Create FFmpeg 结构体学习(二)： AVStream 分析.md

FFmpeg 结构体学习(二)： AVStream 分析.md

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+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。
+```
+