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ffmpeg AVPacket结构体及其相关函数

2019-08-16 07:56:18来源：博客园阅读 ()

ffmpeg AVPacket结构体及其相关函数

0. 简介

AVPacket结构体并不是很复杂, 但是在ffmpeg中用的非常多. 与其相关的函数也是比较多.

AVPacket保存了解复用之后, 解码之前的数据, 和这些数据相关的一些附加信息.

对于视频来说, AVPacket通常包含一个压缩的frame; 而音频可能包含多个压缩的frame. 一个packet也可能是空的, 不包含任何压缩数据data, 而只包含附加数据side data(容器提供的关于packet的一些附加信息, 例如: 在编码结束的时候更新一些流的参数)

AVPacket是FFmpeg中为数不多的结构之一, 其大小是公共abi的一部分, 因此它可以在堆栈上分配, 如果libavcodec和libavformat没有很大的改动, 就不会向其中添加任何新字段.

官方文档:

AVPacket is one of the few structs in FFmpeg,whose size is a part of public ABI.Thus it may be allocated on stack and no new fields can be added to it without libavcodec and libavformat major bump.

1. AVPacket数据结构定义

ffmpeg 版本3.4.1, struct AVPacket定义于<libavcodec/avcodec.h>

struct AVPacket结构体源码:

 1 typedef struct AVPacket {
 2     /**
 3      * A reference to the reference-counted buffer where the packet data is
 4      * stored.
 5      * May be NULL, then the packet data is not reference-counted.
 6      */
 7     AVBufferRef *buf;
 8     /**
 9      * Presentation timestamp in AVStream->time_base units; the time at which
10      * the decompressed packet will be presented to the user.
11      * Can be AV_NOPTS_VALUE if it is not stored in the file.
12      * pts MUST be larger or equal to dts as presentation cannot happen before
13      * decompression, unless one wants to view hex dumps. Some formats misuse
14      * the terms dts and pts/cts to mean something different. Such timestamps
15      * must be converted to true pts/dts before they are stored in AVPacket.
16      */
17     int64_t pts;
18     /**
19      * Decompression timestamp in AVStream->time_base units; the time at which
20      * the packet is decompressed.
21      * Can be AV_NOPTS_VALUE if it is not stored in the file.
22      */
23     int64_t dts;
24     uint8_t *data;
25     int   size;
26     int   stream_index;
27     /**
28      * A combination of AV_PKT_FLAG values
29      */
30     int   flags;
31     /**
32      * Additional packet data that can be provided by the container.
33      * Packet can contain several types of side information.
34      */
35     AVPacketSideData *side_data;
36     int side_data_elems;
37 
38     /**
39      * Duration of this packet in AVStream->time_base units, 0 if unknown.
40      * Equals next_pts - this_pts in presentation order.
41      */
42     int64_t duration;
43 
44     int64_t pos;                            ///< byte position in stream, -1 if unknown
45 
46 #if FF_API_CONVERGENCE_DURATION
47     /**
48      * @deprecated Same as the duration field, but as int64_t. This was required
49      * for Matroska subtitles, whose duration values could overflow when the
50      * duration field was still an int.
51      */
52     attribute_deprecated
53     int64_t convergence_duration;
54 #endif
55 } AVPacket;