AVStream
该结构体描述一个媒体流，定义如下：
typedefstructAVStream {
int index; /**< stream index in AVFormatContext */
int id; /**< format-specific stream 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.
*/
AVRationalr_frame_rate;
......
/**
* 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.
*/
AVRationaltime_base;
......
/**
* Decoding: pts of the first frame of the stream, 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;
#if LIBAVFORMAT_VERSION_INT < (53<<16)
char language[4]; /** ISO 639-2/B 3-letter language code (empty string if undefined) */
#endif
/* av_read_frame() support */
enumAVStreamParseTypeneed_parsing;
structAVCodecParserContext *parser;
......
/* av_seek_frame() support */
AVIndexEntry *index_entries; /**< Only used if the format does not
support seeking natively. */
intnb_index_entries;
unsigned intindex_entries_allocated_size;
int64_t nb_frames; ///< number of frames in this stream if known or 0
......
/**
* Average framerate
*/
AVRationalavg_frame_rate;
......
} AVStream;
主要域的释义如下，其中大部分域的值可以由avformat_open_input根据文件头的信息确定，缺少的信息需要通过调用avformat_find_stream_info读帧及软解码进一步获取：
index/id：index对应流的索引，这个数字是自动生成的，根据index可以从AVFormatContext::streams表中索引到该流；而id则是流的标识，依赖于具体的容器格式。比如对于MPEG TS格式，id就是pid。
time_base：流的时间基准，是一个实数，该流中媒体数据的pts和dts都将以这个时间基准为粒度。通常，使用av_rescale/av_rescale_q可以实现不同时间基准的转换。
start_time：流的起始时间，以流的时间基准为单位，通常是该流中第一个帧的pts。
duration：流的总时间，以流的时间基准为单位。
need_parsing：对该流parsing过程的控制域。
nb_frames：流内的帧数目。
r_frame_rate/framerate/avg_frame_rate：帧率相关。
codec：指向该流对应的AVCodecContext结构，调用avformat_open_input时生成。
parser：指向该流对应的AVCodecParserContext结构，调用avformat_find_stream_info时生成。