AVPacket结构体

2020-12-20

AVPacket是存储压缩编码数据相关信息的结构体。AVPacket是FFmpeg中很重要的一个结构体，它保存了压缩后的数据以及这些数据的信息，比如显示时间戳pts,解码时间戳dts,每一帧持续的时间duration以及媒体流的索引等。

对于视频而言一个AVPacket通常是一个压缩的AVFrame，而一个音频AVPacket可能包含若干个AVFrame。当然也有可能AVPacket是一个空的，不包含任何数据仅仅包含sider data。（例如在编码结束时更新一些流的参数）。根据官方文档，AVPacket的大小是公开ABI的一部分，这样的结构体在FFmpeg中很少，因此它可能在栈中分配空间，并且如果libavcodec和libavformat没有较大的改动，那么AVPacket将不会添加新的字段。

AVPacket的定义如下（在libavcodec/packet.h文件中定义）：

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.
     */
    int64_t duration;

    int64_t pos;                            ///< byte position in stream, -1 if unknown

#if FF_API_CONVERGENCE_DURATION
    /**
     * @deprecated Same as the duration field, but as int64_t. This was required
     * for Matroska subtitles, whose duration values could overflow when the
     * duration field was still an int.
     */
    attribute_deprecated
    int64_t convergence_duration;
#endif
} AVPacket;

AVPacket相比其他的结构体而言简单，但却是非常常用的一个。在AVPacket结构体中，重要的变量有以下几个：

AVBufferRef *buf; 指向引用的数据，如果如NULL,那么packet中的data没被使用
int64_t pts; 显示时间戳
int64_t dts; 解码时间戳
uint8_t *data;压缩的编码数据
int size;data的大小
int stream_index; packet所在的流索引
int flags; 标志，其中最低为1表示该数据是一个关键帧
AVPacketSideData *side_data;Packet是一个容器，此字段说明额外的数据可以被packet提供
int64_t duration;数据的持续时长
int64_t pos; 数据在流中位置，-1表示未知