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200 lines
6.1 KiB
200 lines
6.1 KiB
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。
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# 一、源码整理
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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)：
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```
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/**
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* Stream structure.
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* New fields can be added to the end with minor version bumps.
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* Removal, reordering and changes to existing fields require a major
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* version bump.
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* sizeof(AVStream) must not be used outside libav*.
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*/
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typedef struct AVStream {
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int index; /**< stream index in AVFormatContext */
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/**
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* Format-specific stream ID.
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* decoding: set by libavformat
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* encoding: set by the user
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*/
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int id;
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AVCodecContext *codec; /**< codec context */
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/**
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* Real base framerate of the stream.
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* This is the lowest framerate with which all timestamps can be
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* represented accurately (it is the least common multiple of all
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* framerates in the stream). Note, this value is just a guess!
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* For example, if the time base is 1/90000 and all frames have either
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* approximately 3600 or 1800 timer ticks, then r_frame_rate will be 50/1.
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*/
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AVRational r_frame_rate;
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void *priv_data;
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/**
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* encoding: pts generation when outputting stream
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*/
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struct AVFrac pts;
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/**
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* This is the fundamental unit of time (in seconds) in terms
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* of which frame timestamps are represented. For fixed-fps content,
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* time base should be 1/framerate and timestamp increments should be 1.
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* decoding: set by libavformat
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* encoding: set by libavformat in av_write_header
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*/
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AVRational time_base;
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/**
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* Decoding: pts of the first frame of the stream in presentation order, in stream time base.
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* Only set this if you are absolutely 100% sure that the value you set
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* it to really is the pts of the first frame.
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* This may be undefined (AV_NOPTS_VALUE).
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* @note The ASF header does NOT contain a correct start_time the ASF
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* demuxer must NOT set this.
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*/
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int64_t start_time;
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/**
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* Decoding: duration of the stream, in stream time base.
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* If a source file does not specify a duration, but does specify
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* a bitrate, this value will be estimated from bitrate and file size.
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*/
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int64_t duration;
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int64_t nb_frames; ///< number of frames in this stream if known or 0
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int disposition; /**< AV_DISPOSITION_* bit field */
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enum AVDiscard discard; ///< Selects which packets can be discarded at will and do not need to be demuxed.
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/**
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* sample aspect ratio (0 if unknown)
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* - encoding: Set by user.
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* - decoding: Set by libavformat.
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*/
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AVRational sample_aspect_ratio;
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AVDictionary *metadata;
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/**
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* Average framerate
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*/
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AVRational avg_frame_rate;
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/**
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* For streams with AV_DISPOSITION_ATTACHED_PIC disposition, this packet
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* will contain the attached picture.
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*
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* decoding: set by libavformat, must not be modified by the caller.
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* encoding: unused
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*/
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AVPacket attached_pic;
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/*****************************************************************
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* All fields below this line are not part of the public API. They
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* may not be used outside of libavformat and can be changed and
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* removed at will.
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* New public fields should be added right above.
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*****************************************************************
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*/
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/**
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* Stream information used internally by av_find_stream_info()
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*/
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#define MAX_STD_TIMEBASES (60*12+5)
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struct {
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int64_t last_dts;
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int64_t duration_gcd;
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int duration_count;
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double duration_error[2][2][MAX_STD_TIMEBASES];
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int64_t codec_info_duration;
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int nb_decoded_frames;
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int found_decoder;
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} *info;
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int pts_wrap_bits; /**< number of bits in pts (used for wrapping control) */
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// Timestamp generation support:
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/**
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* Timestamp corresponding to the last dts sync point.
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*
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* Initialized when AVCodecParserContext.dts_sync_point >= 0 and
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* a DTS is received from the underlying container. Otherwise set to
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* AV_NOPTS_VALUE by default.
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*/
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int64_t reference_dts;
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int64_t first_dts;
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int64_t cur_dts;
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int64_t last_IP_pts;
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int last_IP_duration;
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/**
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* Number of packets to buffer for codec probing
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*/
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#define MAX_PROBE_PACKETS 2500
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int probe_packets;
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/**
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* Number of frames that have been demuxed during av_find_stream_info()
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*/
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int codec_info_nb_frames;
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/**
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* Stream Identifier
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* This is the MPEG-TS stream identifier +1
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* 0 means unknown
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*/
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int stream_identifier;
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int64_t interleaver_chunk_size;
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int64_t interleaver_chunk_duration;
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/* av_read_frame() support */
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enum AVStreamParseType need_parsing;
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struct AVCodecParserContext *parser;
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/**
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* last packet in packet_buffer for this stream when muxing.
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*/
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struct AVPacketList *last_in_packet_buffer;
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AVProbeData probe_data;
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#define MAX_REORDER_DELAY 16
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int64_t pts_buffer[MAX_REORDER_DELAY+1];
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AVIndexEntry *index_entries; /**< Only used if the format does not
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support seeking natively. */
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int nb_index_entries;
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unsigned int index_entries_allocated_size;
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/**
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* flag to indicate that probing is requested
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* NOT PART OF PUBLIC API
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*/
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int request_probe;
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} AVStream;
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```
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# 二、AVStream 重点字段
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```
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int index：标识该视频/音频流
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AVCodecContext *codec：指向该视频/音频流的AVCodecContext（它们是一一对应的关系）
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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
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int64_t duration：该视频/音频流长度
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AVDictionary *metadata：元数据信息
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AVRational avg_frame_rate：帧率（注：对视频来说，这个挺重要的）
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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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```
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