在上文FFmpeg 结构体学习(二): AVStream 分析我们学习了AVStream结构体的相关内容。本文,我们将讲述一下AVPacket。
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。