在上文FFmpeg 结构体学习(一)： AVFormatContext 分析我们学习了AVFormatContext结构体的相关内容。本文，我们将讲述一下AVStream。
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 0 int 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 2500 int 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 16 int64_t pts_buffer[MAX_REORDER_DELAY+1]; AVIndexEntry *index_entries; /**< Only used if the format does not support 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。
