1.測試拿過來個視訊，發現用ijk播放器與系統播放器（mediaplayer）播放感覺不一樣，用ijk播放感覺播放頁面卡頓一點，沒有系統播放器那麼流暢。
下面看一下這個問題，這個問題的原因其實很簡單，由於我丟幀值設定的是5，改成1就可以感覺2個播放器在體驗上感覺差不多了。（丟5幀人眼就可以看出來差別了！）。
2.在高通660的機器上播放一個4k（30fps）視訊，但是無法正常播放，實際一秒的解碼幀只有20幀，實際播放只有4幀這樣。導致畫面卡頓，音視訊不同步。
後面發現是由於這個視訊，在高通機器上很多幀解碼都比較慢，導致視訊一直比音訊慢，在硬解碼丟幀時，判斷視訊一直比音訊慢，導致視訊一直在丟幀，也就出現看上去的卡頓。
丟幀原理
首先需要明白丟幀需要丟哪裡的幀，丟什麼幀呢？
丟幀可以丟解碼前的幀也可以丟解碼後的幀，
丟解碼前的幀需要判斷幀型別來丟，可以選擇丟b，p幀，如果需要丟I幀就需要把整個gop全部丟掉防止花屏。
丟解碼後的幀可以不用按照幀型別來丟，由於幀都是解碼後的資料了，如yuv，直接按照pts來判斷音視訊是否不同步來丟就可以了。
1.ffplay中丟幀設計
我們看下ffplay中丟幀的設計：ffplay丟的是解碼後的視訊幀
在video_thread解碼執行緒中，我們可以看到get_video_frame函式主要用於解碼獲取解碼後的資料avframe，然後來檢測判斷丟幀。
static int get_video_frame(VideoState *is, AVFrame *frame)
{
int got_picture;
//解碼獲取解碼後的資料avframe
if ((got_picture = decoder_decode_frame(&is->viddec, frame, NULL)) < 0)
return -1;
if (got_picture) {
double dpts = NAN;
if (frame->pts != AV_NOPTS_VALUE)
dpts = av_q2d(is->video_st->time_base) * frame->pts;//視訊的pts轉換為ms,也就是當前進度時間
//視訊的寬高比
frame->sample_aspect_ratio = av_guess_sample_aspect_ratio(is->ic, is->video_st, frame);
//丟幀數大於0且同步不是按照video
if (framedrop>0 || (framedrop && get_master_sync_type(is) != AV_SYNC_VIDEO_MASTER)) {
if (frame->pts != AV_NOPTS_VALUE) {
//frame_last_filter_delay 預設是0
//diff小於0，視訊比音訊慢，丟幀
//diff大於0,視訊比音訊快，不丟幀
double diff = dpts - get_master_clock(is);
// AV_NOSYNC_THRESHOLD：同步閾值。如果誤差太大，則不進行校正，也不丟幀來做同步了
if (!isnan(diff) && fabs(diff) < AV_NOSYNC_THRESHOLD &&
diff - is->frame_last_filter_delay < 0 &&
is->viddec.pkt_serial == is->vidclk.serial &&
is->videoq.nb_packets) {
is->frame_drops_early++;
av_frame_unref(frame);//丟幀
got_picture = 0;
}
}
}
}
return got_picture;
}
複製程式碼2.ijk中丟幀設計
ijk中丟幀丟的也是解碼後的視訊幀，分為硬解碼丟幀和軟解碼丟幀。
2.1.硬解碼丟幀設計
在ffpipenode_android_mediacodec_vdec.c檔案中， func_run_sync函式主要用來處理整個硬解碼的實現邏輯。
/**
*硬解碼處理流程
**/
static int func_run_sync(IJKFF_Pipenode *node)
{
JNIEnv *env = NULL;
IJKFF_Pipenode_Opaque *opaque = node->opaque;
FFPlayer *ffp = opaque->ffp;
VideoState *is = ffp->is;
Decoder *d = &is->viddec;
PacketQueue *q = d->queue;
int ret = 0;
int dequeue_count = 0;
AVFrame *frame = NULL;
int got_frame = 0;
AVRational tb = is->video_st->time_base;
AVRational frame_rate = av_guess_frame_rate(is->ic, is->video_st, NULL);
double duration;
double pts;
if (!opaque->acodec) {
return ffp_video_thread(ffp);
}
if (JNI_OK != SDL_JNI_SetupThreadEnv(&env)) {
ALOGE("%s: SetupThreadEnv failed\n", __func__);
return -1;
}
frame = av_frame_alloc();
if (!frame)
goto fail;
//建立資料入隊執行緒enqueue_thread_func
opaque->enqueue_thread = SDL_CreateThreadEx(&opaque->_enqueue_thread, enqueue_thread_func, node, "amediacodec_input_thread");
if (!opaque->enqueue_thread) {
ALOGE("%s: SDL_CreateThreadEx failed\n", __func__);
ret = -1;
goto fail;
}
//迴圈拉取解碼資料
while (!q->abort_request) {
int64_t timeUs = opaque->acodec_first_dequeue_output_request ? 0 : AMC_OUTPUT_TIMEOUT_US;
got_frame = 0;
//硬解獲取frame
ret = drain_output_buffer(env, node, timeUs, &dequeue_count, frame, &got_frame);
if (opaque->acodec_first_dequeue_output_request) {
SDL_LockMutex(opaque->acodec_first_dequeue_output_mutex);
opaque->acodec_first_dequeue_output_request = false;
SDL_CondSignal(opaque->acodec_first_dequeue_output_cond);
SDL_UnlockMutex(opaque->acodec_first_dequeue_output_mutex);
}
//資料拉取出錯
if (ret != 0) {
ret = -1;
if (got_frame && frame->opaque) //release buffer false通知MediaCodec丟棄這一幀
{
SDL_VoutAndroid_releaseBufferProxyP(opaque->weak_vout, (SDL_AMediaCodecBufferProxy **)&frame->opaque, false);
}
goto fail;
}
if (got_frame) {
duration = (frame_rate.num && frame_rate.den ? av_q2d((AVRational){frame_rate.den, frame_rate.num}) : 0);
pts = (frame->pts == AV_NOPTS_VALUE) ? NAN : frame->pts * av_q2d(tb);
//設定的丟幀大於0或者 丟幀數不等於0同時主時鐘不是視訊時鐘
if (ffp->framedrop > 0 || (ffp->framedrop && ffp_get_master_sync_type(is) != AV_SYNC_VIDEO_MASTER)) {
ffp->stat.decode_frame_count++;//解碼幀計數
if (frame->pts != AV_NOPTS_VALUE) {
double dpts = pts;//如視訊某幀pts
double diff = dpts - ffp_get_master_clock(is);//視訊幀與音訊幀的差值（如果主時鐘是音訊的話）
//frame_last_filter_delay 這個時間是0，所以diff大於0，表示視訊比音訊快，不需要丟幀
//如果diff小於0，表示視訊比音訊慢，需要丟幀
//音訊比視訊快，且差距小於最大同步值，超過該值則不做同步處理了
if (!isnan(diff) && fabs(diff) < AV_NOSYNC_THRESHOLD &&
diff - is->frame_last_filter_delay < 0 &&
is->viddec.pkt_serial == is->vidclk.serial &&
is->videoq.nb_packets) {//解碼器中視訊幀的包序列等於視訊時鐘中的序列號，視訊佇列還有視訊幀
is->frame_drops_early++;
is->continuous_frame_drops_early++;//初始值是0
if (is->continuous_frame_drops_early > ffp->framedrop) {//如果continuous_frame_drops_early變數大於丟幀數，初始化ontinuous_frame_drops_early為0
is->continuous_frame_drops_early = 0;
} else {
ffp->stat.drop_frame_count++;//丟幀數加加
//丟幀率 = 丟幀數 / 解碼幀數
ffp->stat.drop_frame_rate = (float)(ffp->stat.drop_frame_count) / (float)(ffp->stat.decode_frame_count);
if (frame->opaque) {//通知mediacodec，釋放掉，不顯示
SDL_VoutAndroid_releaseBufferProxyP(opaque->weak_vout, (SDL_AMediaCodecBufferProxy **)&frame->opaque, false);
}
av_frame_unref(frame);//釋放掉frame
continue;
}
}
}
}
//幀入隊，放置解碼後視訊佇列中，在video_refresh中處理
ret = ffp_queue_picture(ffp, frame, pts, duration, av_frame_get_pkt_pos(frame), is->viddec.pkt_serial);
if (ret) {//入隊出錯，release buffer false通知MediaCodec丟棄這一幀，不顯示
if (frame->opaque)
SDL_VoutAndroid_releaseBufferProxyP(opaque->weak_vout, (SDL_AMediaCodecBufferProxy **)&frame->opaque, false);
}
av_frame_unref(frame);
}
}
fail:
av_frame_free(&frame);
opaque->abort = true;
SDL_WaitThread(opaque->enqueue_thread, NULL);
SDL_AMediaCodecFake_abort(opaque->acodec);
if (opaque->n_buf_out) {
free(opaque->amc_buf_out);
opaque->n_buf_out = 0;
opaque->amc_buf_out = NULL;
opaque->off_buf_out = 0;
opaque->last_queued_pts = AV_NOPTS_VALUE;
}
if (opaque->acodec) {
SDL_VoutAndroid_invalidateAllBuffers(opaque->weak_vout);
SDL_LockMutex(opaque->acodec_mutex);
SDL_UnlockMutex(opaque->acodec_mutex);
}
SDL_AMediaCodec_stop(opaque->acodec);
SDL_AMediaCodec_decreaseReferenceP(&opaque->acodec);
ALOGI("MediaCodec: %s: exit: %d", __func__, ret);
return ret;
#if 0 //硬解出錯，走軟解
fallback_to_ffplay:
ALOGW("fallback to ffplay decoder\n");
return ffp_video_thread(opaque->ffp);
#endif
}
複製程式碼2.2軟解碼丟幀設計
static int get_video_frame(FFPlayer *ffp, AVFrame *frame)
{
VideoState *is = ffp->is;
int got_picture;
//視訊流buffer載入快取統計
ffp_video_statistic_l(ffp);
//軟解耗時測試
//int64_t starttime = av_gettime_relative();
//解碼獲取解碼後的資料avframe
if ((got_picture = decoder_decode_frame(ffp, &is->viddec, frame, NULL)) < 0)
return -1;
/*
if(frame->key_frame) {//關鍵幀軟解耗時測試
int64_t endtime = av_gettime_relative();
int usetime = endtime - starttime;
ALOGE("zmlruan>>>>>>usetime:%d",usetime);
}*/
if (got_picture) {//解碼成功，拿到解碼資料
double dpts = NAN;
if (frame->pts != AV_NOPTS_VALUE)
dpts = av_q2d(is->video_st->time_base) * frame->pts;//視訊的pts轉換為ms,也就是當前進度時間
//視訊的寬高比
frame->sample_aspect_ratio = av_guess_sample_aspect_ratio(is->ic, is->video_st, frame);
//丟幀數大於0且同步不是按照video
if (ffp->framedrop>0 || (ffp->framedrop && get_master_sync_type(is) != AV_SYNC_VIDEO_MASTER)) {
ffp->stat.decode_frame_count++;//解碼數
if (frame->pts != AV_NOPTS_VALUE) {//diff = 視訊時間戳 減去主時鐘時間戳，（這裡看音訊時間戳）
double diff = dpts - get_master_clock(is); // AV_NOSYNC_THRESHOLD：同步閾值。如果誤差太大，則不進行校正
if (!isnan(diff) && fabs(diff) < AV_NOSYNC_THRESHOLD &&
diff - is->frame_last_filter_delay < 0 &&
is->viddec.pkt_serial == is->vidclk.serial &&
is->videoq.nb_packets) {
is->frame_drops_early++;
is->continuous_frame_drops_early++;
if (is->continuous_frame_drops_early > ffp->framedrop) {
is->continuous_frame_drops_early = 0;
} else {
ffp->stat.drop_frame_count++;//丟幀數加加
//丟幀率 = 丟幀數 / 解碼幀數
ffp->stat.drop_frame_rate = (float)(ffp->stat.drop_frame_count) / (float)(ffp->stat.decode_frame_count);
av_frame_unref(frame);//丟幀
got_picture = 0;//修改返回引數，表示沒獲取到視訊幀，丟掉了
}
}
}
}
}
return got_picture;
}
複製程式碼這樣我們就可以看到ijk中丟幀的實現邏輯了。