FFmpeg 結構體學習(三)： AVPacket 分析

在上文FFmpeg 結構體學習(二)： AVStream 分析我們學習了AVStream結構體的相關內容。本文，我們將講述一下AVPacket。

AVPacket是存儲壓縮編碼數據相關信息的結構體。下面我們來分析一下該結構體里重要變量的含義和作用。

一、源碼整理

首先我們先看一下結構體AVPacket的定義的結構體源碼(位於libavcodec/avcodec.h)：

typedef struct AVPacket {
    /**
     * 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.
     */
    struct {
        uint8_t *data;
        int      size;
        enum AVPacketSideDataType type;
    } *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;
    void  (*destruct)(struct AVPacket *);
    void  *priv;
    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;

二、AVPacket 重點字段

uint8_t *data：壓縮編碼的數據。

int   size：data的大小

int64_t pts：顯示時間戳

int64_t dts：解碼時間戳

int   stream_index：標識該AVPacket所屬的視頻/音頻流。

針對data做一下說明：對於H.264格式來說，在使用FFMPEG進行視音頻處理的時候，我們常常可以將得到的AVPacket的data數據直接寫成文件，從而得到視音頻的碼流文件。