ffmpeg-AVFormatConext解析

AVFormatContext是个贯穿全局的数据结构,很多函数都要用它作为参数。FFmpeg代码中对这个数据结构的注释是 format I/O context,此结构包含了一个视频流的格式内容。其中存AVInputFormat(AVOutputFormat,但是同一时间 AVFormatContext内只能存在其中一).AVStream,AVPacket区儿个重要的数据结构以及一些其他的相关信息,比如 title、Hthor,copyright等.另外,企有一些可能在编解码中会用到的信息,诸如 duration,file size,bit_rate等.

由于 AVFormatConext 结构包含许多信息,因此初始化过程是分步完成的,而且其中有些变量如果没有值可用,也可不初始化。 但是由于一般声明都使用指针,因此分配内存的过程不可少.

如下代码初始化了 AVFormatContext:

AVFormatContext *pFormatCtx;
pFormatCtx=avformat_alloc_context();

结构体定义如下:

//mux：视频/音频封装 复用
//demux：解封装视频/音频 解复用
typedef struct AVFormatContext {
    //与logging及avoptions相关的class
    const AVClass *av_class;

    //输入数据的封装格式。仅解封装用，由avformat_open_input()设置
    struct AVInputFormat *iformat;

    //输出数据的封装格式。仅封装用，调用者在avformat_write_header()之前设置
    struct AVOutputFormat *oformat;

    /**
     * 私有数据 是一个 AVOptions-enabled struct
     * 当且仅当iformat/oformat.priv_class 不为 NULL.
     *
     * - muxing: set by avformat_write_header()
     * - demuxing: set by avformat_open_input()
     */
    void *priv_data;

    /**
     * I/O上下文
     * 解封装：由用户在avformat_open_input()之前设置
     * （然后用户必须手动关闭它）或通过 avformat_open_input()设置
     * 
     * 封装：由用户在avformat_write_header()之前设置。
     * 调用者必须注意关闭/释放IO上下文
     * /
    AVIOContext *pb;

    //流的数量
    unsigned int nb_streams;

    //文件中所有流的列表 可以通过avformat_new_stream创建新的stream
    AVStream **streams;

#if FF_API_FORMAT_FILENAME
    /**
     * input or output filename
     *
     * - demuxing: set by avformat_open_input()
     * - muxing: may be set by the caller before avformat_write_header()
     *
     * @deprecated Use url instead.
     */
    attribute_deprecated
    char filename[1024];
#endif

    /**
     * input or output URL. Unlike the old filename field, this field has no
     * length restriction.
     *
     * - demuxing: set by avformat_open_input(), initialized to an empty
     *             string if url parameter was NULL in avformat_open_input().
     * - muxing: may be set by the caller before calling avformat_write_header()
     *           (or avformat_init_output() if that is called first) to a string
     *           which is freeable by av_free(). Set to an empty string if it
     *           was NULL in avformat_init_output().
     *
     * Freed by libavformat in avformat_free_context().
     */
    char *url;

    /**
     * 第一帧的位置
     *
     * Demuxing only, set by libavformat.
     */
    int64_t start_time;

    /**
     * Stream的时长
     *
     * Demuxing only, set by libavformat.
     */
    int64_t duration;

    /**
     * 整个流的比特率
     */
    int64_t bit_rate;

    unsigned int packet_size;
    int max_delay;

    /**
     * Demuxer/Muxer的状态
     * Set by the user before avformat_open_input() / avformat_write_header().
     */
    int flags;

    /**
     * 从输入读取的用于确定输入容器格式的数据的最大大小
     * 仅封装用，由调用者在avformat_open_input()之前设置
     * Demuxing only, set by the caller before avformat_open_input().
     */
    int64_t probesize;

    /**
     * 通过avformat_find_stream_info()读取最大时长
     * Can be set to 0 to let avformat choose using a heuristic.
     */
    int64_t max_analyze_duration;

    const uint8_t *key;
    int keylen;

    unsigned int nb_programs;
    AVProgram **programs;

    /**
     * Forced video codec_id.
     * Demuxing: Set by user.
     */
    enum AVCodecID video_codec_id;

    /**
     * Forced audio codec_id.
     * Demuxing: Set by user.
     */
    enum AVCodecID audio_codec_id;

    /**
     * 字幕 codec_id.
     * Demuxing: Set by user.
     */
    enum AVCodecID subtitle_codec_id;

    /**
     * 每条流的最大内存字节数
     */
    unsigned int max_index_size;

    /**
     * buffering frames中的最大的内存字节数
     */
    unsigned int max_picture_buffer;

    /**
     * Number of chapters in AVChapter array.
     * When muxing, chapters are normally written in the file header,
     * so nb_chapters should normally be initialized before write_header
     * is called. Some muxers (e.g. mov and mkv) can also write chapters
     * in the trailer.  To write chapters in the trailer, nb_chapters
     * must be zero when write_header is called and non-zero when
     * write_trailer is called.
     * - muxing: set by user
     * - demuxing: set by libavformat
     */
    unsigned int nb_chapters;
    AVChapter **chapters;

    /**
     * 整个文件的元数据.
     *
     * - demuxing: set by libavformat in avformat_open_input()
     * - muxing: may be set by the caller before avformat_write_header()
     *
     * Freed by libavformat in avformat_free_context().
     */
    AVDictionary *metadata;

    /**
     * 起始时间,从PST=0开始
     */
    int64_t start_time_realtime;

    /**
     * 在avformat_find_stream_info()中,用于确定帧数.
     * Demuxing only, set by the caller before avformat_find_stream_info().
     */
    int fps_probe_size;

    /**
     * 错误检测.
     * Demuxing only, set by the caller before avformat_open_input().
     */
    int error_recognition;

    /**
     * 自定义的终端callback
     */
    AVIOInterruptCB interrupt_callback;

    /**
     * 标记是否debugging.
     */
    int debug;
#define FF_FDEBUG_TS        0x0001

    /**
     * 最大交叉Buffering(缓冲数据)时长,在muxing(复用)时使用
     * Muxing only, set by the caller before avformat_write_header().
     */
    int64_t max_interleave_delta;

    int strict_std_compliance;

    /**
     * 时间flag
     */
    int event_flags;
#define AVFMT_EVENT_FLAG_METADATA_UPDATED 0x0001 ///< The call resulted in updated metadata.

    /**
     * 解码第一帧时读取的最大Packet数目
     */
    int max_ts_probe;

    /**
     * 在muxing时避免无效时间戳
     * - muxing: Set by user
     * - demuxing: unused
     */
    int avoid_negative_ts;
#define AVFMT_AVOID_NEG_TS_AUTO             -1 ///< Enabled when required by target format
#define AVFMT_AVOID_NEG_TS_MAKE_NON_NEGATIVE 1 ///< Shift timestamps so they are non negative
#define AVFMT_AVOID_NEG_TS_MAKE_ZERO         2 ///< Shift timestamps so that they start at 0

    /**
     * Transport stream id.
     * This will be moved into demuxer private options. Thus no API/ABI compatibility
     */
    int ts_id;

    /**
     * 音频预加载
     * - encoding: Set by user
     * - decoding: unused
     */
    int audio_preload;

    /**
     * 最大chunk时长,不是所有格式都支持chunk方式
     * - encoding: Set by user
     * - decoding: unused
     */
    int max_chunk_duration;

    /**
     * 最大chunk 以字节为单位
     * - encoding: Set by user
     * - decoding: unused
     */
    int max_chunk_size;

    /**
     * 强制使用wallclock时间戳作为数据包的PTS/DTS,B帧存在时,未定义
     * - encoding: unused
     * - decoding: Set by user
     */
    int use_wallclock_as_timestamps;

    /**
     * avio flags, used to force AVIO_FLAG_DIRECT.
     * - encoding: unused
     * - decoding: Set by user
     */
    int avio_flags;

    /**
     * 可以通过不同的方法估计持续时间字段
     * - encoding: unused
     * - decoding: Read by user
     */
    enum AVDurationEstimationMethod duration_estimation_method;

    /**
     * 在打开流时,跳过初始字节
     * - encoding: unused
     * - decoding: Set by user
     */
    int64_t skip_initial_bytes;

    /**
     * 纠正单个时间戳溢出
     * - encoding: unused
     * - decoding: Set by user
     */
    unsigned int correct_ts_overflow;

    /**
     * 强制快进/快退到任意帧
     * - encoding: unused
     * - decoding: Set by user
     */
    int seek2any;

    /**
     * 刷新 I/O context after each packet.
     * - encoding: Set by user
     * - decoding: unused
     */
    int flush_packets;

    /**
     * 格式探测评分,最大评分是 AVPROBE_SCORE_MAX
     * - encoding: unused
     * - decoding: set by avformat, read by user
     */
    int probe_score;

    /**
     * 读取最大的字节数以确定格式.
     * - encoding: unused
     * - decoding: set by user
     */
    int format_probesize;

    /**
     * ',' 分隔,所有可用的Decoder
     */
    char *codec_whitelist;

    /**
     * ',' 分隔,所有可用的Demuxer
     */
    char *format_whitelist;

    /**
     * An opaque field for libavformat internal usage.
     * Must not be accessed in any way by callers.
     */
    AVFormatInternal *internal;

    /**
     * IO 更改的标志
     */
    int io_repositioned;

    /**
     * 特殊解码器或相同codec_id的视频Codec
     */
    AVCodec *video_codec;

    /**
     * 特殊解码器或相同codec_id的音频Codec
     */
    AVCodec *audio_codec;

    /**
     * 特殊解码器或相同codec_id的字幕Codec
     */
    AVCodec *subtitle_codec;

    /**
     * 特殊解码器或相同codec_id的数据Codec
     */
    AVCodec *data_codec;

    /**
     * metadata头设置的padding值
     */
    int metadata_header_padding;

    /**
     * User data.
     * This is a place for some private data of the user.
     */
    void *opaque;

    /**
     * 设备和应用通信的callback
     */
    av_format_control_message control_message_cb;

    /**
     * 输出时间戳偏移值
     */
    int64_t output_ts_offset;

    /**
     * dump format separator.
     * can be ", " or "\n      " or anything else
     * - muxing: Set by user.
     * - demuxing: Set by user.
     */
    uint8_t *dump_separator;

    /**
     * Forced Data codec_id.
     * Demuxing: Set by user.
     */
    enum AVCodecID data_codec_id;

#if FF_API_OLD_OPEN_CALLBACKS
    /**
     * 使用av_format_set_open_cb()代替
     *
     * Demuxing: Set by user.
     *
     * @deprecated Use io_open and io_close.
     */
    attribute_deprecated
    int (*open_cb)(struct AVFormatContext *s, AVIOContext **p, const char *url, int flags, const AVIOInterruptCB *int_cb, AVDictionary **options);
#endif

    /**
     * ',' 分隔,协议白名单
     */
    char *protocol_whitelist;

    /**
     * 当IO流打开时,demux操作的回调函数
     */
    int (*io_open)(struct AVFormatContext *s, AVIOContext **pb, const char *url,
                   int flags, AVDictionary **options);

    /**
     * AVFormatContext.io_open() 操作的回调.
     */
    void (*io_close)(struct AVFormatContext *s, AVIOContext *pb);

    /**
     * ','分隔,协议黑名单
     */
    char *protocol_blacklist;

    /**
     * The maximum number of streams.
     * - encoding: unused
     * - decoding: set by user
     */
    int max_streams;

    /**
     * Skip duration calcuation in estimate_timings_from_pts.
     * - encoding: unused
     * - decoding: set by user
     */
    int skip_estimate_duration_from_pts;
} AVFormatContext;