diff --git a/FFmpeg 结构体学习(二)： AVStream 分析.md b/FFmpeg 结构体学习(二)： AVStream 分析.md new file mode 100644 index 0000000..dfcb4ee --- /dev/null +++ b/FFmpeg 结构体学习(二)： AVStream 分析.md @@ -0,0 +1,200 @@ +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。 +``` +