FFmpeg 结构体学习(二): AVStream 分析

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。




 * 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;
    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
    int request_probe;
} AVStream;
View Code

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。
*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。


posted @ 2018-08-14 16:27  灰色飘零  阅读(1237)  评论(0编辑  收藏  举报