Create FFmpeg 结构体学习(三)： AVPacket 分析.md

FFmpeg 结构体学习(三)： AVPacket 分析.md

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+AVPacket是存储压缩编码数据相关信息的结构体。下面我们来分析一下该结构体里重要变量的含义和作用。
+
+# 一、源码整理
+
+首先我们先看一下结构体AVPacket的定义的结构体源码(位于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 重点字段
+
+
+
+```
+uint8_t *data：压缩编码的数据。
+
+int   size：data的大小
+
+int64_t pts：显示时间戳
+
+int64_t dts：解码时间戳
+
+int   stream_index：标识该AVPacket所属的视频/音频流。
+```
+
+
+
+针对data做一下说明：对于H.264格式来说，在使用FFMPEG进行视音频处理的时候，我们常常可以将得到的AVPacket的data数据直接写成文件，从而得到视音频的码流文件。