typedef struct VideoState {
    SDL_Thread *read_tid;           // demux解复用线程
    AVInputFormat *iformat;
    int abort_request;
    int force_refresh;
    int paused;
    int last_paused;
    int queue_attachments_req;
    int seek_req;                   // 标识一次SEEK请求
    int seek_flags;                 // SEEK标志，诸如AVSEEK_FLAG_BYTE等
    int64_t seek_pos;               // SEEK的目标位置(当前位置+增量)
    int64_t seek_rel;               // 本次SEEK的位置增量
    int read_pause_return;
    AVFormatContext *ic;
    int realtime;

    Clock audclk;                   // 音频时钟
    Clock vidclk;                   // 视频时钟
    Clock extclk;                   // 外部时钟

    FrameQueue pictq;               // 视频frame队列
    FrameQueue subpq;               // 字幕frame队列
    FrameQueue sampq;               // 音频frame队列

    Decoder auddec;                 // 音频解码器
    Decoder viddec;                 // 视频解码器
    Decoder subdec;                 // 字幕解码器

    int audio_stream;               // 音频流索引

    int av_sync_type;

    double audio_clock;             // 每个音频帧更新一下此值，以pts形式表示
    int audio_clock_serial;         // 播放序列，seek可改变此值
    double audio_diff_cum; /* used for AV difference average computation */
    double audio_diff_avg_coef;
    double audio_diff_threshold;
    int audio_diff_avg_count;
    AVStream *audio_st;             // 音频流
    PacketQueue audioq;             // 音频packet队列
    int audio_hw_buf_size;          // SDL音频缓冲区大小(单位字节)
    uint8_t *audio_buf;             // 指向待播放的一帧音频数据，指向的数据区将被拷入SDL音频缓冲区。若经过重采样则指向audio_buf1，否则指向frame中的音频
    uint8_t *audio_buf1;            // 音频重采样的输出缓冲区
    unsigned int audio_buf_size; /* in bytes */ // 待播放的一帧音频数据(audio_buf指向)的大小
    unsigned int audio_buf1_size;   // 申请到的音频缓冲区audio_buf1的实际尺寸
    int audio_buf_index; /* in bytes */ // 当前音频帧中已拷入SDL音频缓冲区的位置索引(指向第一个待拷贝字节)
    int audio_write_buf_size;       // 当前音频帧中尚未拷入SDL音频缓冲区的数据量，audio_buf_size = audio_buf_index + audio_write_buf_size
    int audio_volume;               // 音量
    int muted;                      // 静音状态
    struct AudioParams audio_src;   // 音频frame的参数
#if CONFIG_AVFILTER
    struct AudioParams audio_filter_src;
#endif
    struct AudioParams audio_tgt;   // SDL支持的音频参数，重采样转换：audio_src->audio_tgt
    struct SwrContext *swr_ctx;     // 音频重采样context
    int frame_drops_early;          // 丢弃视频packet计数
    int frame_drops_late;           // 丢弃视频frame计数

    enum ShowMode {
        SHOW_MODE_NONE = -1, SHOW_MODE_VIDEO = 0, SHOW_MODE_WAVES, SHOW_MODE_RDFT, SHOW_MODE_NB
    } show_mode;
    int16_t sample_array[SAMPLE_ARRAY_SIZE];
    int sample_array_index;
    int last_i_start;
    RDFTContext *rdft;
    int rdft_bits;
    FFTSample *rdft_data;
    int xpos;
    double last_vis_time;
    SDL_Texture *vis_texture;
    SDL_Texture *sub_texture;
    SDL_Texture *vid_texture;

    int subtitle_stream;                // 字幕流索引
    AVStream *subtitle_st;              // 字幕流
    PacketQueue subtitleq;              // 字幕packet队列

    double frame_timer;                 // 记录最后一帧播放的时刻
    double frame_last_returned_time;
    double frame_last_filter_delay;
    int video_stream;
    AVStream *video_st;                 // 视频流
    PacketQueue videoq;                 // 视频队列
    double max_frame_duration;      // maximum duration of a frame - above this, we consider the jump a timestamp discontinuity
    struct SwsContext *img_convert_ctx;
    struct SwsContext *sub_convert_ctx;
    int eof;

    char *filename;
    int width, height, xleft, ytop;
    int step;

#if CONFIG_AVFILTER
    int vfilter_idx;
    AVFilterContext *in_video_filter;   // the first filter in the video chain
    AVFilterContext *out_video_filter;  // the last filter in the video chain
    AVFilterContext *in_audio_filter;   // the first filter in the audio chain
    AVFilterContext *out_audio_filter;  // the last filter in the audio chain
    AVFilterGraph *agraph;              // audio filter graph
#endif

    int last_video_stream, last_audio_stream, last_subtitle_stream;

    SDL_cond *continue_read_thread;
} VideoState;

typedef struct Clock {
    // 当前帧(待播放)显示时间戳，播放后，当前帧变成上一帧
    double pts;           /* clock base */
    // 当前帧显示时间戳与当前系统时钟时间的差值
    double pts_drift;     /* clock base minus time at which we updated the clock */
    // 当前时钟(如视频时钟)最后一次更新时间，也可称当前时钟时间
    double last_updated;
    // 时钟速度控制，用于控制播放速度
    double speed;
    // 播放序列，所谓播放序列就是一段连续的播放动作，一个seek操作会启动一段新的播放序列
    int serial;           /* clock is based on a packet with this serial */
    // 暂停标志
    int paused;
    // 指向packet_serial
    int *queue_serial;    /* pointer to the current packet queue serial, used for obsolete clock detection */
} Clock;

typedef struct PacketQueue {
    MyAVPacketList *first_pkt, *last_pkt;
    int nb_packets;                 // 队列中packet的数量
    int size;                       // 队列所占内存空间大小
    int64_t duration;               // 队列中所有packet总的播放时长
    int abort_request;
    int serial;                     // 播放序列，所谓播放序列就是一段连续的播放动作，一个seek操作会启动一段新的播放序列
    SDL_mutex *mutex;
    SDL_cond *cond;
} PacketQueue;

typedef struct FrameQueue {
    Frame queue[FRAME_QUEUE_SIZE];
    int rindex;                     // 读索引。待播放时读取此帧进行播放，播放后此帧成为上一帧
    int windex;                     // 写索引
    int size;                       // 总帧数
    int max_size;                   // 队列可存储最大帧数
    int keep_last;                  // 是否保留已播放的最后一帧使能标志
    int rindex_shown;               // 是否保留已播放的最后一帧实现手段
    SDL_mutex *mutex;
    SDL_cond *cond;
    PacketQueue *pktq;              // 指向对应的packet_queue
} FrameQueue;

static int frame_queue_init(FrameQueue *f, PacketQueue *pktq, int max_size, int keep_last)
{
    int i;
    memset(f, 0, sizeof(FrameQueue));
    if (!(f->mutex = SDL_CreateMutex())) {
        av_log(NULL, AV_LOG_FATAL, "SDL_CreateMutex(): %sn", SDL_GetError());
        return AVERROR(ENOMEM);
    }
    if (!(f->cond = SDL_CreateCond())) {
        av_log(NULL, AV_LOG_FATAL, "SDL_CreateCond(): %sn", SDL_GetError());
        return AVERROR(ENOMEM);
    }
    f->pktq = pktq;
    f->max_size = FFMIN(max_size, FRAME_QUEUE_SIZE);
    f->keep_last = !!keep_last;
    for (i = 0; i < f->max_size; i++)
        if (!(f->queue[i].frame = av_frame_alloc()))
            return AVERROR(ENOMEM);
    return 0;
}

static void frame_queue_destory(FrameQueue *f)
{
    int i;
    for (i = 0; i < f->max_size; i++) {
        Frame *vp = &f->queue[i];
        frame_queue_unref_item(vp);     // 释放对vp->frame中的数据缓冲区的引用，注意不是释放frame对象本身
        av_frame_free(&vp->frame);      // 释放vp->frame对象
    }
    SDL_DestroyMutex(f->mutex);
    SDL_DestroyCond(f->cond);
}

frame_queue_peek_writable()     // 获取写指针
frame_queue_push()              // 更新写指针

static int queue_picture(VideoState *is, AVFrame *src_frame, double pts, double duration, int64_t pos, int serial)
{
    Frame *vp;

    if (!(vp = frame_queue_peek_writable(&is->pictq)))
        return -1;

    vp->sar = src_frame->sample_aspect_ratio;
    vp->uploaded = 0;

    vp->width = src_frame->width;
    vp->height = src_frame->height;
    vp->format = src_frame->format;

    vp->pts = pts;
    vp->duration = duration;
    vp->pos = pos;
    vp->serial = serial;

    set_default_window_size(vp->width, vp->height, vp->sar);

    av_frame_move_ref(vp->frame, src_frame);
    frame_queue_push(&is->pictq);
    return 0;
}

static Frame *frame_queue_peek_writable(FrameQueue *f)
{
    /* wait until we have space to put a new frame */
    SDL_LockMutex(f->mutex);
    while (f->size >= f->max_size &&
           !f->pktq->abort_request) {
        SDL_CondWait(f->cond, f->mutex);
    }
    SDL_UnlockMutex(f->mutex);

    if (f->pktq->abort_request)
        return NULL;

    return &f->queue[f->windex];
}

static void frame_queue_push(FrameQueue *f)
{
    if (++f->windex == f->max_size)
        f->windex = 0;
    SDL_LockMutex(f->mutex);
    f->size++;
    SDL_CondSignal(f->cond);
    SDL_UnlockMutex(f->mutex);
}

frame_queue_peek_readable()     // 获取读指针(若读空则等待)
frame_queue_peek()              // 获取当前节点指针
frame_queue_peek_next()         // 获取下一节点指针
frame_queue_peek_last()         // 获取上一节点指针
frame_queue_next()              // 更新读指针(同时删除旧节点)

static void video_refresh(void *opaque, double *remaining_time)
{
    ......
    if (frame_queue_nb_remaining(&is->pictq) == 0) {    // 所有帧已显示
        // nothing to do, no picture to display in the queue
    } else {
        Frame *vp, *lastvp;
        lastvp = frame_queue_peek_last(&is->pictq);     // 上一帧：上次已显示的帧
        vp = frame_queue_peek(&is->pictq);              // 当前帧：当前待显示的帧
        frame_queue_next(&is->pictq);                   // 删除上一帧，并更新rindex
        video_display(is)-->video_image_display()-->frame_queue_peek_last();
    }
    ......
}

/* return the number of undisplayed frames in the queue */
static int frame_queue_nb_remaining(FrameQueue *f)
{
    return f->size - f->rindex_shown;
}

static void frame_queue_next(FrameQueue *f)
{
    if (f->keep_last && !f->rindex_shown) {
        f->rindex_shown = 1;
        return;
    }
    frame_queue_unref_item(&f->queue[f->rindex]);
    if (++f->rindex == f->max_size)
        f->rindex = 0;
    SDL_LockMutex(f->mutex);
    f->size--;
    SDL_CondSignal(f->cond);
    SDL_UnlockMutex(f->mutex);
}

static Frame *frame_queue_peek_readable(FrameQueue *f)
{
    /* wait until we have a readable a new frame */
    SDL_LockMutex(f->mutex);
    while (f->size - f->rindex_shown <= 0 &&
           !f->pktq->abort_request) {
        SDL_CondWait(f->cond, f->mutex);
    }
    SDL_UnlockMutex(f->mutex);

    if (f->pktq->abort_request)
        return NULL;

    return &f->queue[(f->rindex + f->rindex_shown) % f->max_size];
}

static Frame *frame_queue_peek(FrameQueue *f)
{
    return &f->queue[(f->rindex + f->rindex_shown) % f->max_size];
}

static Frame *frame_queue_peek_next(FrameQueue *f)
{
    return &f->queue[(f->rindex + f->rindex_shown + 1) % f->max_size];
}

// 取出此帧进行播放，只读取不删除，不删除是因为此帧需要缓存下来供下一次使用。播放后，此帧变为上一帧
static Frame *frame_queue_peek_last(FrameQueue *f)
{
    return &f->queue[f->rindex];
}