1 static void refresh_loop_wait_event(VideoState *is, SDL_Event *event) { 2     double remaining_time = 0.0; 3      SDL_PumpEvents();/*不停的循环内部更新消息*/ 4     while (!SDL_PeepEvents(event, 1, SDL_GETEVENT, SDL_ALLEVENTS)) {/*check the event queue for messages*/ 5         if (!cursor_hidden && av_gettime() - cursor_last_shown > CURSOR_HIDE_DELAY) { 6             SDL_ShowCursor(0); 7             cursor_hidden = 1; 8         } 9         if (remaining_time > 0.0)10             av_usleep((int64_t)(remaining_time * 1000000.0));/*使用这个函数来休眠,取代之前版本中的定时器*/11         remaining_time = REFRESH_RATE;/*10ms*/12         if (is->show_mode != SHOW_MODE_NONE && (!is->paused || is->force_refresh))13             video_refresh(is, &remaining_time);14         SDL_PumpEvents();15     }16 }

  1 /* called to display each frame */  2 static void video_refresh(void *opaque, double *remaining_time)  3 {  4     VideoState *is = opaque;  5     VideoPicture *vp;  6     double time;  7   8     SubPicture *sp, *sp2;  9  10     if (!is->paused && get_master_sync_type(is) == AV_SYNC_EXTERNAL_CLOCK && is->realtime)/*如果用外部时钟同步的话*/ 11         check_external_clock_speed(is); 12  13     if (!display_disable && is->show_mode != SHOW_MODE_VIDEO && is->audio_st) { 14         time = av_gettime() / 1000000.0; 15         if (is->force_refresh || is->last_vis_time + rdftspeed < time) {/*强制刷新视频*/ 16             video_display(is); 17             is->last_vis_time = time;/*记录本次的时间*/ 18         } 19         *remaining_time = FFMIN(*remaining_time, is->last_vis_time + rdftspeed - time); 20     } 21  22     if (is->video_st) { 23         int redisplay = 0; 24         if (is->force_refresh) 25             redisplay = pictq_prev_picture(is); 26 retry: 27         if (is->pictq_size == 0) {/*如果缓冲区没有数据*/ 28             SDL_LockMutex(is->pictq_mutex); 29             if (is->frame_last_dropped_pts != AV_NOPTS_VALUE && is->frame_last_dropped_pts > is->frame_last_pts) { 30                 update_video_pts(is, is->frame_last_dropped_pts, is->frame_last_dropped_pos, is->frame_last_dropped_serial); 31                 is->frame_last_dropped_pts = AV_NOPTS_VALUE; 32             } 33             SDL_UnlockMutex(is->pictq_mutex); 34             // nothing to do, no picture to display in the queue 35         } else { 36             double last_duration, duration, delay; 37             /* dequeue the picture */ 38             vp = &is->pictq[is->pictq_rindex]; 39  40             if (vp->serial != is->videoq.serial) { 41                 pictq_next_picture(is); 42                 redisplay = 0; 43                 goto retry; 44             } 45  46             if (is->paused) 47                 goto display; 48  49             /* compute nominal last_duration *//*通过计算当前要显示的帧和上一帧pts的差来预测当期帧显示时间---预测--->下一帧的到来时间*/ 50             last_duration = vp->pts - is->frame_last_pts;/*计算上一帧的显示时间(名义上)*/ 51             if (!isnan(last_duration) && last_duration > 0 && last_duration < is->max_frame_duration) {/*判断上一帧显示的时间是否在范围内*/ 52                 /* if duration of the last frame was sane, update last_duration in video state */ 53                 is->frame_last_duration = last_duration;/*更新一帧的持续显示时间*/ 54             } 55             if (redisplay) 56                 delay = 0.0; 57             else 58                 delay = compute_target_delay(is->frame_last_duration, is);/*通过上一帧的情况来预测本次的情况,这样可以得到下一帧的到来时间*/ 59  60             time= av_gettime()/1000000.0; 61             if (time < is->frame_timer + delay && !redisplay) {/**/ 62                 *remaining_time = FFMIN(is->frame_timer + delay - time, *remaining_time); 63                 return; 64             } 65  66             is->frame_timer += delay; 67             if (delay > 0 && time - is->frame_timer > AV_SYNC_THRESHOLD_MAX) 68                 is->frame_timer = time; 69  70             SDL_LockMutex(is->pictq_mutex); 71             if (!redisplay && !isnan(vp->pts)) 72                 update_video_pts(is, vp->pts, vp->pos, vp->serial);/*更新当前帧pts和pos*/ 73             SDL_UnlockMutex(is->pictq_mutex); 74  75             if (is->pictq_size > 1) {/*如果缓冲中帧数比较多的时候,例如下一帧也已经到了*/ 76                 VideoPicture *nextvp = &is->pictq[(is->pictq_rindex + 1) % VIDEO_PICTURE_QUEUE_SIZE]; 77                 duration = nextvp->pts - vp->pts;/*这个时候,应该用已经在缓存中的下一帧pts-当前pts来真实计算当前持续显示时间*/ 78                 if(!is->step && (redisplay || framedrop>0 || (framedrop && get_master_sync_type(is) != AV_SYNC_VIDEO_MASTER)) && time > is->frame_timer + duration){/*如果延迟时间超过一帧了,就采取丢掉当前帧*/ 79                     if (!redisplay) 80                         is->frame_drops_late++; 81                     pictq_next_picture(is);/*采取丢帧策略,丢弃迟来的帧,取下一帧*/ 82                     redisplay = 0; 83                     goto retry; 84                 } 85             } 86  87             if (is->subtitle_st) { 88                     while (is->subpq_size > 0) { 89                         sp = &is->subpq[is->subpq_rindex]; 90  91                         if (is->subpq_size > 1) 92                             sp2 = &is->subpq[(is->subpq_rindex + 1) % SUBPICTURE_QUEUE_SIZE]; 93                         else 94                             sp2 = NULL; 95  96                         if (sp->serial != is->subtitleq.serial 97                                 || (is->vidclk.pts > (sp->pts + ((float) sp->sub.end_display_time / 1000))) 98                                 || (sp2 && is->vidclk.pts > (sp2->pts + ((float) sp2->sub.start_display_time / 1000)))) 99                         {100                             free_subpicture(sp);101 102                             /* update queue size and signal for next picture */103                             if (++is->subpq_rindex == SUBPICTURE_QUEUE_SIZE)104                                 is->subpq_rindex = 0;105 106                             SDL_LockMutex(is->subpq_mutex);107                             is->subpq_size--;108                             SDL_CondSignal(is->subpq_cond);109                             SDL_UnlockMutex(is->subpq_mutex);110                         } else {111                             break;112                         }113                     }114             }115 116 display:117             /* display picture */118             if (!display_disable && is->show_mode == SHOW_MODE_VIDEO)119                 video_display(is);120 121             pictq_next_picture(is);122 123             if (is->step && !is->paused)124                 stream_toggle_pause(is);125         }126     }127     is->force_refresh = 0;128     if (show_status) {/*显示状态*/129         static int64_t last_time;130         int64_t cur_time;131         int aqsize, vqsize, sqsize;132         double av_diff;133 134         cur_time = av_gettime();135         if (!last_time || (cur_time - last_time) >= 30000) {136             aqsize = 0;137             vqsize = 0;138             sqsize = 0;139             if (is->audio_st)140                 aqsize = is->audioq.size;141             if (is->video_st)142                 vqsize = is->videoq.size;143             if (is->subtitle_st)144                 sqsize = is->subtitleq.size;145             av_diff = 0;146             if (is->audio_st && is->video_st)147                 av_diff = get_clock(&is->audclk) - get_clock(&is->vidclk);148             else if (is->video_st)149                 av_diff = get_master_clock(is) - get_clock(&is->vidclk);150             else if (is->audio_st)151                 av_diff = get_master_clock(is) - get_clock(&is->audclk);152             av_log(NULL, AV_LOG_INFO,153                    "%7.2f %s:%7.3f fd=%4d aq=%5dKB vq=%5dKB sq=%5dB f=%"PRId64"/%"PRId64"   \r",154                    get_master_clock(is),155                    (is->audio_st && is->video_st) ? "A-V" : (is->video_st ? "M-V" : (is->audio_st ? "M-A" : "   ")),156                    av_diff,157                    is->frame_drops_early + is->frame_drops_late,158                    aqsize / 1024,159                    vqsize / 1024,160                    sqsize,161                    is->video_st ? is->video_st->codec->pts_correction_num_faulty_dts : 0,162                    is->video_st ? is->video_st->codec->pts_correction_num_faulty_pts : 0);163             fflush(stdout);164             last_time = cur_time;165         }166     }167 }

 1 static double compute_target_delay(double delay, VideoState *is) 2 { 3     double sync_threshold, diff; 4  5     /* update delay to follow master synchronisation source */ 6     if (get_master_sync_type(is) != AV_SYNC_VIDEO_MASTER) { 7         /* if video is slave, we try to correct big delays by 8            duplicating or deleting a frame *//*我们通过复制和删除一帧来纠正大的延时*/ 9         diff = get_clock(&is->vidclk) - get_master_clock(is);10 11         /* skip or repeat frame. We take into account the12            delay to compute the threshold. I still don‘t know13            if it is the best guess */14         sync_threshold = FFMAX(AV_SYNC_THRESHOLD_MIN, FFMIN(AV_SYNC_THRESHOLD_MAX, delay));15         if (!isnan(diff) && fabs(diff) < is->max_frame_duration) {16             if (diff <= -sync_threshold)/*当前视频帧落后于主时钟源*/17             {18                 delay = FFMAX(0, delay + diff);19             }20             else if (diff >= sync_threshold && delay > AV_SYNC_FRAMEDUP_THRESHOLD)/*视频帧超前,但If a frame duration is longer than this, it will not be duplicated to compensate AV sync*/21             {    /*大概意思是:本来当视频帧超前的时候,22                 我们应该要选择重复该帧或者下面的2倍延时(即加重延时的策略),23                 但因为该帧的显示时间大于显示更新门槛,24                 所以这个时候不应该以该帧做同步*/25                 delay = delay + diff;26             }27             else if (diff >= sync_threshold)28             {29                 delay = 2 * delay;/*采取加倍延时*/30             }31         }32     }33 34     av_dlog(NULL, "video: delay=%0.3f A-V=%f\n",35             delay, -diff);36 37     return delay;38 }