FFmpeg结构体：AVStream

1.描述

AVStream是存储每一个视频/音频流信息的结构体，位于avformat.h文件中。

2.结构体定义

typedef struct AVStream {
    int index;    /**< stream index in AVFormatContext */
    /**
     * Format-specific stream ID.
     * decoding: set by libavformat
     * encoding: set by the user, replaced by libavformat if left unset
     */
    int id;
#if FF_API_LAVF_AVCTX
    /**
     * @deprecated use the codecpar struct instead
     */
    attribute_deprecated
    AVCodecContext *codec;
#endif
    void *priv_data;

#if FF_API_LAVF_FRAC
    /**
     * @deprecated this field is unused
     */
    attribute_deprecated
    struct AVFrac pts;
#endif

    /**
     * This is the fundamental unit of time (in seconds) in terms
     * of which frame timestamps are represented.
     *
     * decoding: set by libavformat
     * encoding: May be set by the caller before avformat_write_header() to
     *           provide a hint to the muxer about the desired timebase. In
     *           avformat_write_header(), the muxer will overwrite this field
     *           with the timebase that will actually be used for the timestamps
     *           written into the file (which may or may not be related to the
     *           user-provided one, depending on the format).
     */
    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
     *
     * - demuxing: May be set by libavformat when creating the stream or in
     *             avformat_find_stream_info().
     * - muxing: May be set by the caller before avformat_write_header().
     */
    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;

    /**
     * An array of side data that applies to the whole stream (i.e. the
     * container does not allow it to change between packets).
     *
     * There may be no overlap between the side data in this array and side data
     * in the packets. I.e. a given side data is either exported by the muxer
     * (demuxing) / set by the caller (muxing) in this array, then it never
     * appears in the packets, or the side data is exported / sent through
     * the packets (always in the first packet where the value becomes known or
     * changes), then it does not appear in this array.
     *
     * - demuxing: Set by libavformat when the stream is created.
     * - muxing: May be set by the caller before avformat_write_header().
     *
     * Freed by libavformat in avformat_free_context().
     *
     * @see av_format_inject_global_side_data()
     */
    AVPacketSideData *side_data;
    /**
     * The number of elements in the AVStream.side_data array.
     */
    int            nb_side_data;

    /**
     * Flags for the user to detect events happening on the stream. Flags must
     * be cleared by the user once the event has been handled.
     * A combination of AVSTREAM_EVENT_FLAG_*.
     */
    int event_flags;
#define AVSTREAM_EVENT_FLAG_METADATA_UPDATED 0x0001 ///< The call resulted in updated metadata.

    /*
     * Codec parameters associated with this stream. Allocated and freed by
     * libavformat in avformat_new_stream() and avformat_free_context()
     * respectively.
     *
     * - demuxing: filled by libavformat on stream creation or in
     *             avformat_find_stream_info()
     * - muxing: filled by the caller before avformat_write_header()
     */
    AVCodecParameters *codecpar;

    /*****************************************************************
     * 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 (30*12+30+3+6)
    struct {
        int64_t last_dts;
        int64_t duration_gcd;
        int duration_count;
        int64_t rfps_duration_sum;
        double (*duration_error)[2][MAX_STD_TIMEBASES];
        int64_t codec_info_duration;
        int64_t codec_info_duration_fields;

        /**
         * 0  -> decoder has not been searched for yet.
         * >0 -> decoder found
         * <0 -> decoder with codec_id == -found_decoder has not been found
         */
        int found_decoder;

        int64_t last_duration;

        /**
         * Those are used for average framerate estimation.
         */
        int64_t fps_first_dts;
        int     fps_first_dts_idx;
        int64_t fps_last_dts;
        int     fps_last_dts_idx;

    } *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 first_dts;
    int64_t cur_dts;
    int64_t last_IP_pts;
    int last_IP_duration;

    /**
     * Number of packets to buffer for codec probing
     */
    int probe_packets;

    /**
     * Number of frames that have been demuxed during av_find_stream_info()
     */
    int codec_info_nb_frames;

    /* 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;

    /**
     * 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.
     *
     * Code outside avformat should access this field using:
     * av_stream_get/set_r_frame_rate(stream)
     */
    AVRational r_frame_rate;

    /**
     * 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;

    /**
     * stream probing state
     * -1   -> probing finished
     *  0   -> no probing requested
     * rest -> perform probing with request_probe being the minimum score to accept.
     * NOT PART OF PUBLIC API
     */
    int request_probe;
    /**
     * Indicates that everything up to the next keyframe
     * should be discarded.
     */
    int skip_to_keyframe;

    /**
     * Number of samples to skip at the start of the frame decoded from the next packet.
     */
    int skip_samples;

    /**
     * If not 0, the number of samples that should be skipped from the start of
     * the stream (the samples are removed from packets with pts==0, which also
     * assumes negative timestamps do not happen).
     * Intended for use with formats such as mp3 with ad-hoc gapless audio
     * support.
     */
    int64_t start_skip_samples;

    /**
     * If not 0, the first audio sample that should be discarded from the stream.
     * This is broken by design (needs global sample count), but can't be
     * avoided for broken by design formats such as mp3 with ad-hoc gapless
     * audio support.
     */
    int64_t first_discard_sample;

    /**
     * The sample after last sample that is intended to be discarded after
     * first_discard_sample. Works on frame boundaries only. Used to prevent
     * early EOF if the gapless info is broken (considered concatenated mp3s).
     */
    int64_t last_discard_sample;

    /**
     * Number of internally decoded frames, used internally in libavformat, do not access
     * its lifetime differs from info which is why it is not in that structure.
     */
    int nb_decoded_frames;

    /**
     * Timestamp offset added to timestamps before muxing
     * NOT PART OF PUBLIC API
     */
    int64_t mux_ts_offset;

    /**
     * Internal data to check for wrapping of the time stamp
     */
    int64_t pts_wrap_reference;

    /**
     * Options for behavior, when a wrap is detected.
     *
     * Defined by AV_PTS_WRAP_ values.
     *
     * If correction is enabled, there are two possibilities:
     * If the first time stamp is near the wrap point, the wrap offset
     * will be subtracted, which will create negative time stamps.
     * Otherwise the offset will be added.
     */
    int pts_wrap_behavior;

    /**
     * Internal data to prevent doing update_initial_durations() twice
     */
    int update_initial_durations_done;

    /**
     * Internal data to generate dts from pts
     */
    int64_t pts_reorder_error[MAX_REORDER_DELAY+1];
    uint8_t pts_reorder_error_count[MAX_REORDER_DELAY+1];

    /**
     * Internal data to analyze DTS and detect faulty mpeg streams
     */
    int64_t last_dts_for_order_check;
    uint8_t dts_ordered;
    uint8_t dts_misordered;

    /**
     * Internal data to inject global side data
     */
    int inject_global_side_data;

    /**
     * String containing paris of key and values describing recommended encoder configuration.
     * Paris are separated by ','.
     * Keys are separated from values by '='.
     */
    char *recommended_encoder_configuration;

    /**
     * display aspect ratio (0 if unknown)
     * - encoding: unused
     * - decoding: Set by libavformat to calculate sample_aspect_ratio internally
     */
    AVRational display_aspect_ratio;

    struct FFFrac *priv_pts;

    /**
     * An opaque field for libavformat internal usage.
     * Must not be accessed in any way by callers.
     */
    AVStreamInternal *internal;
} AVStream;

3.常见变量及其作用

int index; //在AVFormatContext中的索引，这个数字是自动生成的，可以通过这个数字从AVFormatContext::streams表中索引到该流。
int id;//流的标识，依赖于具体的容器格式。解码：由libavformat设置。编码：由用户设置，如果未设置则由libavformat替换。
AVCodecContext *codec;//指向该流对应的AVCodecContext结构，调用avformat_open_input时生成。
AVRational time_base;//这是表示帧时间戳的基本时间单位（以秒为单位）。该流中媒体数据的pts和dts都将以这个时间基准为粒度。
int64_t start_time;//流的起始时间，以流的时间基准为单位。如需设置，100％确保你设置它的值真的是第一帧的pts。
int64_t duration;//解码：流的持续时间。如果源文件未指定持续时间，但指定了比特率，则将根据比特率和文件大小估计该值。
int64_t nb_frames; //此流中的帧数（如果已知）或0。
enum AVDiscard discard;//选择哪些数据包可以随意丢弃，不需要去demux。
AVRational sample_aspect_ratio;//样本长宽比（如果未知，则为0）。
AVDictionary *metadata;//元数据信息。
AVRational avg_frame_rate;//平均帧速率。解封装：可以在创建流时设置为libavformat，也可以在avformat_find_stream_info（）中设置。封装：可以由调用者在avformat_write_header（）之前设置。
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。
int probe_packets;//编解码器用于probe的包的个数。
int codec_info_nb_frames;//在av_find_stream_info（）期间已经解封装的帧数。
int request_probe;//流探测状态，1表示探测完成，0表示没有探测请求，rest 执行探测。
int skip_to_keyframe;//表示应丢弃直到下一个关键帧的所有内容。
int skip_samples;//在从下一个数据包解码的帧开始时要跳过的采样数。
int64_t start_skip_samples;//如果不是0，则应该从流的开始跳过的采样的数目。
int64_t first_discard_sample;//如果不是0，则应该从流中丢弃第一个音频样本。

int64_t pts_reorder_error[MAX_REORDER_DELAY+1];
uint8_t pts_reorder_error_count[MAX_REORDER_DELAY+1];//内部数据，从pts生成dts。

int64_t last_dts_for_order_check;
uint8_t dts_ordered;
uint8_t dts_misordered;//内部数据，用于分析dts和检测故障mpeg流。
AVRational display_aspect_ratio;//显示宽高比。