1.描述

AVPacket用于存儲壓縮數據，位于avcodec.h文件中。
通常由demuxer導出，然后作為輸入傳遞給解碼器；或者作為編碼器的輸出,然后傳遞給muxer。
對于視頻，它通常應包含一個壓縮幀。 對于音頻，它可能包含幾個壓縮幀。 允許編碼器輸出空分組，沒有壓縮數據，僅包含輔助數據（例如，在編碼結束時更新一些參數）。

2.結構體定義

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;

3.常見變量及其作用

AVPacket這個結構體變量分析：

AVBufferRef *buf; //緩沖包數據存儲位置。
int64_t pts;           //顯示時間戳，以AVStream->time_base為單位。
int64_t dts;        //解碼時間戳，以AVStream->time_base為單位。
uint8_t *data;     //壓縮編碼的數據。
int   size;//data的大小。
int   stream_index;//給出所屬媒體流的索引。
int   flags;//標識域，其中，最低位置1表示該數據是一個關鍵幀。
AVPacketSideData *side_data;//容器提供的額外數據包。
int64_t duration;//Packet持續的時間，以AVStream->time_base為單位。
int64_t pos;//表示該數據在媒體流中的字節偏移量。

AVPacket結構本身只是一個容器，它使用data成員引用實際的數據緩沖區。這個緩沖區通常是由av_new_packet創建的，但也可能由FFmpeg的API創建（如av_read_frame）。當某個AVPacket結構的數據緩沖區不再被使用時，要需要通過調用av_free_packet釋放。
這個結構體在FFmpeg中非常常見，需要重點掌握。