C/C++音视频库ffmpeg的数据包AVPacket分析

C/C++音视频库ffmpeg的数据包AVPacket分析

AVPacket是ffmpeg用来存放编码后的视频帧数据，我们来分析一下这个结构体，先贴出ffmpeg3.2中AVPacket声明的源代码：

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;

我们依次进行分析

AVBufferRef *buf;

用来存放引用计数的数据，如果没有使用引用计数，值就是NULL，当你多个packet对象引用同一帧数据的时候用到。

int64_t pts;

本帧数据显示的时间，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。

转换方式，比如转为毫秒

AVFormatContext *ic = NULL;

static double r2d(AVRational r)

{

return r.num == 0 || r.den == 0 ? 0. : (double)r.num / (double)r.den;

}

//。。。

int pts = (pkt->pts *r2d(ic->streams[pkt->stream_index]->time_base)) * 1000;

int64_t dts;

基本属性等同于pts，区别就是dts对应的是解码时间不是显示时间，解码后会放入缓冲，比如h264，如果有b帧，则要先解码后面的b帧，再解码之前的帧。

uint8_t *data; int size;

帧的数据和数据大小

int stream_index;

帧数据所属流的索引，用来区分音频，视频，和字幕数据。

int flags;

标志，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;

int side_data_elems;

容器提供的一些附加数据

int64_t duration;

下一帧pts - 当前帧pts ，也就表示两帧时间间隔。

int64_t pos;

当前帧数据在文件中的位置（字节为单位），如果做文件移位用到，如果rtsp就没有此数据。

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