在prepare的stream_open过程中，不仅会启动read_thread，而且会启动video_refresh_thread。今天就来看看这个video_refresh_thread干了什么。

static int video_refresh_thread(void *arg){    FFPlayer *ffp = arg;    VideoState *is = ffp->is;    double remaining_time = 0.0;    while (!is->abort_request) {        if (remaining_time > 0.0)            av_usleep((int)(int64_t)(remaining_time * 1000000.0));        remaining_time = REFRESH_RATE;        if (is->show_mode != SHOW_MODE_NONE && (!is->paused || is->force_refresh))            video_refresh(ffp, &remaining_time);    }    return 0;}

非暂停或强制刷新的时候，循环调用video_refresh。

static void video_refresh(FFPlayer *opaque, double *remaining_time){    ......    if (!ffp->display_disable && is->show_mode != SHOW_MODE_VIDEO && is->audio_st) {        time = av_gettime_relative() / 1000000.0;        if (is->force_refresh || is->last_vis_time + ffp->rdftspeed < time) {            video_display2(ffp);            is->last_vis_time = time;        }        *remaining_time = FFMIN(*remaining_time, is->last_vis_time + ffp->rdftspeed - time);    }    ......}

video_display2的调用较为关键，之前有外部时钟同步和一些时间检测等。video_display2里直接调用了video_image_display2。

static void video_image_display2(FFPlayer *ffp){    VideoState *is = ffp->is;    Frame *vp;    Frame *sp = NULL;    vp = frame_queue_peek_last(&is->pictq);    int latest_seek_load_serial = __atomic_exchange_n(&(is->latest_seek_load_serial), -1, memory_order_seq_cst);    if (latest_seek_load_serial == vp->serial)        ffp->stat.latest_seek_load_duration = (av_gettime() - is->latest_seek_load_start_at) / 1000;    if (vp->bmp) {        if (is->subtitle_st) {            if (frame_queue_nb_remaining(&is->subpq) > 0) {                sp = frame_queue_peek(&is->subpq);                if (vp->pts >= sp->pts + ((float) sp->sub.start_display_time / 1000)) {                    if (!sp->uploaded) {                        if (sp->sub.num_rects > 0) {                            char buffered_text[4096];                            if (sp->sub.rects[0]->text) {                                strncpy(buffered_text, sp->sub.rects[0]->text, 4096);                            }                            else if (sp->sub.rects[0]->ass) {                                parse_ass_subtitle(sp->sub.rects[0]->ass, buffered_text);                            }                            ffp_notify_msg4(ffp, FFP_MSG_TIMED_TEXT, 0, 0, buffered_text, sizeof(buffered_text));                        }                        sp->uploaded = 1;                    }                }            }        }        SDL_VoutDisplayYUVOverlay(ffp->vout, vp->bmp);        ffp->stat.vfps = SDL_SpeedSamplerAdd(&ffp->vfps_sampler, FFP_SHOW_VFPS_FFPLAY, "vfps[ffplay]");        if (!ffp->first_video_frame_rendered) {            ffp->first_video_frame_rendered = 1;            ffp_notify_msg1(ffp, FFP_MSG_VIDEO_RENDERING_START);        }    }}

frame_queue_peek_last获取队列最后一帧，如果帧中的图像正常，继续走sdl渲染yup画面SDL_VoutDisplayYUVOverlay。最后通过消息通知开始渲染ffp_notify_msg1。

double last_duration, duration, delay;            Frame *vp, *lastvp;            /* dequeue the picture */            lastvp = frame_queue_peek_last(&is->pictq);            vp = frame_queue_peek(&is->pictq);            if (vp->serial != is->videoq.serial) {                frame_queue_next(&is->pictq);                goto retry;            }            if (lastvp->serial != vp->serial)                is->frame_timer = av_gettime_relative() / 1000000.0;            if (is->paused)                goto display;            /* compute nominal last_duration */            last_duration = vp_duration(is, lastvp, vp);            delay = compute_target_delay(ffp, last_duration, is);            time= av_gettime_relative()/1000000.0;            if (isnan(is->frame_timer) || time < is->frame_timer)                is->frame_timer = time;            if (time < is->frame_timer + delay) {                *remaining_time = FFMIN(is->frame_timer + delay - time, *remaining_time);                goto display;            }            is->frame_timer += delay;            if (delay > 0 && time - is->frame_timer > AV_SYNC_THRESHOLD_MAX)                is->frame_timer = time;            SDL_LockMutex(is->pictq.mutex);            if (!isnan(vp->pts))                update_video_pts(is, vp->pts, vp->pos, vp->serial);            SDL_UnlockMutex(is->pictq.mutex);            if (frame_queue_nb_remaining(&is->pictq) > 1) {                Frame *nextvp = frame_queue_peek_next(&is->pictq);                duration = vp_duration(is, vp, nextvp);                if(!is->step && (ffp->framedrop > 0 || (ffp->framedrop && get_master_sync_type(is) != AV_SYNC_VIDEO_MASTER)) && time > is->frame_timer + duration) {                    frame_queue_next(&is->pictq);                    goto retry;                }            }

首先取出上一帧(lastvp)和当前帧(vp)，然后有个判断是看从video的队列中的序列是否与当前的帧是相同的，如果不是挨个查找下帧，然后跳转到retry，再次执行。我理解的是从FrameQueue队列中找到播放序列相同的这一帧，然后进行后续的操作。下面就是vp_duration了，这里做了一个减法，计算出了这一帧持续的时间。那么后面的delay = compute_target_delay(ffp, last_duration, is);有什么作用呢？

static double compute_target_delay(FFPlayer *ffp, double delay, VideoState *is){    double sync_threshold, diff = 0;    /* update delay to follow master synchronisation source */    if (get_master_sync_type(is) != AV_SYNC_VIDEO_MASTER) {        /* if video is slave, we try to correct big delays by           duplicating or deleting a frame */        diff = get_clock(&is->vidclk) - get_master_clock(is);        /* skip or repeat frame. We take into account the           delay to compute the threshold. I still don't know           if it is the best guess */        sync_threshold = FFMAX(AV_SYNC_THRESHOLD_MIN, FFMIN(AV_SYNC_THRESHOLD_MAX, delay));        /* -- by bbcallen: replace is->max_frame_duration with AV_NOSYNC_THRESHOLD */        if (!isnan(diff) && fabs(diff) < AV_NOSYNC_THRESHOLD) {            if (diff <= -sync_threshold)                delay = FFMAX(0, delay + diff);            else if (diff >= sync_threshold && delay > AV_SYNC_FRAMEDUP_THRESHOLD)                delay = delay + diff;            else if (diff >= sync_threshold)                delay = 2 * delay;        }    }    if (ffp) {        ffp->stat.avdelay = delay;        ffp->stat.avdiff  = diff;    }#ifdef FFP_SHOW_AUDIO_DELAY    av_log(NULL, AV_LOG_TRACE, "video: delay=%0.3f A-V=%f\n",            delay, -diff);#endif    return delay;}

首先是一个AV_SYNC_VIDEO_MASTER的判断，如果发现主时钟不是video，那么计算当前视频时钟与主时钟的差值，diff = get_clock(&is->vidclk) - get_master_clock(is);计算视频时钟与当前的音频时钟之间的差值。然后根据根据偏差的范围进行了调整延迟时间（视频比音频快，加大下一帧的渲染时间，否则缩短时间），这里有3种情况判断：如果当前视频时间落后于主时钟，需要减小下一帧画面的等待时间；如果视频帧超前了，并且显示时间大于一个阈值（AV_SYNC_FRAMEDUP_THRESHOLD），则显示下一帧的时间为超前的时间差加上上一帧的显示时间；如果视频超前了，并且上一帧的显示时间小于这个阈值，则加倍延时。之后设置到了ffp->stat中，并返回。这个返回的偏差值就是后面进行是否抛帧或sleep的判断依据。

回到video_refresh，is->frame_timer = av_gettime_relative() / 1000000.0;这里的frame_time实际上就是上一帧显示的时间，is->frame_timer + delay其实就是当前这一帧显示的时间。那么看这个判断：

if (time < is->frame_timer + delay) {                *remaining_time = FFMIN(is->frame_timer + delay - time, *remaining_time);                goto display;            }

如果播放的时间还没有到达当前这一帧的显示时间，那么直接跳到display，在display中将is->force_refresh变量为0，不显示当前帧,

这里的意思大约是还没到播放的时间，那么跳转出去到display，然后函数就返回了。那么这个函数的调用者video_refresh_thread里面可是循环调用的，所以后面会经过sleep然后再次调用到这个里面。好吧，这里我就简单的理解为如果没有到达显示的时间点，就sleep。后面是如果time - is->frame_timer + delay超过了AV_SYNC_THRESHOLD_MAX，就将is->frame_timer设置为当前时间。这里的目的还不是特别清楚，不过可以感觉到，是为了后面要做判断，估计是要对一些延迟比较高的帧考虑抛帧处理吧。

if (frame_queue_nb_remaining(&is->pictq) > 1) {                Frame *nextvp = frame_queue_peek_next(&is->pictq);                duration = vp_duration(is, vp, nextvp);                if(!is->step && (ffp->framedrop > 0 || (ffp->framedrop && get_master_sync_type(is) != AV_SYNC_VIDEO_MASTER)) && time > is->frame_timer + duration) {                    frame_queue_next(&is->pictq);                    goto retry;                }            }

如果视频还有下一帧，则拿出下一帧来，计算差值。下面要注意：time > is->frame_timer + duration的判断，如果下一帧的时间点比当前的系统时间慢，也就是说不仅当前的慢了，下一帧的也慢了，慢了2帧，那么久触发丢帧，丢掉当前帧。通过frame_queue_next将缓存游标挪到下一帧，然后goto到retry，重新进行上面的渲染判断。