
/*
 *雷霄骅
 *leixiaohua1020@126.com
 *中国传媒大学/数字电视技术
 */
/**
 * Audio Video Frame.
 * New fields can be added to the end of AVFRAME with minor version
 * bumps. Similarly fields that are marked as to be only accessed by
 * av_opt_ptr() can be reordered. This allows 2 forks to add fields
 * without breaking compatibility with each other.
 * Removal, reordering and changes in the remaining cases require
 * a major version bump.
 * sizeof(AVFrame) must not be used outside libavcodec.
 */
typedef struct AVFrame {
#define AV_NUM_DATA_POINTERS 8
    /**图像数据
     * pointer to the picture/channel planes.
     * This might be different from the first allocated byte
     * - encoding: Set by user
     * - decoding: set by AVCodecContext.get_buffer()
     */
    //解码后原始数据，针对视频可能是（YUV，RGB），音频的数据为（PCM）
    uint8_t *data[AV_NUM_DATA_POINTERS];
 
    /**
     * Size, in bytes, of the data for each picture/channel plane.
     *
     * For audio, only linesize[0] may be set. For planar audio, each channel
     * plane must be the same size.
     *
     * - encoding: Set by user
     * - decoding: set by AVCodecContext.get_buffer()
     */
    // data中每行数据的大小
    int linesize[AV_NUM_DATA_POINTERS];
 
    /**
     * pointers to the data planes/channels.
     *
     * For video, this should simply point to data[].
     *
     * For planar audio, each channel has a separate data pointer, and
     * linesize[0] contains the size of each channel buffer.
     * For packed audio, there is just one data pointer, and linesize[0]
     * contains the total size of the buffer for all channels.
     *
     * Note: Both data and extended_data will always be set by get_buffer(),
     * but for planar audio with more channels that can fit in data,
     * extended_data must be used by the decoder in order to access all
     * channels.
     *
     * encoding: unused
     * decoding: set by AVCodecContext.get_buffer()
     */
    //扩展数据，指向数据平面/通道指针，对于音频的舒缓每个通道一个但对于的数据指针
    //对于视频数据应该指向data[]
    uint8_t **extended_data;
 
    /**宽高
     * width and height of the video frame
     * - encoding: unused
     * - decoding: Read by user.
     */
    //视频数据的宽度和高度
    int width, height;
 
    /**
     * number of audio samples (per channel) described by this frame
     * - encoding: Set by user
     * - decoding: Set by libavcodec
     */
    //音频的一个AVframe中的包含多个视频帧
    int nb_samples;
 
    /**
     * format of the frame, -1 if unknown or unset
     * Values correspond to enum AVPixelFormat for video frames,
     * enum AVSampleFormat for audio)
     * - encoding: unused
     * - decoding: Read by user.
     */
    // 解码后原始数据的类型（YUV420，yuv422，RBG24）
    int format;
 
    /**
     * 1 -> keyframe, 0-> not
     * - encoding: Set by libavcodec.
     * - decoding: Set by libavcodec.
     */
 
    //是否是关键帧
    int key_frame;
 
    /**帧类型（I,B,P）
     * Picture type of the frame, see ?_TYPE below.
     * - encoding: Set by libavcodec. for coded_picture (and set by user for input).
     * - decoding: Set by libavcodec.
     */
    // 视频数据帧的类型
    enum AVPictureType pict_type;
 
    /**
     * pointer to the first allocated byte of the picture. Can be used in get_buffer/release_buffer.
     * This isn't used by libavcodec unless the default get/release_buffer() is used.
     * - encoding:
     * - decoding:
     */
    uint8_t *base[AV_NUM_DATA_POINTERS];
 
    /**
     * sample aspect ratio for the video frame, 0/1 if unknown/unspecified
     * - encoding: unused
     * - decoding: Read by user.
     */
    //视频数据宽高的比率
    AVRational sample_aspect_ratio;
 
    /**
     * presentation timestamp in time_base units (time when frame should be shown to user)
     * If AV_NOPTS_VALUE then frame_rate = 1/time_base will be assumed.
     * - encoding: MUST be set by user.
     * - decoding: Set by libavcodec.
     */
    // 显示事件戳
    int64_t pts;
 
    /**
     * reordered pts from the last AVPacket that has been input into the decoder
     * - encoding: unused
     * - decoding: Read by user.
     */
    // 
    int64_t pkt_pts;
 
    /**
     * dts from the last AVPacket that has been input into the decoder
     * - encoding: unused
     * - decoding: Read by user.
     */
    int64_t pkt_dts;
 
    /**
     * picture number in bitstream order
     * - encoding: set by
     * - decoding: Set by libavcodec.
     */
    int coded_picture_number;
    /**
     * picture number in display order
     * - encoding: set by
     * - decoding: Set by libavcodec.
     */
    int display_picture_number;
 
    /**
     * quality (between 1 (good) and FF_LAMBDA_MAX (bad))
     * - encoding: Set by libavcodec. for coded_picture (and set by user for input).
     * - decoding: Set by libavcodec.
     */
    int quality;
 
    /**
     * is this picture used as reference
     * The values for this are the same as the MpegEncContext.picture_structure
     * variable, that is 1->top field, 2->bottom field, 3->frame/both fields.
     * Set to 4 for delayed, non-reference frames.
     * - encoding: unused
     * - decoding: Set by libavcodec. (before get_buffer() call)).
     */
    int reference;
 
    /**QP表
     * QP table
     * - encoding: unused
     * - decoding: Set by libavcodec.
     */
    int8_t *qscale_table;
    /**
     * QP store stride
     * - encoding: unused
     * - decoding: Set by libavcodec.
     */
    int qstride;
 
    /**
     *
     */
    int qscale_type;
 
    /**跳过宏块表
     * mbskip_table[mb]>=1 if MB didn't change
     * stride= mb_width = (width+15)>>4
     * - encoding: unused
     * - decoding: Set by libavcodec.
     */
    uint8_t *mbskip_table;
 
    /**运动矢量表
     * motion vector table
     * @code
     * example:
     * int mv_sample_log2= 4 - motion_subsample_log2;
     * int mb_width= (width+15)>>4;
     * int mv_stride= (mb_width << mv_sample_log2) + 1;
     * motion_val[direction][x + y*mv_stride][0->mv_x, 1->mv_y];
     * @endcode
     * - encoding: Set by user.
     * - decoding: Set by libavcodec.
     */
    int16_t (*motion_val[2])[2];
 
    /**宏块类型表
     * macroblock type table
     * mb_type_base + mb_width + 2
     * - encoding: Set by user.
     * - decoding: Set by libavcodec.
     */
    uint32_t *mb_type;
 
    /**DCT系数
     * DCT coefficients
     * - encoding: unused
     * - decoding: Set by libavcodec.
     */
    short *dct_coeff;
 
    /**参考帧列表
     * motion reference frame index
     * the order in which these are stored can depend on the codec.
     * - encoding: Set by user.
     * - decoding: Set by libavcodec.
     */
    int8_t *ref_index[2];
 
    /**
     * for some private data of the user
     * - encoding: unused
     * - decoding: Set by user.
     */
    void *opaque;
 
    /**
     * error
     * - encoding: Set by libavcodec. if flags&CODEC_FLAG_PSNR.
     * - decoding: unused
     */
    uint64_t error[AV_NUM_DATA_POINTERS];
 
    /**
     * type of the buffer (to keep track of who has to deallocate data[*])
     * - encoding: Set by the one who allocates it.
     * - decoding: Set by the one who allocates it.
     * Note: User allocated (direct rendering) & internal buffers cannot coexist currently.
     */
    int type;
 
    /**
     * When decoding, this signals how much the picture must be delayed.
     * extra_delay = repeat_pict / (2*fps)
     * - encoding: unused
     * - decoding: Set by libavcodec.
     */
    int repeat_pict;
 
    /**
     * The content of the picture is interlaced.
     * - encoding: Set by user.
     * - decoding: Set by libavcodec. (default 0)
     */
    int interlaced_frame;
 
    /**
     * If the content is interlaced, is top field displayed first.
     * - encoding: Set by user.
     * - decoding: Set by libavcodec.
     */
    int top_field_first;
 
    /**
     * Tell user application that palette has changed from previous frame.
     * - encoding: ??? (no palette-enabled encoder yet)
     * - decoding: Set by libavcodec. (default 0).
     */
    int palette_has_changed;
 
    /**
     * codec suggestion on buffer type if != 0
     * - encoding: unused
     * - decoding: Set by libavcodec. (before get_buffer() call)).
     */
    int buffer_hints;
 
    /**
     * Pan scan.
     * - encoding: Set by user.
     * - decoding: Set by libavcodec.
     */
    AVPanScan *pan_scan;
 
    /**
     * reordered opaque 64bit (generally an integer or a double precision float
     * PTS but can be anything).
     * The user sets AVCodecContext.reordered_opaque to represent the input at
     * that time,
     * the decoder reorders values as needed and sets AVFrame.reordered_opaque
     * to exactly one of the values provided by the user through AVCodecContext.reordered_opaque
     * @deprecated in favor of pkt_pts
     * - encoding: unused
     * - decoding: Read by user.
     */
    int64_t reordered_opaque;
 
    /**
     * hardware accelerator private data (FFmpeg-allocated)
     * - encoding: unused
     * - decoding: Set by libavcodec
     */
    void *hwaccel_picture_private;
 
    /**
     * the AVCodecContext which ff_thread_get_buffer() was last called on
     * - encoding: Set by libavcodec.
     * - decoding: Set by libavcodec.
     */
    struct AVCodecContext *owner;
 
    /**
     * used by multithreading to store frame-specific info
     * - encoding: Set by libavcodec.
     * - decoding: Set by libavcodec.
     */
    void *thread_opaque;
 
    /**
     * log2 of the size of the block which a single vector in motion_val represents:
     * (4->16x16, 3->8x8, 2-> 4x4, 1-> 2x2)
     * - encoding: unused
     * - decoding: Set by libavcodec.
     */
    uint8_t motion_subsample_log2;
 
    /**（音频）采样率
     * Sample rate of the audio data.
     *
     * - encoding: unused
     * - decoding: read by user
     */
    int sample_rate;
 
    /**
     * Channel layout of the audio data.
     *
     * - encoding: unused
     * - decoding: read by user.
     */
    uint64_t channel_layout;
 
    /**
     * frame timestamp estimated using various heuristics, in stream time base
     * Code outside libavcodec should access this field using:
     * av_frame_get_best_effort_timestamp(frame)
     * - encoding: unused
     * - decoding: set by libavcodec, read by user.
     */
    int64_t best_effort_timestamp;
 
    /**
     * reordered pos from the last AVPacket that has been input into the decoder
     * Code outside libavcodec should access this field using:
     * av_frame_get_pkt_pos(frame)
     * - encoding: unused
     * - decoding: Read by user.
     */
    int64_t pkt_pos;
 
    /**
     * duration of the corresponding packet, expressed in
     * AVStream->time_base units, 0 if unknown.
     * Code outside libavcodec should access this field using:
     * av_frame_get_pkt_duration(frame)
     * - encoding: unused
     * - decoding: Read by user.
     */
    int64_t pkt_duration;
 
    /**
     * metadata.
     * Code outside libavcodec should access this field using:
     * av_frame_get_metadata(frame)
     * - encoding: Set by user.
     * - decoding: Set by libavcodec.
     */
    AVDictionary *metadata;
 
    /**
     * decode error flags of the frame, set to a combination of
     * FF_DECODE_ERROR_xxx flags if the decoder produced a frame, but there
     * were errors during the decoding.
     * Code outside libavcodec should access this field using:
     * av_frame_get_decode_error_flags(frame)
     * - encoding: unused
     * - decoding: set by libavcodec, read by user.
     */
    int decode_error_flags;
#define FF_DECODE_ERROR_INVALID_BITSTREAM   1
#define FF_DECODE_ERROR_MISSING_REFERENCE   2
 
    /**
     * number of audio channels, only used for audio.
     * Code outside libavcodec should access this field using:
     * av_frame_get_channels(frame)
     * - encoding: unused
     * - decoding: Read by user.
     */
    int64_t channels;
} AVFrame;

AVPacket存储未解码的数据


typedef struct AVPacket {
     //对数据包数据所在数缓冲区的引用
     AVBufferRef *buf;
     /**
      * Presentation timestamp in AVStream->time_base units; the time at which
      * the decompressed packet will be presented to the user.
      * Can be AV_NOPTS_VALUE if it is not stored in the file.
      * pts MUST be larger or equal to dts as presentation cannot happen before
      * decompression, unless one wants to view hex dumps. Some formats misuse
      * the terms dts and pts/cts to mean something different. Such timestamps
      * must be converted to true pts/dts before they are stored in AVPacket.
      */
     // 显示时间戳
     int64_t pts;
     /**
      * Decompression timestamp in AVStream->time_base units; the time at which
      * the packet is decompressed.
      * Can be AV_NOPTS_VALUE if it is not stored in the file.
      */
     // 解码事件戳
     int64_t dts;
     //保存压缩数据的指针
     uint8_t *data;
     //压缩收据的大小
     int   size;
     // 包存在数据的的数据流的ID
     int   stream_index;
     /**
      * A combination of AV_PKT_FLAG values
      */
     // 数据为最低为1表示该数据为的关键帧率
     int   flags;
     /**
      * Additional packet data that can be provided by the container.
      * Packet can contain several types of side information.
      */
     AVPacketSideData *side_data;
     int side_data_elems;
  
     
     //单位数据包的持续时间
     int64_t duration;
     //在数据流中的具体位置
     int64_t pos;                            ///< byte position in stream, -1 if unknown
  
     //保存用户的私有数据
     void *opaque;
  
     /**
      * AVBufferRef for free use by the API user. FFmpeg will never check the
      * contents of the buffer ref. FFmpeg calls av_buffer_unref() on it when
      * the packet is unreferenced. av_packet_copy_props() calls create a new
      * reference with av_buffer_ref() for the target packet's opaque_ref field.
      *
      * This is unrelated to the opaque field, although it serves a similar
      * purpose.
      */
     AVBufferRef *opaque_ref;
  
     /**
      * Time base of the packet's timestamps.
      * In the future, this field may be set on packets output by encoders or
      * demuxers, but its value will be by default ignored on input to decoders
      * or muxers.
      */
     // 数据包的时间基
     AVRational time_base;
 } AVPacket;


typedef struct AVBufferRef {
     AVBuffer *buffer;
  
     /**
      * The data buffer. It is considered writable if and only if
      * this is the only reference to the buffer, in which case
      * av_buffer_is_writable() returns 1.
      */
     uint8_t *data;
     /**
      * Size of data in bytes.
      */
     size_t   size;
 } AVBufferRef;

AVBuffer：数据缓冲区


struct AVBuffer {
    //指向一段长度为size的neicun
    uint8_t *data; /**< data described by this buffer */
    size_t size; /**< size of data in bytes */
 
    // 引用计数，表示有多山个AVBufferRef对象引用了自己
    atomic_uint refcount;
 
    /**
     * a callback for freeing the data
     */
    void (*free)(void *opaque, uint8_t *data);
 
    //一个指针，指向一个用户自定义类型的对象，也可为空，FFmpeg 不关心。
 
    void *opaque;
 
   //一些辅助自身状态的标志变量
    int flags;
    int flags_internal;
};

AVFormatcontext

此结构包含了一个视频流的格式内容。其中AVInputFormat或者AVOutputFormat，但是同一时间AVFormatContext内只能存在其中一个），AVStream，AVPacket这几个重要的结构以及一些其他信息，比如title，author，copyright等，后还有一些可能在编解码中会用到的信息，比如 duration、file_size、 bit_rate等。


typedef struct AVFormatContext
{
    /**
     * A class for logging and @ref avoptions. Set by avformat_alloc_context().
     * Exports (de)muxer private options if they exist.
     */
    const AVClass *av_class;
 
    // 输入容器的格式
    // avformat_open_input() 设置
    ff_const59 struct AVInputFormat *iformat;
 
    // 输出容器的个数
    // avformat_write_header()设置
    ff_const59 struct AVOutputFormat *oformat;
 
    //  * - muxing: set by avformat_write_header()
    //  * - demuxing: set by avformat_open_input()
    // 格式私有数据
    void *priv_data;
 
    // I/O context. 在解复用时候，在avformat_open_input()调用之前设置
    AVIOContext *pb;
 
    // 流信息
    int ctx_flags;
 
    // 流的数量
    unsigned int nb_streams;
 
    // 文件流的链表
    AVStream **streams;
 
#if FF_API_FORMAT_FILENAME
    // 流所属的文件名
    char filename[1024];
#endif
 
    // 流的url地址
    char *url;
 
    // 开始帧的位置，只有在解复用的时候用到
    int64_t start_time;
 
    // 流的时长
    int64_t duration;
 
    // 整个流的比特率
    int64_t bit_rate;
    // 整个packet的大小
    unsigned int packet_size;
    // 最大的延迟
    int max_delay;
 
    // 解码器的状态
    int flags;
#define AVFMT_FLAG_GENPTS 0x0001          ///< Generate missing pts even if it requires parsing future frames.
#define AVFMT_FLAG_IGNIDX 0x0002          ///< Ignore index.
#define AVFMT_FLAG_NONBLOCK 0x0004        ///< Do not block when reading packets from input.
#define AVFMT_FLAG_IGNDTS 0x0008          ///< Ignore DTS on frames that contain both DTS & PTS
#define AVFMT_FLAG_NOFILLIN 0x0010        ///< Do not infer any values from other values, just return what is stored in the container
#define AVFMT_FLAG_NOPARSE 0x0020         ///< Do not use AVParsers, you also must set AVFMT_FLAG_NOFILLIN as the fillin code works on frames and no parsing -> no frames. Also seeking to frames can not work if parsing to find frame boundaries has been disabled
#define AVFMT_FLAG_NOBUFFER 0x0040        ///< Do not buffer frames when possible
#define AVFMT_FLAG_CUSTOM_IO 0x0080       ///< The caller has supplied a custom AVIOContext, don't avio_close() it.
#define AVFMT_FLAG_DISCARD_CORRUPT 0x0100 ///< Discard frames marked corrupted
#define AVFMT_FLAG_FLUSH_PACKETS 0x0200   ///< Flush the AVIOContext every packet.
/**
 * When muxing, try to avoid writing any random/volatile data to the output.
 * This includes any random IDs, real-time timestamps/dates, muxer version, etc.
 *
 * This flag is mainly intended for testing.
 */
#define AVFMT_FLAG_BITEXACT 0x0400
#if FF_API_LAVF_MP4A_LATM
#define AVFMT_FLAG_MP4A_LATM 0x8000 ///< Deprecated, does nothing.
#endif
#define AVFMT_FLAG_SORT_DTS 0x10000 ///< try to interleave outputted packets by dts (using this flag can slow demuxing down)
#if FF_API_LAVF_PRIV_OPT
#define AVFMT_FLAG_PRIV_OPT 0x20000 ///< Enable use of private options by delaying codec open (deprecated, will do nothing once av_demuxer_open() is removed)
#endif
#if FF_API_LAVF_KEEPSIDE_FLAG
#define AVFMT_FLAG_KEEP_SIDE_DATA 0x40000 ///< Deprecated, does nothing.
#endif
#define AVFMT_FLAG_FAST_SEEK 0x80000 ///< Enable fast, but inaccurate seeks for some formats
#define AVFMT_FLAG_SHORTEST 0x100000 ///< Stop muxing when the shortest stream stops.
#define AVFMT_FLAG_AUTO_BSF 0x200000 ///< Add bitstream filters as requested by the muxer
 
    // 从输入中读取最大字节数用来确定流的属性
    int64_t probesize;
 
    // 读取数据的最大持续时间
    int64_t max_analyze_duration;
 
    const uint8_t *key;
    int keylen;
 
    unsigned int nb_programs;
    AVProgram **programs;
 
    // 视屏编码数据编码器类型 Id
    enum AVCodecID video_codec_id;
 
    // 音频编码器的类型 Id
    enum AVCodecID audio_codec_id;
 
    // 字幕类型编码器的 ID
    enum AVCodecID subtitle_codec_id;
 
    // 每条流中最大内存字节数
    unsigned int max_index_size;
 
    // Buffering frames的最大内存字节数
    unsigned int max_picture_buffer;
 
    // AVChapters array的chapters的数量
    unsigned int nb_chapters;
    AVChapter **chapters;
 
    /**
     * Metadata that applies to the whole file.
     *
     * - 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().
     */
    // 元数据，应用整个数据过程
    AVDictionary *metadata;
 
    // 起始时间，从PTS=0开始
    int64_t start_time_realtime;
 
    // 帧率
    int fps_probe_size;
 
    // 错误检测
    int error_recognition;
 
    /**
     * Custom interrupt callbacks for the I/O layer.
     *
     * demuxing: set by the user before avformat_open_input().
     * muxing: set by the user before avformat_write_header()
     * (mainly useful for AVFMT_NOFILE formats). The callback
     * should also be passed to avio_open2() if it's used to
     * open the file.
     */
    AVIOInterruptCB interrupt_callback;
 
    /**
     * Flags to enable debugging.
     */
    int debug;
#define FF_FDEBUG_TS 0x0001
 
    /**
     * Maximum buffering duration for interleaving.
     *
     * To ensure all the streams are interleaved correctly,
     * av_interleaved_write_frame() will wait until it has at least one packet
     * for each stream before actually writing any packets to the output file.
     * When some streams are "sparse" (i.e. there are large gaps between
     * successive packets), this can result in excessive buffering.
     *
     * This field specifies the maximum difference between the timestamps of the
     * first and the last packet in the muxing queue, above which libavformat
     * will output a packet regardless of whether it has queued a packet for all
     * the streams.
     *
     * Muxing only, set by the caller before avformat_write_header().
     */
    int64_t max_interleave_delta;
 
    /**
     * Allow non-standard and experimental extension
     * @see AVCodecContext.strict_std_compliance
     */
    int strict_std_compliance;
 
    /**
     * Flags indicating events happening on the file, a combination of
     * AVFMT_EVENT_FLAG_*.
     *
     * - demuxing: may be set by the demuxer in avformat_open_input(),
     *   avformat_find_stream_info() and av_read_frame(). Flags must be cleared
     *   by the user once the event has been handled.
     * - muxing: may be set by the user after avformat_write_header() to
     *   indicate a user-triggered event.  The muxer will clear the flags for
     *   events it has handled in av_[interleaved]_write_frame().
     */
    int event_flags;
/**
 * - demuxing: the demuxer read new metadata from the file and updated
 *   AVFormatContext.metadata accordingly
 * - muxing: the user updated AVFormatContext.metadata and wishes the muxer to
 *   write it into the file
 */
#define AVFMT_EVENT_FLAG_METADATA_UPDATED 0x0001
 
    /**
     * Maximum number of packets to read while waiting for the first timestamp.
     * Decoding only.
     */
    int max_ts_probe;
 
    /**
     * Avoid negative timestamps during muxing.
     * Any value of the AVFMT_AVOID_NEG_TS_* constants.
     * Note, this only works when using av_interleaved_write_frame. (interleave_packet_per_dts is in use)
     * - 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
 
    // 数据传输过程中的流ID
    int ts_id;
 
    // 音视频提前加载，不是所有格式都支持
    int audio_preload;
 
    // 最大chunk时长，不是所有格式都支持
    int max_chunk_duration;
 
    // 最大chunk以字节为单位
    int max_chunk_size;
 
    // 强制使用wallclock时间戳作为数据包的pts/dts
    int use_wallclock_as_timestamps;
 
    /**
     * avio flags, used to force AVIO_FLAG_DIRECT.
     * - encoding: unused
     * - decoding: Set by user
     */
    int avio_flags;
 
    // 可以通过不同的方式估计持续时间字段
    enum AVDurationEstimationMethod duration_estimation_method;
 
    // 当字节流打开流的时候，跳过初始化字节
    int64_t skip_initial_bytes;
 
    // 纠正单个时间戳的溢出
    unsigned int correct_ts_overflow;
 
    // 强制seek到任意一帧
    int seek2any;
 
    // 在每个packet之后
    int flush_packets;
 
    // 格式探测评分
    int probe_score;
 
    // 读取最大的字节数来确定格式
    int format_probesize;
 
    // 由','分隔的所有可用的decoder（解码器）
    char *codec_whitelist;
 
    // 由‘，’分隔的所有可用的demuxers（解复用器）
    char *format_whitelist;
 
    // libavformat内部私有成员
    AVFormatInternal *internal;
 
    // I/O 更改的标志
    int io_repositioned;
 
    // 特殊解码器或者相同codec_id的视频Codec
    AVCodec *video_codec;
 
    // 特殊解码器或者相同codec_id的音频Codec
    AVCodec *audio_codec;
 
    // 特殊解码器或者相同codec_id的字幕Codec
    AVCodec *subtitle_codec;
 
    // 特殊解码器或者相同codec_id的数据Codec
    AVCodec *data_codec;
 
    /**
     * Number of bytes to be written as padding in a metadata header.
     * Demuxing: Unused.
     * Muxing: Set by user via av_format_set_metadata_header_padding.
     */
    int metadata_header_padding;
 
    // 用户私有数据
    void *opaque;
 
    /**
     * Callback used by devices to communicate with application.
     */
    av_format_control_message control_message_cb;
 
    /**
     * Output timestamp offset, in microseconds.
     * Muxing: set by user
     */
    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
    /**
     * Called to open further IO contexts when needed for demuxing.
     *
     * This can be set by the user application to perform security checks on
     * the URLs before opening them.
     * The function should behave like avio_open2(), AVFormatContext is provided
     * as contextual information and to reach AVFormatContext.opaque.
     *
     * If NULL then some simple checks are used together with avio_open2().
     *
     * Must not be accessed directly from outside avformat.
     * @See av_format_set_open_cb()
     *
     * 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
 
    /**
     * ',' separated list of allowed protocols.
     * - encoding: unused
     * - decoding: set by user
     */
    // 协议白名单
    char *protocol_whitelist;
 
    /**
     * A callback for opening new IO streams.
     *
     * Whenever a muxer or a demuxer needs to open an IO stream (typically from
     * avformat_open_input() for demuxers, but for certain formats can happen at
     * other times as well), it will call this callback to obtain an IO context.
     *
     * @param s the format context
     * @param pb on success, the newly opened IO context should be returned here
     * @param url the url to open
     * @param flags a combination of AVIO_FLAG_*
     * @param options a dictionary of additional options, with the same
     *                semantics as in avio_open2()
     * @return 0 on success, a negative AVERROR code on failure
     *
     * @note Certain muxers and demuxers do nesting, i.e. they open one or more
     * additional internal format contexts. Thus the AVFormatContext pointer
     * passed to this callback may be different from the one facing the caller.
     * It will, however, have the same 'opaque' field.
     */
    int (*io_open)(struct AVFormatContext *s, AVIOContext **pb, const char *url,
                   int flags, AVDictionary **options);
 
    /**
     * A callback for closing the streams opened with AVFormatContext.io_open().
     */
    void (*io_close)(struct AVFormatContext *s, AVIOContext *pb);
 
    // 协议的黑名单
    char *protocol_blacklist;
 
    // 流的最大数量
    int max_streams;
 
    /**
     * Skip duration calcuation in estimate_timings_from_pts.
     * - encoding: unused
     * - decoding: set by user
     */
    int skip_estimate_duration_from_pts;
 
    // 多少个packet用于检测
    int max_probe_packets;
} AVFormatContext;

AVFormatInput

VInputFormat是解复用器（解封装）作用时读取媒体文件并将其拆分为数据块（数据包）。每个数据包，包含一个或者多个编码帧


typedef struct AVInputFormat
{
    // 封装格式的短名
    const char *name;
 
    // 封装格式的长名字
    const char *long_name;
 
    /**
     * Can use flags: AVFMT_NOFILE, AVFMT_NEEDNUMBER, AVFMT_SHOW_IDS,
     * AVFMT_NOTIMESTAMPS, AVFMT_GENERIC_INDEX, AVFMT_TS_DISCONT, AVFMT_NOBINSEARCH,
     * AVFMT_NOGENSEARCH, AVFMT_NO_BYTE_SEEK, AVFMT_SEEK_TO_PTS.
     */
    int flags;
 
    //   如果扩展被定义就不会进行类型格式的探查
    const char *extensions;
    //  编码器标签
    const struct AVCodecTag *const *codec_tag;
    //  私有数据
    const AVClass *priv_class; ///< AVClass for the private context
 
    // mime类型，比如video/avc，在probing（探测）时候需要检查
    const char *mime_type;
 
    /*****************************************************************
     * 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.
     *****************************************************************
     */
#if FF_API_NEXT
    ff_const59 struct AVInputFormat *next;
#endif
 
    // 原始Demuxer存储的codec i
    int raw_codec_id;
 
    // 魔种格式文件的数据大小
    int priv_data_size;
 
    /**
     * Tell if a given file has a chance of being parsed as this format.
     * The buffer provided is guaranteed to be AVPROBE_PADDING_SIZE bytes
     * big so you do not have to check for that unless you need more.
     */
    int (*read_probe)(const AVProbeData *);
 
    /**
     * Read the format header and initialize the AVFormatContext
     * structure. Return 0 if OK. 'avformat_new_stream' should be
     * called to create new streams.
     */
    int (*read_header)(struct AVFormatContext *);
 
    /**
     * Read one packet and put it in 'pkt'. pts and flags are also
     * set. 'avformat_new_stream' can be called only if the flag
     * AVFMTCTX_NOHEADER is used and only in the calling thread (not in a
     * background thread).
     * @return 0 on success, < 0 on error.
     *         Upon returning an error, pkt must be unreferenced by the caller.
     */
    int (*read_packet)(struct AVFormatContext *, AVPacket *pkt);
 
    /**
     * Close the stream. The AVFormatContext and AVStreams are not
     * freed by this function
     */
    int (*read_close)(struct AVFormatContext *);
 
    /**
     * Seek to a given timestamp relative to the frames in
     * stream component stream_index.
     * @param stream_index Must not be -1.
     * @param flags Selects which direction should be preferred if no exact
     *              match is available.
     * @return >= 0 on success (but not necessarily the new offset)
     */
    int (*read_seek)(struct AVFormatContext *,
                     int stream_index, int64_t timestamp, int flags);
 
    /**
     * Get the next timestamp in stream[stream_index].time_base units.
     * @return the timestamp or AV_NOPTS_VALUE if an error occurred
     */
    int64_t (*read_timestamp)(struct AVFormatContext *s, int stream_index,
                              int64_t *pos, int64_t pos_limit);
 
    /**
     * Start/resume playing - only meaningful if using a network-based format
     * (RTSP).
     */
    int (*read_play)(struct AVFormatContext *);
 
    /**
     * Pause playing - only meaningful if using a network-based format
     * (RTSP).
     */
    int (*read_pause)(struct AVFormatContext *);
 
    /**
     * Seek to timestamp ts.
     * Seeking will be done so that the point from which all active streams
     * can be presented successfully will be closest to ts and within min/max_ts.
     * Active streams are all streams that have AVStream.discard < AVDISCARD_ALL.
     */
    int (*read_seek2)(struct AVFormatContext *s, int stream_index, int64_t min_ts, int64_t ts, int64_t max_ts, int 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);
 
#if LIBAVFORMAT_VERSION_MAJOR < 59
    /**
     * 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);
#endif
} AVInputFormat;

AVOutputFormat


typedef struct AVOutputFormat
{
    // 封装格式名称
    const char *name;
    // 封装格式的长名称
    const char *long_name;
    
    const char *mime_type;
    const char *extensions; /**< comma-separated filename extensions */
    /* output support */
    enum AVCodecID audio_codec;    /**< default audio codec */
    enum AVCodecID video_codec;    /**< default video codec */
    enum AVCodecID subtitle_codec; /**< default subtitle codec */
    /**
     * 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
     */
    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
#if FF_API_NEXT
    ff_const59 struct AVOutputFormat *next;
#endif
    /**
     * 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.
     */
    int (*write_packet)(struct AVFormatContext *, AVPacket *pkt);
    int (*write_trailer)(struct AVFormatContext *);
    /**
     * A format-specific function for interleavement.
     * If unset, packets will be interleaved by dts.
     */
    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.
     */
    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.
     */
    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);
#if LIBAVFORMAT_VERSION_MAJOR < 59
    /**
     * 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);
#endif
    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().
     */
    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.
     */
    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.
     */
    int (*check_bitstream)(struct AVFormatContext *, const AVPacket *pkt);
} AVOutputFormat;

AVCodecContext


typedef struct AVCodecContext
{
    // 一个用来记录和指向avoptions的类。由avformat_all_context()设置。如果(de)muxer存在私有option也会输出
    const AVClass *av_class;
    int log_level_offset;
    // 解码器的类型（视频，音频...）
    enum AVMediaType codec_type; /* see AVMEDIA_TYPE_xxx */
    // 采用编码器的AVcodec
    const struct AVCodec *codec;
    // 编码器的id
    enum AVCodecID codec_id; /* see AV_CODEC_ID_xxx */
 
    /**
     * fourcc (LSB first, so "ABCD" -> ('D'<<24) + ('C'<<16) + ('B'<<8) + 'A').
     * This is used to work around some encoder bugs.
     * A demuxer should set this to what is stored in the field used to identify the codec.
     * If there are multiple such fields in a container then the demuxer should choose the one
     * which maximizes the information about the used codec.
     * If the codec tag field in a container is larger than 32 bits then the demuxer should
     * remap the longer ID to 32 bits with a table or other structure. Alternatively a new
     * extra_codec_tag + size could be added but for this a clear advantage must be demonstrated
     * first.
     * - encoding: Set by user, if not then the default based on codec_id will be used.
     * - decoding: Set by user, will be converted to uppercase by libavcodec during init.
     */
    unsigned int codec_tag;
 
    // 私有数据
    void *priv_data;
 
    /**
     * Private context used for internal data.
     *
     * Unlike priv_data, this is not codec-specific. It is used in general
     * libavcodec functions.
     */
    struct AVCodecInternal *internal;
 
    /**
     * Private data of the user, can be used to carry app specific stuff.
     * - encoding: Set by user.
     * - decoding: Set by user.
     */
    void *opaque;
 
    //  平均比特率
    int64_t bit_rate;
 
    // 	比特流允许偏离参考的比特数
    int bit_rate_tolerance;
 
    /**
     * Global quality for codecs which cannot change it per frame.
     * This should be proportional to MPEG-1/2/4 qscale.
     * - encoding: Set by user.
     * - decoding: unused
     */
    int global_quality;
 
    /**
     * - encoding: Set by user.
     * - decoding: unused
     */
    int compression_level;
#define FF_COMPRESSION_DEFAULT -1
 
    /**
     * AV_CODEC_FLAG_*.
     * - encoding: Set by user.
     * - decoding: Set by user.
     */
    int flags;
 
    /**
     * AV_CODEC_FLAG2_*
     * - encoding: Set by user.
     * - decoding: Set by user.
     */
    int flags2;
 
    /**
     * some codecs need / can use extradata like Huffman tables.
     * MJPEG: Huffman tables
     * rv10: additional flags
     * MPEG-4: global headers (they can be in the bitstream or here)
     * The allocated memory should be AV_INPUT_BUFFER_PADDING_SIZE bytes larger
     * than extradata_size to avoid problems if it is read with the bitstream reader.
     * The bytewise contents of extradata must not depend on the architecture or CPU endianness.
     * Must be allocated with the av_malloc() family of functions.
     * - encoding: Set/allocated/freed by libavcodec.
     * - decoding: Set/allocated/freed by user.
     */
    uint8_t *extradata;
    int extradata_size;
 
    /**
     * This is the fundamental unit of time (in seconds) in terms
     * of which frame timestamps are represented. For fixed-fps content,
     * timebase should be 1/framerate and timestamp increments should be
     * identically 1.
     * This often, but not always is the inverse of the frame rate or field rate
     * for video. 1/time_base is not the average frame rate if the frame rate is not
     * constant.
     *
     * Like containers, elementary streams also can store timestamps, 1/time_base
     * is the unit in which these timestamps are specified.
     * As example of such codec time base see ISO/IEC 14496-2:2001(E)
     * vop_time_increment_resolution and fixed_vop_rate
     * (fixed_vop_rate == 0 implies that it is different from the framerate)
     *
     * - encoding: MUST be set by user.
     * - decoding: the use of this field for decoding is deprecated.
     *             Use framerate instead.
     */
    AVRational time_base;
 
    /**
     * For some codecs, the time base is closer to the field rate than the frame rate.
     * Most notably, H.264 and MPEG-2 specify time_base as half of frame duration
     * if no telecine is used ...
     *
     * Set to time_base ticks per frame. Default 1, e.g., H.264/MPEG-2 set it to 2.
     */
    int ticks_per_frame;
 
    /**
     * Codec delay.
     *
     * Encoding: Number of frames delay there will be from the encoder input to
     *           the decoder output. (we assume the decoder matches the spec)
     * Decoding: Number of frames delay in addition to what a standard decoder
     *           as specified in the spec would produce.
     *
     * Video:
     *   Number of frames the decoded output will be delayed relative to the
     *   encoded input.
     *
     * Audio:
     *   For encoding, this field is unused (see initial_padding).
     *
     *   For decoding, this is the number of samples the decoder needs to
     *   output before the decoder's output is valid. When seeking, you should
     *   start decoding this many samples prior to your desired seek point.
     *
     * - encoding: Set by libavcodec.
     * - decoding: Set by libavcodec.
     */
    int delay;
 
    // 视屏图像的尺寸
    int width, height;
 
    //比特流的宽高，当解码帧裁剪之后输出，所以可能与width, height值不一样
    int coded_width, coded_height;
 
    // 在一组gop中图像中的数量
    int gop_size;
 
    // 图像的格式
    enum AVPixelFormat pix_fmt;
 
    /**
     * If non NULL, 'draw_horiz_band' is called by the libavcodec
     * decoder to draw a horizontal band. It improves cache usage. Not
     * all codecs can do that. You must check the codec capabilities
     * beforehand.
     * When multithreading is used, it may be called from multiple threads
     * at the same time; threads might draw different parts of the same AVFrame,
     * or multiple AVFrames, and there is no guarantee that slices will be drawn
     * in order.
     * The function is also used by hardware acceleration APIs.
     * It is called at least once during frame decoding to pass
     * the data needed for hardware render.
     * In that mode instead of pixel data, AVFrame points to
     * a structure specific to the acceleration API. The application
     * reads the structure and can change some fields to indicate progress
     * or mark state.
     * - encoding: unused
     * - decoding: Set by user.
     * @param height the height of the slice
     * @param y the y position of the slice
     * @param type 1->top field, 2->bottom field, 3->frame
     * @param offset offset into the AVFrame.data from which the slice should be read
     */
    void (*draw_horiz_band)(struct AVCodecContext *s,
                            const AVFrame *src, int offset[AV_NUM_DATA_POINTERS],
                            int y, int type, int height);
 
    /**
     * callback to negotiate the pixelFormat
     * @param fmt is the list of formats which are supported by the codec,
     * it is terminated by -1 as 0 is a valid format, the formats are ordered by quality.
     * The first is always the native one.
     * @note The callback may be called again immediately if initialization for
     * the selected (hardware-accelerated) pixel format failed.
     * @warning Behavior is undefined if the callback returns a value not
     * in the fmt list of formats.
     * @return the chosen format
     * - encoding: unused
     * - decoding: Set by user, if not set the native format will be chosen.
     */
    enum AVPixelFormat (*get_format)(struct AVCodecContext *s, const enum AVPixelFormat *fmt);
 
    // 非 B 帧之间的最大 B 帧数
    int max_b_frames;
 
    // IP 和 B 帧之间的 qscale 因子 如果 > 0，则将使用最后一个 P 帧量化器
    float b_quant_factor;
 
#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated int b_frame_strategy;
#endif
 
    // qscale offset between IP and B-frames
    float b_quant_offset;
 
    // 解码器中帧重新排序缓冲区的大小
    int has_b_frames;
 
#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated int mpeg_quant;
#endif
 
    /**
     * qscale factor between P- and I-frames
     * If > 0 then the last P-frame quantizer will be used (q = lastp_q * factor + offset).
     * If < 0 then normal ratecontrol will be done (q= -normal_q*factor+offset).
     * - encoding: Set by user.
     * - decoding: unused
     */
    float i_quant_factor;
 
    /**
     * qscale offset between P and I-frames
     * - encoding: Set by user.
     * - decoding: unused
     */
    float i_quant_offset;
 
    /**
     * luminance masking (0-> disabled)
     * - encoding: Set by user.
     * - decoding: unused
     */
    float lumi_masking;
 
    /**
     * temporary complexity masking (0-> disabled)
     * - encoding: Set by user.
     * - decoding: unused
     */
    float temporal_cplx_masking;
 
    /**
     * spatial complexity masking (0-> disabled)
     * - encoding: Set by user.
     * - decoding: unused
     */
    float spatial_cplx_masking;
 
    /**
     * p block masking (0-> disabled)
     * - encoding: Set by user.
     * - decoding: unused
     */
    float p_masking;
 
    /**
     * darkness masking (0-> disabled)
     * - encoding: Set by user.
     * - decoding: unused
     */
    float dark_masking;
 
    /**
     * slice count
     * - encoding: Set by libavcodec.
     * - decoding: Set by user (or 0).
     */
    int slice_count;
 
#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated int prediction_method;
#define FF_PRED_LEFT 0
#define FF_PRED_PLANE 1
#define FF_PRED_MEDIAN 2
#endif
 
    /**
     * slice offsets in the frame in bytes
     * - encoding: Set/allocated by libavcodec.
     * - decoding: Set/allocated by user (or NULL).
     */
    int *slice_offset;
 
    /**
     * sample aspect ratio (0 if unknown)
     * That is the width of a pixel divided by the height of the pixel.
     * Numerator and denominator must be relatively prime and smaller than 256 for some video standards.
     * - encoding: Set by user.
     * - decoding: Set by libavcodec.
     */
    AVRational sample_aspect_ratio;
 
    /**
     * motion estimation comparison function
     * - encoding: Set by user.
     * - decoding: unused
     */
    int me_cmp;
    /**
     * subpixel motion estimation comparison function
     * - encoding: Set by user.
     * - decoding: unused
     */
    int me_sub_cmp;
    /**
     * macroblock comparison function (not supported yet)
     * - encoding: Set by user.
     * - decoding: unused
     */
    int mb_cmp;
    /**
     * interlaced DCT comparison function
     * - encoding: Set by user.
     * - decoding: unused
     */
    int ildct_cmp;
#define FF_CMP_SAD 0
#define FF_CMP_SSE 1
#define FF_CMP_SATD 2
#define FF_CMP_DCT 3
#define FF_CMP_PSNR 4
#define FF_CMP_BIT 5
#define FF_CMP_RD 6
#define FF_CMP_ZERO 7
#define FF_CMP_VSAD 8
#define FF_CMP_VSSE 9
#define FF_CMP_NSSE 10
#define FF_CMP_W53 11
#define FF_CMP_W97 12
#define FF_CMP_DCTMAX 13
#define FF_CMP_DCT264 14
#define FF_CMP_MEDIAN_SAD 15
#define FF_CMP_CHROMA 256
 
    /**
     * ME diamond size & shape
     * - encoding: Set by user.
     * - decoding: unused
     */
    int dia_size;
 
    /**
     * amount of previous MV predictors (2a+1 x 2a+1 square)
     * - encoding: Set by user.
     * - decoding: unused
     */
    int last_predictor_count;
 
#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated int pre_me;
#endif
 
    /**
     * motion estimation prepass comparison function
     * - encoding: Set by user.
     * - decoding: unused
     */
    int me_pre_cmp;
 
    /**
     * ME prepass diamond size & shape
     * - encoding: Set by user.
     * - decoding: unused
     */
    int pre_dia_size;
 
    /**
     * subpel ME quality
     * - encoding: Set by user.
     * - decoding: unused
     */
    int me_subpel_quality;
 
    /**
     * maximum motion estimation search range in subpel units
     * If 0 then no limit.
     *
     * - encoding: Set by user.
     * - decoding: unused
     */
    int me_range;
 
    /**
     * slice flags
     * - encoding: unused
     * - decoding: Set by user.
     */
    int slice_flags;
#define SLICE_FLAG_CODED_ORDER 0x0001 ///< draw_horiz_band() is called in coded order instead of display
#define SLICE_FLAG_ALLOW_FIELD 0x0002 ///< allow draw_horiz_band() with field slices (MPEG-2 field pics)
#define SLICE_FLAG_ALLOW_PLANE 0x0004 ///< allow draw_horiz_band() with 1 component at a time (SVQ1)
 
    /**
     * macroblock decision mode
     * - encoding: Set by user.
     * - decoding: unused
     */
    int mb_decision;
#define FF_MB_DECISION_SIMPLE 0 ///< uses mb_cmp
#define FF_MB_DECISION_BITS 1   ///< chooses the one which needs the fewest bits
#define FF_MB_DECISION_RD 2     ///< rate distortion
 
    /**
     * custom intra quantization matrix
     * Must be allocated with the av_malloc() family of functions, and will be freed in
     * avcodec_free_context().
     * - encoding: Set/allocated by user, freed by libavcodec. Can be NULL.
     * - decoding: Set/allocated/freed by libavcodec.
     */
    uint16_t *intra_matrix;
 
    /**
     * custom inter quantization matrix
     * Must be allocated with the av_malloc() family of functions, and will be freed in
     * avcodec_free_context().
     * - encoding: Set/allocated by user, freed by libavcodec. Can be NULL.
     * - decoding: Set/allocated/freed by libavcodec.
     */
    uint16_t *inter_matrix;
 
#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated int scenechange_threshold;
 
    /** @deprecated use encoder private options instead */
    attribute_deprecated int noise_reduction;
#endif
 
    /**
     * precision of the intra DC coefficient - 8
     * - encoding: Set by user.
     * - decoding: Set by libavcodec
     */
    int intra_dc_precision;
 
    /**
     * Number of macroblock rows at the top which are skipped.
     * - encoding: unused
     * - decoding: Set by user.
     */
    int skip_top;
 
    /**
     * Number of macroblock rows at the bottom which are skipped.
     * - encoding: unused
     * - decoding: Set by user.
     */
    int skip_bottom;
 
    /**
     * minimum MB Lagrange multiplier
     * - encoding: Set by user.
     * - decoding: unused
     */
    int mb_lmin;
 
    /**
     * maximum MB Lagrange multiplier
     * - encoding: Set by user.
     * - decoding: unused
     */
    int mb_lmax;
 
#if FF_API_PRIVATE_OPT
    /**
     * @deprecated use encoder private options instead
     */
    attribute_deprecated int me_penalty_compensation;
#endif
 
    /**
     * - encoding: Set by user.
     * - decoding: unused
     */
    int bidir_refine;
 
#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated int brd_scale;
#endif
 
    /**
     * minimum GOP size
     * - encoding: Set by user.
     * - decoding: unused
     */
    int keyint_min;
 
    /**
     * number of reference frames
     * - encoding: Set by user.
     * - decoding: Set by lavc.
     */
    int refs;
 
#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated int chromaoffset;
#endif
 
    /**
     * Note: Value depends upon the compare function used for fullpel ME.
     * - encoding: Set by user.
     * - decoding: unused
     */
    int mv0_threshold;
 
#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated int b_sensitivity;
#endif
 
    /**
     * Chromaticity coordinates of the source primaries.
     * - encoding: Set by user
     * - decoding: Set by libavcodec
     */
    enum AVColorPrimaries color_primaries;
 
    /**
     * Color Transfer Characteristic.
     * - encoding: Set by user
     * - decoding: Set by libavcodec
     */
    enum AVColorTransferCharacteristic color_trc;
 
    /**
     * YUV colorspace type.
     * - encoding: Set by user
     * - decoding: Set by libavcodec
     */
    enum AVColorSpace colorspace;
 
    /**
     * MPEG vs JPEG YUV range.
     * - encoding: Set by user
     * - decoding: Set by libavcodec
     */
    enum AVColorRange color_range;
 
    /**
     * This defines the location of chroma samples.
     * - encoding: Set by user
     * - decoding: Set by libavcodec
     */
    enum AVChromaLocation chroma_sample_location;
 
    /**
     * Number of slices.
     * Indicates number of picture subdivisions. Used for parallelized
     * decoding.
     * - encoding: Set by user
     * - decoding: unused
     */
    int slices;
 
    /** Field order
     * - encoding: set by libavcodec
     * - decoding: Set by user.
     */
    enum AVFieldOrder field_order;
 
    /* audio only */
    int sample_rate; ///< samples per second
    int channels;    ///< number of audio channels
 
    /**
     * audio sample format
     * - encoding: Set by user.
     * - decoding: Set by libavcodec.
     */
    enum AVSampleFormat sample_fmt; ///< sample format
 
    /* The following data should not be initialized. */
    /**
     * Number of samples per channel in an audio frame.
     *
     * - encoding: set by libavcodec in avcodec_open2(). Each submitted frame
     *   except the last must contain exactly frame_size samples per channel.
     *   May be 0 when the codec has AV_CODEC_CAP_VARIABLE_FRAME_SIZE set, then the
     *   frame size is not restricted.
     * - decoding: may be set by some decoders to indicate constant frame size
     */
    int frame_size;
 
    /**
     * Frame counter, set by libavcodec.
     *
     * - decoding: total number of frames returned from the decoder so far.
     * - encoding: total number of frames passed to the encoder so far.
     *
     *   @note the counter is not incremented if encoding/decoding resulted in
     *   an error.
     */
    int frame_number;
 
    /**
     * number of bytes per packet if constant and known or 0
     * Used by some WAV based audio codecs.
     */
    int block_align;
 
    /**
     * Audio cutoff bandwidth (0 means "automatic")
     * - encoding: Set by user.
     * - decoding: unused
     */
    int cutoff;
 
    /**
     * Audio channel layout.
     * - encoding: set by user.
     * - decoding: set by user, may be overwritten by libavcodec.
     */
    uint64_t channel_layout;
 
    /**
     * Request decoder to use this channel layout if it can (0 for default)
     * - encoding: unused
     * - decoding: Set by user.
     */
    uint64_t request_channel_layout;
 
    /**
     * Type of service that the audio stream conveys.
     * - encoding: Set by user.
     * - decoding: Set by libavcodec.
     */
    enum AVAudioServiceType audio_service_type;
 
    /**
     * desired sample format
     * - encoding: Not used.
     * - decoding: Set by user.
     * Decoder will decode to this format if it can.
     */
    enum AVSampleFormat request_sample_fmt;
 
    /**
     * This callback is called at the beginning of each frame to get data
     * buffer(s) for it. There may be one contiguous buffer for all the data or
     * there may be a buffer per each data plane or anything in between. What
     * this means is, you may set however many entries in buf[] you feel necessary.
     * Each buffer must be reference-counted using the AVBuffer API (see description
     * of buf[] below).
     *
     * The following fields will be set in the frame before this callback is
     * called:
     * - format
     * - width, height (video only)
     * - sample_rate, channel_layout, nb_samples (audio only)
     * Their values may differ from the corresponding values in
     * AVCodecContext. This callback must use the frame values, not the codec
     * context values, to calculate the required buffer size.
     *
     * This callback must fill the following fields in the frame:
     * - data[]
     * - linesize[]
     * - extended_data:
     *   * if the data is planar audio with more than 8 channels, then this
     *     callback must allocate and fill extended_data to contain all pointers
     *     to all data planes. data[] must hold as many pointers as it can.
     *     extended_data must be allocated with av_malloc() and will be freed in
     *     av_frame_unref().
     *   * otherwise extended_data must point to data
     * - buf[] must contain one or more pointers to AVBufferRef structures. Each of
     *   the frame's data and extended_data pointers must be contained in these. That
     *   is, one AVBufferRef for each allocated chunk of memory, not necessarily one
     *   AVBufferRef per data[] entry. See: av_buffer_create(), av_buffer_alloc(),
     *   and av_buffer_ref().
     * - extended_buf and nb_extended_buf must be allocated with av_malloc() by
     *   this callback and filled with the extra buffers if there are more
     *   buffers than buf[] can hold. extended_buf will be freed in
     *   av_frame_unref().
     *
     * If AV_CODEC_CAP_DR1 is not set then get_buffer2() must call
     * avcodec_default_get_buffer2() instead of providing buffers allocated by
     * some other means.
     *
     * Each data plane must be aligned to the maximum required by the target
     * CPU.
     *
     * @see avcodec_default_get_buffer2()
     *
     * Video:
     *
     * If AV_GET_BUFFER_FLAG_REF is set in flags then the frame may be reused
     * (read and/or written to if it is writable) later by libavcodec.
     *
     * avcodec_align_dimensions2() should be used to find the required width and
     * height, as they normally need to be rounded up to the next multiple of 16.
     *
     * Some decoders do not support linesizes changing between frames.
     *
     * If frame multithreading is used, this callback may be called from a
     * different thread, but not from more than one at once. Does not need to be
     * reentrant.
     *
     * @see avcodec_align_dimensions2()
     *
     * Audio:
     *
     * Decoders request a buffer of a particular size by setting
     * AVFrame.nb_samples prior to calling get_buffer2(). The decoder may,
     * however, utilize only part of the buffer by setting AVFrame.nb_samples
     * to a smaller value in the output frame.
     *
     * As a convenience, av_samples_get_buffer_size() and
     * av_samples_fill_arrays() in libavutil may be used by custom get_buffer2()
     * functions to find the required data size and to fill data pointers and
     * linesize. In AVFrame.linesize, only linesize[0] may be set for audio
     * since all planes must be the same size.
     *
     * @see av_samples_get_buffer_size(), av_samples_fill_arrays()
     *
     * - encoding: unused
     * - decoding: Set by libavcodec, user can override.
     */
    int (*get_buffer2)(struct AVCodecContext *s, AVFrame *frame, int flags);
 
#if FF_API_OLD_ENCDEC
    /**
     * If non-zero, the decoded audio and video frames returned from
     * avcodec_decode_video2() and avcodec_decode_audio4() are reference-counted
     * and are valid indefinitely. The caller must free them with
     * av_frame_unref() when they are not needed anymore.
     * Otherwise, the decoded frames must not be freed by the caller and are
     * only valid until the next decode call.
     *
     * This is always automatically enabled if avcodec_receive_frame() is used.
     *
     * - encoding: unused
     * - decoding: set by the caller before avcodec_open2().
     */
    attribute_deprecated int refcounted_frames;
#endif
 
    /* - encoding parameters */
    float qcompress; ///< amount of qscale change between easy & hard scenes (0.0-1.0)
    float qblur;     ///< amount of qscale smoothing over time (0.0-1.0)
 
    /**
     * minimum quantizer
     * - encoding: Set by user.
     * - decoding: unused
     */
    int qmin;
 
    /**
     * maximum quantizer
     * - encoding: Set by user.
     * - decoding: unused
     */
    int qmax;
 
    /**
     * maximum quantizer difference between frames
     * - encoding: Set by user.
     * - decoding: unused
     */
    int max_qdiff;
 
    /**
     * decoder bitstream buffer size
     * - encoding: Set by user.
     * - decoding: unused
     */
    int rc_buffer_size;
 
    /**
     * ratecontrol override, see RcOverride
     * - encoding: Allocated/set/freed by user.
     * - decoding: unused
     */
    int rc_override_count;
    RcOverride *rc_override;
 
    /**
     * maximum bitrate
     * - encoding: Set by user.
     * - decoding: Set by user, may be overwritten by libavcodec.
     */
    int64_t rc_max_rate;
 
    /**
     * minimum bitrate
     * - encoding: Set by user.
     * - decoding: unused
     */
    int64_t rc_min_rate;
 
    /**
     * Ratecontrol attempt to use, at maximum, <value> of what can be used without an underflow.
     * - encoding: Set by user.
     * - decoding: unused.
     */
    float rc_max_available_vbv_use;
 
    /**
     * Ratecontrol attempt to use, at least, <value> times the amount needed to prevent a vbv overflow.
     * - encoding: Set by user.
     * - decoding: unused.
     */
    float rc_min_vbv_overflow_use;
 
    /**
     * Number of bits which should be loaded into the rc buffer before decoding starts.
     * - encoding: Set by user.
     * - decoding: unused
     */
    int rc_initial_buffer_occupancy;
 
#if FF_API_CODER_TYPE
#define FF_CODER_TYPE_VLC 0
#define FF_CODER_TYPE_AC 1
#define FF_CODER_TYPE_RAW 2
#define FF_CODER_TYPE_RLE 3
    /**
     * @deprecated use encoder private options instead
     */
    attribute_deprecated int coder_type;
#endif /* FF_API_CODER_TYPE */
 
#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated int context_model;
#endif
 
#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated int frame_skip_threshold;
 
    /** @deprecated use encoder private options instead */
    attribute_deprecated int frame_skip_factor;
 
    /** @deprecated use encoder private options instead */
    attribute_deprecated int frame_skip_exp;
 
    /** @deprecated use encoder private options instead */
    attribute_deprecated int frame_skip_cmp;
#endif /* FF_API_PRIVATE_OPT */
 
    /**
     * trellis RD quantization
     * - encoding: Set by user.
     * - decoding: unused
     */
    int trellis;
 
#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated int min_prediction_order;
 
    /** @deprecated use encoder private options instead */
    attribute_deprecated int max_prediction_order;
 
    /** @deprecated use encoder private options instead */
    attribute_deprecated
        int64_t timecode_frame_start;
#endif
 
#if FF_API_RTP_CALLBACK
    /**
     * @deprecated unused
     */
    /* The RTP callback: This function is called    */
    /* every time the encoder has a packet to send. */
    /* It depends on the encoder if the data starts */
    /* with a Start Code (it should). H.263 does.   */
    /* mb_nb contains the number of macroblocks     */
    /* encoded in the RTP payload.                  */
    attribute_deprecated void (*rtp_callback)(struct AVCodecContext *avctx, void *data, int size, int mb_nb);
#endif
 
#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated int rtp_payload_size; /* The size of the RTP payload: the coder will  */
                                               /* do its best to deliver a chunk with size     */
                                               /* below rtp_payload_size, the chunk will start */
                                               /* with a start code on some codecs like H.263. */
                                               /* This doesn't take account of any particular  */
                                               /* headers inside the transmitted RTP payload.  */
#endif
 
#if FF_API_STAT_BITS
    /* statistics, used for 2-pass encoding */
    attribute_deprecated int mv_bits;
    attribute_deprecated int header_bits;
    attribute_deprecated int i_tex_bits;
    attribute_deprecated int p_tex_bits;
    attribute_deprecated int i_count;
    attribute_deprecated int p_count;
    attribute_deprecated int skip_count;
    attribute_deprecated int misc_bits;
 
    /** @deprecated this field is unused */
    attribute_deprecated int frame_bits;
#endif
 
    /**
     * pass1 encoding statistics output buffer
     * - encoding: Set by libavcodec.
     * - decoding: unused
     */
    char *stats_out;
 
    /**
     * pass2 encoding statistics input buffer
     * Concatenated stuff from stats_out of pass1 should be placed here.
     * - encoding: Allocated/set/freed by user.
     * - decoding: unused
     */
    char *stats_in;
 
    /**
     * Work around bugs in encoders which sometimes cannot be detected automatically.
     * - encoding: Set by user
     * - decoding: Set by user
     */
    int workaround_bugs;
#define FF_BUG_AUTODETECT 1 ///< autodetection
#define FF_BUG_XVID_ILACE 4
#define FF_BUG_UMP4 8
#define FF_BUG_NO_PADDING 16
#define FF_BUG_AMV 32
#define FF_BUG_QPEL_CHROMA 64
#define FF_BUG_STD_QPEL 128
#define FF_BUG_QPEL_CHROMA2 256
#define FF_BUG_DIRECT_BLOCKSIZE 512
#define FF_BUG_EDGE 1024
#define FF_BUG_HPEL_CHROMA 2048
#define FF_BUG_DC_CLIP 4096
#define FF_BUG_MS 8192 ///< Work around various bugs in Microsoft's broken decoders.
#define FF_BUG_TRUNCATED 16384
#define FF_BUG_IEDGE 32768
 
    /**
     * strictly follow the standard (MPEG-4, ...).
     * - encoding: Set by user.
     * - decoding: Set by user.
     * Setting this to STRICT or higher means the encoder and decoder will
     * generally do stupid things, whereas setting it to unofficial or lower
     * will mean the encoder might produce output that is not supported by all
     * spec-compliant decoders. Decoders don't differentiate between normal,
     * unofficial and experimental (that is, they always try to decode things
     * when they can) unless they are explicitly asked to behave stupidly
     * (=strictly conform to the specs)
     */
    int strict_std_compliance;
#define FF_COMPLIANCE_VERY_STRICT 2 ///< Strictly conform to an older more strict version of the spec or reference software.
#define FF_COMPLIANCE_STRICT 1      ///< Strictly conform to all the things in the spec no matter what consequences.
#define FF_COMPLIANCE_NORMAL 0
#define FF_COMPLIANCE_UNOFFICIAL -1   ///< Allow unofficial extensions
#define FF_COMPLIANCE_EXPERIMENTAL -2 ///< Allow nonstandardized experimental things.
 
    /**
     * error concealment flags
     * - encoding: unused
     * - decoding: Set by user.
     */
    int error_concealment;
#define FF_EC_GUESS_MVS 1
#define FF_EC_DEBLOCK 2
#define FF_EC_FAVOR_INTER 256
 
    /**
     * debug
     * - encoding: Set by user.
     * - decoding: Set by user.
     */
    int debug;
#define FF_DEBUG_PICT_INFO 1
#define FF_DEBUG_RC 2
#define FF_DEBUG_BITSTREAM 4
#define FF_DEBUG_MB_TYPE 8
#define FF_DEBUG_QP 16
#define FF_DEBUG_DCT_COEFF 0x00000040
#define FF_DEBUG_SKIP 0x00000080
#define FF_DEBUG_STARTCODE 0x00000100
#define FF_DEBUG_ER 0x00000400
#define FF_DEBUG_MMCO 0x00000800
#define FF_DEBUG_BUGS 0x00001000
#define FF_DEBUG_BUFFERS 0x00008000
#define FF_DEBUG_THREADS 0x00010000
#define FF_DEBUG_GREEN_MD 0x00800000
#define FF_DEBUG_NOMC 0x01000000
 
    /**
     * Error recognition; may misdetect some more or less valid parts as errors.
     * - encoding: Set by user.
     * - decoding: Set by user.
     */
    int err_recognition;
 
/**
 * Verify checksums embedded in the bitstream (could be of either encoded or
 * decoded data, depending on the codec) and print an error message on mismatch.
 * If AV_EF_EXPLODE is also set, a mismatching checksum will result in the
 * decoder returning an error.
 */
#define AV_EF_CRCCHECK (1 << 0)
#define AV_EF_BITSTREAM (1 << 1) ///< detect bitstream specification deviations
#define AV_EF_BUFFER (1 << 2)    ///< detect improper bitstream length
#define AV_EF_EXPLODE (1 << 3)   ///< abort decoding on minor error detection
 
#define AV_EF_IGNORE_ERR (1 << 15) ///< ignore errors and continue
#define AV_EF_CAREFUL (1 << 16)    ///< consider things that violate the spec, are fast to calculate and have not been seen in the wild as errors
#define AV_EF_COMPLIANT (1 << 17)  ///< consider all spec non compliances as errors
#define AV_EF_AGGRESSIVE (1 << 18) ///< consider things that a sane encoder should not do as an error
 
    /**
     * opaque 64-bit number (generally a PTS) that will be reordered and
     * output in AVFrame.reordered_opaque
     * - encoding: Set by libavcodec to the reordered_opaque of the input
     *             frame corresponding to the last returned packet. Only
     *             supported by encoders with the
     *             AV_CODEC_CAP_ENCODER_REORDERED_OPAQUE capability.
     * - decoding: Set by user.
     */
    
    int64_t reordered_opaque;
 
    /**
     * Hardware accelerator in use
     * - encoding: unused.
     * - decoding: Set by libavcodec
     */
    // 硬件加速的器正在使用的
    const struct AVHWAccel *hwaccel;
 
    /**
     * Hardware accelerator context.
     * For some hardware accelerators, a global context needs to be
     * provided by the user. In that case, this holds display-dependent
     * data FFmpeg cannot instantiate itself. Please refer to the
     * FFmpeg HW accelerator documentation to know how to fill this
     * is. e.g. for VA API, this is a struct vaapi_context.
     * - encoding: unused
     * - decoding: Set by user
     */
    // 加速器上下文
    void *hwaccel_context;
 
    /**
     * error
     * - encoding: Set by libavcodec if flags & AV_CODEC_FLAG_PSNR.
     * - decoding: unused
     */
    uint64_t error[AV_NUM_DATA_POINTERS];
 
    /**
     * DCT algorithm, see FF_DCT_* below
     * - encoding: Set by user.
     * - decoding: unused
     */
    int dct_algo;
#define FF_DCT_AUTO 0
#define FF_DCT_FASTINT 1
#define FF_DCT_INT 2
#define FF_DCT_MMX 3
#define FF_DCT_ALTIVEC 5
#define FF_DCT_FAAN 6
 
    /**
     * IDCT algorithm, see FF_IDCT_* below.
     * - encoding: Set by user.
     * - decoding: Set by user.
     */
    int idct_algo;
#define FF_IDCT_AUTO 0
#define FF_IDCT_INT 1
#define FF_IDCT_SIMPLE 2
#define FF_IDCT_SIMPLEMMX 3
#define FF_IDCT_ARM 7
#define FF_IDCT_ALTIVEC 8
#define FF_IDCT_SIMPLEARM 10
#define FF_IDCT_XVID

AVStream

其中AVStream是存储每一个视频/音频流信息的结构体


typedef struct AVStream
{
    // 流的索引在
    int index; /**< stream index in AVFormatContext */
    /**
     * Format-specific stream ID.
     * decoding: set by libavformat
     * encoding: set by the user, replaced by libavformat if left unset
     */
    // 流的id
    int id;
#if FF_API_LAVF_AVCTX
    /**
     * @deprecated use the codecpar struct instead
     */
    attribute_deprecated
    // 封装和解码器耦合在一起
        AVCodecContext *codec;
#endif
    // 私有数据
    void *priv_data;
 
    // 时间基
    AVRational time_base;
 
    // 第一帧的显示时间
    int64_t start_time;
 
    // 该视频或者音频的数据长度
    int64_t duration;
    // 数据帧的多长
    int64_t nb_frames; ///< number of frames in this stream if known or 0
    // 去除的数据帧的位置
    int disposition; /**< AV_DISPOSITION_* bit field */
    // 哪一个数据帧被抛弃
    enum AVDiscard discard; ///< Selects which packets can be discarded at will and do not need to be demuxed.
 
    /**
     * sample aspect ratio (0 if unknown)
     * - encoding: Set by user.
     * - decoding: Set by libavformat.
     */
    // 样本比特率
    AVRational sample_aspect_ratio;
    // 元数据
    AVDictionary *metadata;
 
    /**
     * Average framerate
     *
     * - demuxing: May be set by libavformat when creating the stream or in
     *             avformat_find_stream_info().
     * - muxing: May be set by the caller before avformat_write_header().
     */
    AVRational avg_frame_rate;
 
    /**
     * For streams with AV_DISPOSITION_ATTACHED_PIC disposition, this packet
     * will contain the attached picture.
     *
     * decoding: set by libavformat, must not be modified by the caller.
     * encoding: unused
     */
    AVPacket attached_pic;
 
    /**
     * An array of side data that applies to the whole stream (i.e. the
     * container does not allow it to change between packets).
     *
     * There may be no overlap between the side data in this array and side data
     * in the packets. I.e. a given side data is either exported by the muxer
     * (demuxing) / set by the caller (muxing) in this array, then it never
     * appears in the packets, or the side data is exported / sent through
     * the packets (always in the first packet where the value becomes known or
     * changes), then it does not appear in this array.
     *
     * - demuxing: Set by libavformat when the stream is created.
     * - muxing: May be set by the caller before avformat_write_header().
     *
     * Freed by libavformat in avformat_free_context().
     *
     * @see av_format_inject_global_side_data()
     */
    AVPacketSideData *side_data;
    /**
     * The number of elements in the AVStream.side_data array.
     */
    int nb_side_data;
 
    /**
     * Flags indicating events happening on the stream, a combination of
     * AVSTREAM_EVENT_FLAG_*.
     *
     * - demuxing: may be set by the demuxer in avformat_open_input(),
     *   avformat_find_stream_info() and av_read_frame(). Flags must be cleared
     *   by the user once the event has been handled.
     * - muxing: may be set by the user after avformat_write_header(). to
     *   indicate a user-triggered event.  The muxer will clear the flags for
     *   events it has handled in av_[interleaved]_write_frame().
     */
    int event_flags;
/**
 * - demuxing: the demuxer read new metadata from the file and updated
 *     AVStream.metadata accordingly
 * - muxing: the user updated AVStream.metadata and wishes the muxer to write
 *     it into the file
 */
#define AVSTREAM_EVENT_FLAG_METADATA_UPDATED 0x0001
/**
 * - demuxing: new packets for this stream were read from the file. This
 *   event is informational only and does not guarantee that new packets
 *   for this stream will necessarily be returned from av_read_frame().
 */
#define AVSTREAM_EVENT_FLAG_NEW_PACKETS (1 << 1)
 
    /**
     * Real base framerate of the stream.
     * This is the lowest framerate with which all timestamps can be
     * represented accurately (it is the least common multiple of all
     * framerates in the stream). Note, this value is just a guess!
     * For example, if the time base is 1/90000 and all frames have either
     * approximately 3600 or 1800 timer ticks, then r_frame_rate will be 50/1.
     */
    AVRational r_frame_rate;
 
#if FF_API_LAVF_FFSERVER
    /**
     * String containing pairs of key and values describing recommended encoder configuration.
     * Pairs are separated by ','.
     * Keys are separated from values by '='.
     *
     * @deprecated unused
     */
    attribute_deprecated char *recommended_encoder_configuration;
#endif
 
    /**
     * Codec parameters associated with this stream. Allocated and freed by
     * libavformat in avformat_new_stream() and avformat_free_context()
     * respectively.
     *
     * - demuxing: filled by libavformat on stream creation or in
     *             avformat_find_stream_info()
     * - muxing: filled by the caller before avformat_write_header()
     */
    AVCodecParameters *codecpar;
 
    /*****************************************************************
     * All fields below this line are not part of the public API. They
     * may not be used outside of libavformat and can be changed and
     * removed at will.
     * Internal note: be aware that physically removing these fields
     * will break ABI. Replace removed fields with dummy fields, and
     * add new fields to AVStreamInternal.
     *****************************************************************
     */
 
#if LIBAVFORMAT_VERSION_MAJOR < 59
    // kept for ABI compatibility only, do not access in any way
    void *unused;
#endif
 
    int pts_wrap_bits; /**< number of bits in pts (used for wrapping control) */
 
    // Timestamp generation support:
    /**
     * Timestamp corresponding to the last dts sync point.
     *
     * Initialized when AVCodecParserContext.dts_sync_point >= 0 and
     * a DTS is received from the underlying container. Otherwise set to
     * AV_NOPTS_VALUE by default.
     */
    int64_t first_dts;
    int64_t cur_dts;
    int64_t last_IP_pts;
    int last_IP_duration;
 
    /**
     * Number of packets to buffer for codec probing
     */
    int probe_packets;
 
    /**
     * Number of frames that have been demuxed during avformat_find_stream_info()
     */
    int codec_info_nb_frames;
 
    /* av_read_frame() support */
    enum AVStreamParseType need_parsing;
    struct AVCodecParserContext *parser;
 
#if LIBAVFORMAT_VERSION_MAJOR < 59
    // kept for ABI compatibility only, do not access in any way
    void *unused7;
    AVProbeData unused6;
    int64_t unused5[16 + 1];
#endif
    AVIndexEntry *index_entries; /**< Only used if the format does not
                                    support seeking natively. */
    int nb_index_entries;
    unsigned int index_entries_allocated_size;
 
    /**
     * Stream Identifier
     * This is the MPEG-TS stream identifier +1
     * 0 means unknown
     */
    int stream_identifier;
 
#if LIBAVFORMAT_VERSION_MAJOR < 59
    // kept for ABI compatibility only, do not access in any way
    int unused8;
    int unused9;
    int unused10;
#endif
 
    /**
     * An opaque field for libavformat internal usage.
     * Must not be accessed in any way by callers.
     */
    AVStreamInternal *internal;
} AVStream;

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