no more buffered
        * data.
        */
        // 写一个数据包，如果在标志中设置AVFMT_ALLOW_FLUSH，则pkt可以为NULL
        int (*write_packet)(struct AVFormatContext *, AVPacket *pkt);
        int (*write_trailer)(struct AVFormatContext *);
        /**
        * Currently only used to set pixel format if not YUV420P.
        */
        int (*interleave_packet)(struct AVFormatContext *, AVPacket *out, AVPacket *in, int flush);
        /**
        * Test if the given codec can be stored in this container.
        *
        * @return 1 if the codec is supported, 0 if it is not.
        * A negative number if unknown.
        * MKTAG('A', 'P', 'I', 'C') if the codec is only supported as AV_DISPOSITION_ATTACHED_PIC
        */
        int (*query_codec)(enum AVCodecID id, int std_compliance);
        void (*get_output_timestamp)(struct AVFormatContext *s, int stream, int64_t *dts, int64_t *wall);
        /**
        * Allows sending messages from application to device.
        */
        // 允许从应用程序向设备发送消息
        int (*control_message)(struct AVFormatContext *s, int type, void *data, size_t data_size);
        /**
        * Write an uncoded AVFrame.
        *
        * See av_write_uncoded_frame() for details.
        *
        * The library will free *frame afterwards, but the muxer can prevent it
        * by setting the pointer to NULL.
        */
        // 写一个未编码的AVFrame
        int (*write_uncoded_frame)(struct AVFormatContext *, int stream_index, AVFrame **frame, unsigned flags);
        /**
        * Returns device list with it properties.
        * @see avdevice_list_devices() for more details.
        */
        int (*get_device_list)(struct AVFormatContext *s, struct AVDeviceInfoList *device_list);
        /**
        * Initialize device capabilities submodule.
        * @see avdevice_capabilities_create() for more details.
        */
        int (*create_device_capabilities)(struct AVFormatContext *s, struct AVDeviceCapabilitiesQuery *caps);
        /**
        * Free device capabilities submodule.
        * @see avdevice_capabilities_free() for more details.
        */
        int (*free_device_capabilities)(struct AVFormatContext *s, struct AVDeviceCapabilitiesQuery *caps);
        enum AVCodecID data_codec; /**< default data codec */
        /**
        * Initialize format. May allocate data here, and set any AVFormatContext or
        * AVStream parameters that need to be set before packets are sent.
        * This method must not write output.
        *
        * Return 0 if streams were fully configured, 1 if not, negative AVERROR on failure
        *
        * Any allocations made here must be freed in deinit().
        */
        // 初始化格式，可以在此分配数据，并设置在发送数据包之前需要设置的任何AVFormatContext或AVStream参数
        int (*init)(struct AVFormatContext *);
        /**
        * Deinitialize format. If present, this is called whenever the muxer is being
        * destroyed, regardless of whether or not the header has been written.
        *
        * If a trailer is being written, this is called after write_trailer().
        *
        * This is called if init() fails as well.
        */
        // 取消初始化格式
        void (*deinit)(struct AVFormatContext *);
        /**
        * Set up any necessary bitstream filtering and extract any extra data needed
        * for the global header.
        * Return 0 if more packets from this stream must be checked; 1 if not.
        */
        // 设置任何必要的比特流过滤，并提取全局头部所需的任何额外数据
        int (*check_bitstream)(struct AVFormatContext *, const AVPacket *pkt);
    } AVOutputFormat;

avformat_new_stream

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/**
* Add a new stream to a media file.
* 
* When demuxing, it is called by the demuxer in read_header(). If the
* flag AVFMTCTX_NOHEADER is set in s.ctx_flags, then it may also
* be called in read_packet().
* 
* When muxing, should be called by the user before avformat_write_header().
* 
* User is required to call avcodec_close() and avformat_free_context() to
* clean up the allocation by avformat_new_stream().
* 
* @param s media file handle
* @param c If non-NULL, the AVCodecContext corresponding to the new stream
* will be initialized to use this codec. This is needed for e.g. codec-specific
* defaults to be set, so codec should be provided if it is known.
* 
* @return newly created stream or NULL on error.
*/
AVStream *avformat_new_stream(AVFormatContext *s, const AVCodec *c);
  • AVFormatContext ：unsigned int nb_streams; 记录stream通道数⽬。AVStream **streams; 存储stream通道。
  • AVStream : int index; 在AVFormatContext 中所处的通道索引
  • avformat_new_stream之后便在 AVFormatContext ⾥增加了 AVStream 通道（相关的index已经被设置了）。之后，我们就可以⾃⾏设置 AVStream 的⼀些参数信息。例如 : codec_id , format ,bit_rate ,width , height

av_interleaved_write_frame

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int av_interleaved_write_frame(AVFormatContext *s, AVPacket *pkt);
  • 将数据包写⼊输出媒体⽂件，并确保正确的交织（保持packet dts的增⻓性）。该函数会在内部根据需要缓存packet，以确保输出⽂件中的packet按dts递增的顺序正确交织。如果⾃⼰进⾏交织则应调⽤av_write_frame()。
  • s 媒体文件句柄
  • pkt 要写入的packet，如果packet使⽤引⽤参考计数的内存⽅式，则此函数将获取此引⽤权(可以理解为move了reference)，并在内部在合适的时候进⾏释放。此函数返回后，调⽤者不得通过此引⽤访问数据。如果packet没有引⽤计数，libavformat将进⾏复制。此参数可以为NULL（在任何时候，不仅在结尾），以刷新交织队列。Packet的stream_index字段必须设置为s-> streams中相应流的索引。时间戳记（pts，dts）必须设置为stream’s timebase中的正确值（除⾮输出格式⽤AVFMT_NOTIMESTAMPS标志标记，然后可以将其设置为AV_NOPTS_VALUE）。同⼀stream后续packet的dts必须严格递增（除⾮输出格式⽤AVFMT_TS_NONSTRICT标记，则它们只必须不减少）。duration也应设置(如果已知)。
  • 返回值：成功时为0，错误时为负AVERROR。即使此函数调⽤失败，Libavformat仍将始终释放该packet。

av_compare_ts

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/**
* Compare two timestamps each in its own time base.
* 
* @return One of the following values:
* - -1 if `ts_a` is before `ts_b`
* - 1 if `ts_a` is after `ts_b`
* - 0 if they represent the same position
* 
* @warning
* The result of the function is undefined if one of the timestamps is outside
* the `int64_t` range when represented in the other's timebase.
*/
int av_compare_ts(int64_t ts_a, AVRational tb_a, int64_t ts_b, AVRational tb_b);
  • 返回值：
  • -1 ts_a 在ts_b之前
  • 1 ts_a 在ts_b之后
  • 0 ts_a 在ts_b同⼀位置
2023-12-08 该篇文章被 Cleofwine 归为分类: 音视频

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