文章目录

• 一、源码整理
• 二、AVStream 重点字段

AVStream是存储每一个视频/音频流信息的结构体。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AVStream的定义的结构体源码(位于libavformat/avformat.h)：

/*** Stream structure.* New fields can be added to the end with minor version bumps.* Removal, reordering and changes to existing fields require a major* version bump.* sizeof(AVStream) must not be used outside libav*.*/
typedef struct AVStream {int index;    /**< stream index in AVFormatContext *//*** Format-specific stream ID.* decoding: set by libavformat* encoding: set by the user*/int id;AVCodecContext *codec; /**< codec context *//*** Real base framerate of the stream.* This is the lowest framerate with which all timestamps can be* represented accurately (it is the least common multiple of all* framerates in the stream). Note, this value is just a guess!* For example, if the time base is 1/90000 and all frames have either* approximately 3600 or 1800 timer ticks, then r_frame_rate will be 50/1.*/AVRational r_frame_rate;void *priv_data;/*** encoding: pts generation when outputting stream*/struct AVFrac pts;/*** This is the fundamental unit of time (in seconds) in terms* of which frame timestamps are represented. For fixed-fps content,* time base should be 1/framerate and timestamp increments should be 1.* decoding: set by libavformat* encoding: set by libavformat in av_write_header*/AVRational time_base;/*** Decoding: pts of the first frame of the stream in presentation order, in stream time base.* Only set this if you are absolutely 100% sure that the value you set* it to really is the pts of the first frame.* This may be undefined (AV_NOPTS_VALUE).* @note The ASF header does NOT contain a correct start_time the ASF* demuxer must NOT set this.*/int64_t start_time;/*** Decoding: duration of the stream, in stream time base.* If a source file does not specify a duration, but does specify* a bitrate, this value will be estimated from bitrate and file size.*/int64_t duration;int64_t nb_frames;                 ///< number of frames in this stream if known or 0int disposition; /**< AV_DISPOSITION_* bit field */enum AVDiscard discard; ///< Selects which packets can be discarded at will and do not need to be demuxed./*** sample aspect ratio (0 if unknown)* - encoding: Set by user.* - decoding: Set by libavformat.*/AVRational sample_aspect_ratio;AVDictionary *metadata;/*** Average framerate*/AVRational avg_frame_rate;/*** For streams with AV_DISPOSITION_ATTACHED_PIC disposition, this packet* will contain the attached picture.** decoding: set by libavformat, must not be modified by the caller.* encoding: unused*/AVPacket attached_pic;/****************************************************************** All fields below this line are not part of the public API. They* may not be used outside of libavformat and can be changed and* removed at will.* New public fields should be added right above.******************************************************************//*** Stream information used internally by av_find_stream_info()*/
#define MAX_STD_TIMEBASES (60*12+5)struct {int64_t last_dts;int64_t duration_gcd;int duration_count;double duration_error[2][2][MAX_STD_TIMEBASES];int64_t codec_info_duration;int nb_decoded_frames;int found_decoder;} *info;int pts_wrap_bits; /**< number of bits in pts (used for wrapping control) */// Timestamp generation support:/*** Timestamp corresponding to the last dts sync point.** Initialized when AVCodecParserContext.dts_sync_point >= 0 and* a DTS is received from the underlying container. Otherwise set to* AV_NOPTS_VALUE by default.*/int64_t reference_dts;int64_t first_dts;int64_t cur_dts;int64_t last_IP_pts;int last_IP_duration;/*** Number of packets to buffer for codec probing*/
#define MAX_PROBE_PACKETS 2500int probe_packets;/*** Number of frames that have been demuxed during av_find_stream_info()*/int codec_info_nb_frames;/*** Stream Identifier* This is the MPEG-TS stream identifier +1* 0 means unknown*/int stream_identifier;int64_t interleaver_chunk_size;int64_t interleaver_chunk_duration;/* av_read_frame() support */enum AVStreamParseType need_parsing;struct AVCodecParserContext *parser;/*** last packet in packet_buffer for this stream when muxing.*/struct AVPacketList *last_in_packet_buffer;AVProbeData probe_data;
#define MAX_REORDER_DELAY 16int64_t pts_buffer[MAX_REORDER_DELAY+1];AVIndexEntry *index_entries; /**< Only used if the format does notsupport seeking natively. */int nb_index_entries;unsigned int index_entries_allocated_size;/*** flag to indicate that probing is requested* NOT PART OF PUBLIC API*/int request_probe;
} AVStream;

二、AVStream 重点字段

int index：标识该视频/音频流AVCodecContext *codec：指向该视频/音频流的AVCodecContext（它们是一一对应的关系）AVRational time_base：时基。通过该值可以把PTS，DTS转化为真正的时间。FFMPEG其他结构体中也有这个字段，但是根据我的经验，只有AVStream中的time_base是可用的。
PTS*time_base=真正的时间int64_t duration：该视频/音频流长度AVDictionary *metadata：元数据信息AVRational avg_frame_rate：帧率（注：对视频来说，这个挺重要的）AVPacket attached_pic：附带的图片。比如说一些MP3，AAC音频文件附带的专辑封面。

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