ffmpeg之AVPacket

1. AVPacket简介

   AVPacket用于存放压缩后的数据。
    通常是解复用的输出，解码的输入；或者是编码的输出，复用的输入。
    对于视频数据，AVPacket通常包含一帧视频压缩数据；对于音频数据数据，AVPacket则通常包含了数帧的音频压缩数据。
    其大小是public ABI的一部分，因此可以在栈上分配。
    其数据有效期取决于“buf”或“destruct”，如果字段设置了，数据包的管理由ffmpeg负责，数据包在avt_free_packet（内部调用avt_buffer_unref）调用前都有效；如果字段未设置，数据包由调用者管理。

2. AVPacket定义

typedef struct AVPacket {
    /**
     * A reference to the reference-counted buffer where the packet data is
     * stored.
     * May be NULL, then the packet data is not reference-counted.
     */
    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.
     */
    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.
     */
    int64_t dts;
    uint8_t* data;
    int size;
    int stream_index;
    /**
     * A combination of AV_PKT_FLAG values
     */
    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;

    /**
     * Duration of this packet in AVStream->time_base units, 0 if unknown.
     * Equals next_pts - this_pts in presentation order.
     */
    int duration;
#if FF_API_DESTRUCT_PACKET
    attribute_deprecated
    void (*destruct)(struct AVPacket *);
    attribute_deprecated
    void* priv;
#endif
    int64_t pos; ///< byte position in stream, -1 if unknown

    /**
     * Time difference in AVStream->time_base units from the pts of this
     * packet to the point at which the output from the decoder has converged
     * independent from the availability of previous frames. That is, the
     * frames are virtually identical no matter if decoding started from
     * the very first frame or from this keyframe.
     * Is AV_NOPTS_VALUE if unknown.
     * This field is not the display duration of the current packet.
     * This field has no meaning if the packet does not have AV_PKT_FLAG_KEY
     * set.
     *
     * The purpose of this field is to allow seeking in streams that have no
     * keyframes in the conventional sense. It corresponds to the
     * recovery point SEI in H.264 and match_time_delta in NUT. It is also
     * essential for some types of subtitle streams to ensure that all
     * subtitles are correctly displayed after seeking.
     */
    int64_t convergence_duration;
} AVPacket;

3. AVPacket字段说明

    AVBufferRef* buf ： data的引用计数结构体，可能为NULL。不为NULL时，数据管理由ffmpeg负责，为NULL时由调用者自己负责。
    int64_t pts ： 压缩数据显示时间戳。比较关键的数据，在做seek和播放进度的时候都要用到它，pts只是一个数量，对应于AVStream->time_base，要根据time_base才能转换为具体的时间,音频和视频一般有不同的time_base，所以在做音视频同步一定要做转换，不能直接拿pts做。
    int64_t dts ： 压缩数据解码时间戳。
    uint8_t* data ：压缩数据。
    int   size ：压缩数据大小。
    int   stream_index ：所属媒体流的索引。用来区分音频，视频，和字幕数据。
    int   flags ： 标识域。其中为1表示该数据是一个关键帧，AV_PKT_FLAG_KEY 0x0001 关键帧
    AVPacketSideData* side_data ：附加数据。
    int side_data_elems ：附加数据大小。
    int   duration ：压缩数据时长。
    void  (*destruct)(struct AVPacket *) ：压缩数据释放函数指针。
    void* priv  ：
    int64_t pos ：压缩数据在媒体流中的偏移量，未知为-1。
    int64_t convergence_duration ：