技术标签: 视频  ffmpeg  解码  









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;
     * Codec context associated with this stream. Allocated and freed by
     * libavformat.
     * - decoding: The demuxer exports codec information stored in the headers
     *             here.
     * - encoding: The user sets codec information, the muxer writes it to the
     *             output. Mandatory fields as specified in AVCodecContext
     *             documentation must be set even if this AVCodecContext is
     *             not actually used for encoding.
    AVCodecContext *codec;
    void *priv_data;

     * @deprecated this field is unused
    struct AVFrac pts;

     * 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.

     * 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+7+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;
    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.
    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
    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;
} AVStream;

AVRational time_base:时间基本单位的分数表示。如图:

time_base = 1/90000(单位:秒).

time_base的用法:举个例子,在AVFrame中有成员pkt_pts  // PTS copied from the AVPacket that was decoded to produce this frame.即从packet包中解出的pts。(见ffmpeg重要结构体之AVFrame


3600*(1/90000) = 0.04秒,正好是一个帧率为25的视频的两帧间的时间间隔。

AVRational avg_frame_rate:帧率的分数表示。比如24000/1001.


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