ffmpeg重要结构体之AVStream

ffmpeg重要结构体之AVFrame

ffmpeg重要结构体之AVFormatContext

ffmpeg重要结构体之AVCodecContext

ffmpeg重要结构体之AVCodec

ffmpeg重要结构体之AVIOContext

ffmpeg重要结构体之AVStream

ffmpeg重要结构体之AVPacket

AVStream结构体存储有关流（视频流或者音频流）的信息。此定义位于libavformat/avformat.h中。

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;#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.    /*****************************************************************     * 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;#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;} AVStream;

有关视频解码的主要成员：

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）

某一帧图像的pkt_pts为2965399930，下一帧图像的pkt_pts为2965403530，两者之差为3600，其单位就是time_base。

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.

其他相关的成员仍需要进一步研究