最近在看ffmpeg相關的一些東西，以及一些播放器相關資料和代碼。
然后對於ffmpeg-2.0.1版本下的ffplay進行了大概的代碼閱讀，其中這里把里面的音視頻同步，按個人的理解，暫時在這里作個筆記。
在ffplay2.0.1版本里面，視頻的刷新不再直接使用SDL里面的定時器了，而是在主的循環中event_loop中，通過調用函數refresh_loop_wait_event來等待事件，
同時在這個refresh_loop_wait_event函數里面，通過使用休眠函數av_usleep 來進行定時刷新視頻。
調用視頻更新的代碼：
1 static void refresh_loop_wait_event(VideoState *is, SDL_Event *event) { 2 double remaining_time = 0.0; 3 SDL_PumpEvents();/*不停的循環內部更新消息*/
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));/*使用這個函數來休眠,取代之前版本中的定時器*/ 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 }
然后接下來，我們來看看video_refresh函數里面做了些什么事情吧！
代碼如下：
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)/*如果用外部時鍾同步的話*/
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) {/*強制刷新視頻*/
16 video_display(is); 17 is->last_vis_time = time;/*記錄本次的時間*/
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) {/*如果緩沖區沒有數據*/ 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 *//*通過計算當前要顯示的幀和上一幀pts的差來預測當期幀顯示時間---預測--->下一幀的到來時間*/ 50 last_duration = vp->pts - is->frame_last_pts;/*計算上一幀的顯示時間(名義上)*/ 51 if (!isnan(last_duration) && last_duration > 0 && last_duration < is->max_frame_duration) {/*判斷上一幀顯示的時間是否在范圍內*/ 52 /* if duration of the last frame was sane, update last_duration in video state */ 53 is->frame_last_duration = last_duration;/*更新一幀的持續顯示時間*/ 54 } 55 if (redisplay) 56 delay = 0.0; 57 else 58 delay = compute_target_delay(is->frame_last_duration, is);/*通過上一幀的情況來預測本次的情況,這樣可以得到下一幀的到來時間*/ 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);/*更新當前幀pts和pos*/ 73 SDL_UnlockMutex(is->pictq_mutex); 74 75 if (is->pictq_size > 1) {/*如果緩沖中幀數比較多的時候,例如下一幀也已經到了*/ 76 VideoPicture *nextvp = &is->pictq[(is->pictq_rindex + 1) % VIDEO_PICTURE_QUEUE_SIZE]; 77 duration = nextvp->pts - vp->pts;/*這個時候,應該用已經在緩存中的下一幀pts-當前pts來真實計算當前持續顯示時間*/ 78 if(!is->step && (redisplay || framedrop>0 || (framedrop && get_master_sync_type(is) != AV_SYNC_VIDEO_MASTER)) && time > is->frame_timer + duration){/*如果延遲時間超過一幀了,就采取丟掉當前幀*/ 79 if (!redisplay) 80 is->frame_drops_late++; 81 pictq_next_picture(is);/*采取丟幀策略,丟棄遲來的幀,取下一幀*/ 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) {/*顯示狀態*/
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 }
首先說明一下,這里在ffplay里面默認模式是使用音頻做主時鍾源。
其中上面加紅色的代碼是主要的策略：
他通過計算當前這一幀vp->pts和前面那一幀的pts之差來得到上一幀的顯示時間。
然后再根據這個上面計算得到的上一幀的顯示時間來估算預測計算當前這一幀的顯示時間，這樣就可以得到預測下一幀的pts時間了。
這里預測下一幀的出現時間,刷新時間,調用了compute_target_delay來進行處理：
代碼如下:compute_target_delay
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 *//*我們通過復制和刪除一幀來糾正大的延時*/
9 diff = get_clock(&is->vidclk) - get_master_clock(is); 10
11 /* skip or repeat frame. We take into account the 12 delay to compute the threshold. I still don't know 13 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)/*當前視頻幀落后於主時鍾源*/
17 { 18 delay = FFMAX(0, delay + diff); 19 } 20 else if (diff >= sync_threshold && delay > AV_SYNC_FRAMEDUP_THRESHOLD)/*視頻幀超前,但If a frame duration is longer than this, it will not be duplicated to compensate AV sync*/
21 { /*大概意思是:本來當視頻幀超前的時候, 22 我們應該要選擇重復該幀或者下面的2倍延時(即加重延時的策略), 23 但因為該幀的顯示時間大於顯示更新門檻, 24 所以這個時候不應該以該幀做同步*/
25 delay = delay + diff; 26 } 27 else if (diff >= sync_threshold) 28 { 29 delay = 2 * delay;/*采取加倍延時*/
30 } 31 } 32 } 33
34 av_dlog(NULL, "video: delay=%0.3f A-V=%f\n", 35 delay, -diff); 36
37 return delay; 38 }
在這個函數里面通過得帶視頻時間和參考時間之間的差值diff，然后再結合diff的情況來處理delay。