ffplay源码分析03——视频解码线程

ffplay源码分析03——视频解码线程

打开流：stream_component_open（）

文章末尾有彩蛋福利分享

/openagivenstream.Return0ifOK//*@briefstream_component_open@paramis@paramstream_index流索引@returnReturn0ifOK/staticintstream_component_open(VideoState*is,intstream_index){AVFormatContext*ic=is->ic;AVCodecContext*avctx;AVCodec*codec;constchar*forced_codec_name=NULL;AVDictionary*opts=NULL;AVDictionaryEntry*t=NULL;intsample_rate,nb_channels;int64_tchannel_layout;intret=0;intstream_lowres=lowres;if(stream_index<0||stream_index>=ic->nb_streams)return-1;/为解码器分配一个编解码器上下文结构体/avctx=avcodec_alloc_context3(NULL);if(!avctx)returnAVERROR(ENOMEM);/将码流中的编解码器信息拷贝到新分配的编解码器上下文结构体/ret=avcodec_parameters_to_context(avctx,ic->streams[stream_index]->codecpar);if(ret<0)gotofail;//设置pkt_timebaseavctx->pkt_timebase=ic->streams[stream_index]->time_base;/根据codec_id查找解码器/codec=avcodec_find_decoder(avctx->codec_id);switch(avctx->codec_type){caseAVMEDIA_TYPE_AUDIO:is->last_audio_stream=stream_index;forced_codec_name=audio_codec_name;break;caseAVMEDIA_TYPE_SUBTITLE:is->last_subtitle_stream=stream_index;forced_codec_name=subtitle_codec_name;break;caseAVMEDIA_TYPE_VIDEO:is->last_video_stream=stream_index;forced_codec_name=video_codec_name;break;}if(forced_codec_name)codec=avcodec_find_decoder_by_name(forced_codec_name);if(!codec){if(forced_codec_name)av_log(NULL,AV_LOG_WARNING,“Nocodeccouldbefoundwithname‘%s’\n”,forced_codec_name);elseav_log(NULL,AV_LOG_WARNING,“Nodecodercouldbefoundforcodec%s\n”,avcodec_get_name(avctx->codec_id));ret=AVERROR(EINVAL);gotofail;}avctx->codec_id=codec->id;if(stream_lowres>codec->max_lowres){av_log(avctx,AV_LOG_WARNING,“Themaximumvalueforlowressupportedbythedecoderis%d\n”,codec->max_lowres);stream_lowres=codec->max_lowres;}avctx->lowres=stream_lowres;if(fast)avctx->flags2|=AV_CODEC_FLAG2_FAST;opts=filter_codec_opts(codec_opts,avctx->codec_id,ic,ic->streams[stream_index],codec);if(!av_dict_get(opts,“threads”,NULL,0))av_dict_set(&opts,“threads”,“auto”,0);if(stream_lowres)av_dict_set_int(&opts,“lowres”,stream_lowres,0);if(avctx->codec_type==AVMEDIA_TYPE_VIDEO||avctx->codec_type==AVMEDIA_TYPE_AUDIO)av_dict_set(&opts,“refcounted_frames”,“1”,0);if((ret=avcodec_open2(avctx,codec,&opts))<0){gotofail;}if((t=av_dict_get(opts,“”,NULL,AV_DICT_IGNORE_SUFFIX))){av_log(NULL,AV_LOG_ERROR,“Option%snotfound.\n”,t->key);ret=AVERROR_OPTION_NOT_FOUND;gotofail;}is->eof=0;ic->streams[stream_index]->discard=AVDISCARD_DEFAULT;switch(avctx->codec_type){caseAVMEDIA_TYPE_AUDIO:#ifCONFIG_AVFILTER{AVFilterContext*sink;is->audio_filter_src.freq=avctx->sample_rate;is->audio_filter_src.channels=avctx->channels;is->audio_filter_src.channel_layout=get_valid_channel_layout(avctx->channel_layout,avctx->channels);is->audio_filter_src.fmt=avctx->sample_fmt;if((ret=configure_audio_filters(is,afilters,0))<0)gotofail;sink=is->out_audio_filter;sample_rate=av_buffersink_get_sample_rate(sink);nb_channels=av_buffersink_get_channels(sink);channel_layout=av_buffersink_get_channel_layout(sink);}#elsesample_rate=avctx->sample_rate;nb_channels=avctx->channels;channel_layout=avctx->channel_layout;#endif/prepareaudiooutput准备音频输出/if((ret=audio_open(is,channel_layout,nb_channels,sample_rate,&is->audio_tgt))<0)gotofail;is->audio_hw_buf_size=ret;is->audio_src=is->audio_tgt;is->audio_buf_size=0;is->audio_buf_index=0;/initaveragingfilter初始化averaging滤镜,非audiomaster时使用/is->audio_diff_avg_coef=exp(log(0.01)/AUDIO_DIFF_AVG_NB);is->audio_diff_avg_count=0;/由于我们没有精确的音频数据填充FIFO,故只有在大于该阈值时才进行校正音频同步/is->audio_diff_threshold=(double)(is->audio_hw_buf_size)/is->audio_tgt.bytes_per_sec;is->audio_stream=stream_index;//获取audio的stream索引is->audio_st=ic->streams[stream_index];//获取audio的stream指针//初始化ffplay封装的音频解码器decoder_init(&is->auddec,avctx,&is->audioq,is->continue_read_thread);if((is->ic->iformat->flags&(AVFMT_NOBINSEARCH|AVFMT_NOGENSEARCH|AVFMT_NO_BYTE_SEEK))&&!is->ic->iformat->read_seek){is->auddec.start_pts=is->audio_st->start_time;is->auddec.start_pts_tb=is->audio_st->time_base;}//启动音频解码线程if((ret=decoder_start(&is->auddec,audio_thread,“audio_decoder”,is))<0)gotoout;SDL_PauseAudioDevice(audio_dev,0);break;caseAVMEDIA_TYPE_VIDEO:is->video_stream=stream_index;//获取video的stream索引is->video_st=ic->streams[stream_index];//获取video的stream指针//初始化ffplay封装的视频解码器decoder_init(&is->viddec,avctx,&is->videoq,is->continue_read_thread);//启动视频频解码线程if((ret=decoder_start(&is->viddec,video_thread,“video_decoder”,is))<0)gotoout;is->queue_attachments_req=1;//使能请求mp3、aac等音频文件的封面break;caseAVMEDIA_TYPE_SUBTITLE://视频是类似逻辑处理is->subtitle_stream=stream_index;is->subtitle_st=ic->streams[stream_index];decoder_init(&is->subdec,avctx,&is->subtitleq,is->continue_read_thread);if((ret=decoder_start(&is->subdec,subtitle_thread,“subtitle_decoder”,is))<0)gotoout;break;default:break;}gotoout;fail:avcodec_free_context(&avctx);out:av_dict_free(&opts);returnret;}

启动解码线程：decoder_start()

/创建解码线程,audio/video有独立的线程/staticintdecoder_start(Decoder*d,int(fn)(void),constchar*thread_name,void*arg){packet_queue_start(d->queue);//启用对应的packet队列d->decoder_tid=SDL_CreateThread(fn,thread_name,arg);//创建解码线程if(!d->decoder_tid){av_log(NULL,AV_LOG_ERROR,“SDL_CreateThread():%s\n”,SDL_GetError());returnAVERROR(ENOMEM);}return0;}

启动packet队列packet_queue_start()

staticvoidpacket_queue_start(PacketQueue*q){SDL_LockMutex(q->mutex);q->abort_request=0;packet_queue_put_private(q,&flush_pkt);//这里放入了一个flush_pktSDL_UnlockMutex(q->mutex);}

pakcet包插入队列：packet_queue_put_private()

staticintpacket_queue_put_private(PacketQueue*q,AVPacket*pkt){MyAVPacketList*pkt1;if(q->abort_request)//如果已中止，则放入失败return-1;pkt1=av_malloc(sizeof(MyAVPacketList));//分配节点内存if(!pkt1)//内存不足，则放入失败return-1;//没有做引用计数，那这里也说明av_read_frame不会释放替用户释放buffer。pkt1->pkt=*pkt;//拷贝AVPacket(浅拷贝，AVPacket.data等内存并没有拷贝)pkt1->next=NULL;if(pkt==&flush_pkt)//如果放入的是flush_pkt，需要增加队列的播放序列号，以区分不连续的两段数据{q->serial++;printf(“q->serial=%d\n”,q->serial);}pkt1->serial=q->serial;//用队列序列号标记节点/*队列操作：如果last_pkt为空，说明队列是空的，新增节点为队头；否则，队列有数据，则让原队尾的next为新增节点。最后将队尾指向新增节点/if(!q->last_pkt)q->first_pkt=pkt1;elseq->last_pkt->next=pkt1;q->last_pkt=pkt1;//队列属性操作：增加节点数、cache大小、cache总时长,用来控制队列的大小q->nb_packets++;q->size+=pkt1->pkt.size+sizeof(*pkt1);q->duration+=pkt1->pkt.duration;/XXX:shouldduplicatepacketdatainDVcase///发出信号，表明当前队列中有数据了，通知等待中的读线程可以取数据了SDL_CondSignal(q->cond);return0;}

flush_pkt包用来区分连续不同的流，这里有几个地方用到：队列初始化的时候seek的时候，插入，可以冲刷之前缓存的流

视频解码线程：video_thread()

//视频解码线程staticintvideo_thread(void*arg){VideoState*is=arg;AVFrame*frame=av_frame_alloc();//分配解码帧doublepts;//ptsdoubleduration;//帧持续时间intret;//1获取streamtimebaseAVRationaltb=is->video_st->time_base;//获取streamtimebase//2获取帧率，以便计算每帧picture的durationAVRationalframe_rate=av_guess_frame_rate(is->ic,is->video_st,NULL);if(!frame)returnAVERROR(ENOMEM);for(;;){//循环取出视频解码的帧数据//3获取解码后的视频帧ret=get_video_frame(is,frame);if(ret<0)gotothe_end;//解码结束,什么时候会结束if(!ret)//没有解码得到画面,什么情况下会得不到解后的帧continue;//4计算帧持续时间和换算pts值为秒//1/帧率=duration单位秒,没有帧率时则设置为0,有帧率帧计算出帧间隔duration=(frame_rate.num&&frame_rate.den?av_q2d((AVRational){frame_rate.den,frame_rate.num}):0);//根据AVStreamtimebase计算出pts值,单位为秒pts=(frame->pts==AV_NOPTS_VALUE)?NAN:frame->ptsav_q2d(tb);//5将解码后的视频帧插入队列ret=queue_picture(is,frame,pts,duration,frame->pkt_pos,is->viddec.pkt_serial);//6释放frame对应的数据av_frame_unref(frame);if(ret<0)//返回值小于0则退出线程gotothe_end;}the_end:av_frame_free(&frame);return0;}

获取视频帧：get_video_frame()

/**@brief获取视频帧@paramis@paramframe指向获取的视频帧@return/staticintget_video_frame(VideoState*is,AVFrame*frame){intgot_picture;//1.获取解码后的视频帧if((got_picture=decoder_decode_frame(&is->viddec,frame,NULL))<0){return-1;//返回-1意味着要退出解码线程,所以要分析decoder_decode_frame什么情况下返回-1}if(got_picture){//2.分析获取到的该帧是否要drop掉,该机制的目的是在放入帧队列前先drop掉过时的视频帧doubledpts=NAN;if(frame->pts!=AV_NOPTS_VALUE)dpts=av_q2d(is->video_st->time_base)*frame->pts;//计算出秒为单位的ptsframe->sample_aspect_ratio=av_guess_sample_aspect_ratio(is->ic,is->video_st,frame);if(framedrop>0||//允许drop帧(framedrop&&get_master_sync_type(is)!=AV_SYNC_VIDEO_MASTER))//非视频同步模式{if(frame->pts!=AV_NOPTS_VALUE){//pts值有效doublediff=dpts-get_master_clock(is);if(!isnan(diff)&&//差值有效fabs(diff)<AV_NOSYNC_THRESHOLD&&//差值在可同步范围呢diff-is->frame_last_filter_delay<0&&//和过滤器有关系is->viddec.pkt_serial==is->vidclk.serial&&//同一序列的包is->videoq.nb_packets){//packet队列至少有1帧数据is->frame_drops_early++;printf(“%s(%d)diff:%lfs,dropframe,drops:%d\n”,FUNCTION,LINE,diff,is->frame_drops_early);av_frame_unref(frame);got_picture=0;}}}}returngot_picture;}

decoder_decode_frame()

staticintdecoder_decode_frame(Decoder*d,AVFrame*frame,AVSubtitle*sub){intret=AVERROR(EAGAIN);for(;;){AVPacketpkt;//1.流连续情况下获取解码后的帧if(d->queue->serial==d->pkt_serial){//1.1先判断是否是同一播放序列的数据do{if(d->queue->abort_request)return-1;//是否请求退出//1.2.获取解码帧switch(d->avctx->codec_type){caseAVMEDIA_TYPE_VIDEO:ret=avcodec_receive_frame(d->avctx,frame);//printf(“framepts:%ld,dts:%ld\n”,frame->pts,frame->pkt_dts);if(ret>=0){if(decoder_reorder_pts==-1){frame->pts=frame->best_effort_timestamp;}elseif(!decoder_reorder_pts){frame->pts=frame->pkt_dts;}}break;caseAVMEDIA_TYPE_AUDIO:ret=avcodec_receive_frame(d->avctx,frame);if(ret>=0){AVRationaltb=(AVRational){1,frame->sample_rate};if(frame->pts!=AV_NOPTS_VALUE)frame->pts=av_rescale_q(frame->pts,d->avctx->pkt_timebase,tb);elseif(d->next_pts!=AV_NOPTS_VALUE)frame->pts=av_rescale_q(d->next_pts,d->next_pts_tb,tb);if(frame->pts!=AV_NOPTS_VALUE){d->next_pts=frame->pts+frame->nb_samples;d->next_pts_tb=tb;}}break;}//1.3.检查解码是否已经结束，解码结束返回0if(ret==AVERROR_EOF){d->finished=d->pkt_serial;printf(“avcodec_flush_buffers%s(%d)\n”,FUNCTION,LINE);avcodec_flush_buffers(d->avctx);return0;}//1.4.正常解码返回1if(ret>=0)return1;}while(ret!=AVERROR(EAGAIN));//1.5没帧可读时ret返回EAGIN，需要继续送packet}//2获取一个packet，如果播放序列不一致(数据不连续)则过滤掉“过时”的packetdo{//2.1如果没有数据可读则唤醒read_thread,实际是continue_read_threadSDL_condif(d->queue->nb_packets==0)//没有数据可读SDL_CondSignal(d->empty_queue_cond);//通知read_thread放入packet//2.2如果还有pending的packet则使用它if(d->packet_pending){av_packet_move_ref(&pkt,&d->pkt);d->packet_pending=0;}else{//2.3阻塞式读取packetif(packet_queue_get(d->queue,&pkt,1,&d->pkt_serial)<0)return-1;}if(d->queue->serial!=d->pkt_serial){//darren自己的代码printf(“%s(%d)discontinue:queue->serial:%d,pkt_serial:%d\n”,FUNCTION,LINE,d->queue->serial,d->pkt_serial);av_packet_unref(&pkt);//fixedme?释放要过滤的packet}}while(d->queue->serial!=d->pkt_serial);//如果不是同一播放序列(流不连续)则继续读取//3将packet送入解码器if(pkt.data==flush_pkt.data){////whenseekingorwhenswitchingtoadifferentstreamavcodec_flush_buffers(d->avctx);//清空里面的缓存帧d->finished=0;//重置为0d->next_pts=d->start_pts;//主要用在了audiod->next_pts_tb=d->start_pts_tb;//主要用在了audio}else{if(d->avctx->codec_type==AVMEDIA_TYPE_SUBTITLE){intgot_frame=0;ret=avcodec_decode_subtitle2(d->avctx,sub,&got_frame,&pkt);if(ret<0){ret=AVERROR(EAGAIN);}else{if(got_frame&&!pkt.data){d->packet_pending=1;av_packet_move_ref(&d->pkt,&pkt);}ret=got_frame?0:(pkt.data?AVERROR(EAGAIN):AVERROR_EOF);}}else{if(avcodec_send_packet(d->avctx,&pkt)==AVERROR(EAGAIN)){av_log(d->avctx,AV_LOG_ERROR,“Receive_frameandsend_packetbothreturnedEAGAIN,whichisanAPIviolation.\n”);d->packet_pending=1;av_packet_move_ref(&d->pkt,&pkt);}}av_packet_unref(&pkt);//一定要自己去释放音视频数据}}}

packet_pending含义：我们主要分析这个端代码：

if(avcodec_send_packet(d->avctx,&pkt)==AVERROR(EAGAIN)){av_log(d->avctx,AV_LOG_ERROR,“Receive_frameandsend_packetbothreturnedEAGAIN,whichisanAPIviolation.\n”);d->packet_pending=1;av_packet_move_ref(&d->pkt,&pkt);}

我们看看ffmpeg源码：

intattribute_align_argavcodec_send_packet(AVCodecContext*avctx,constAVPacket*avpkt){AVCodecInternal*avci=avctx->internal;intret;if(!avcodec_is_open(avctx)||!av_codec_is_decoder(avctx->codec))returnAVERROR(EINVAL);if(avctx->internal->draining)returnAVERROR_EOF;if(avpkt&&!avpkt->size&&avpkt->data)returnAVERROR(EINVAL);av_packet_unref(avci->buffer_pkt);if(avpkt&&(avpkt->data||avpkt->side_data_elems)){ret=av_packet_ref(avci->buffer_pkt,avpkt);if(ret<0)returnret;}ret=av_bsf_send_packet(avci->filter.bsfs[0],avci->buffer_pkt);if(ret<0){av_packet_unref(avci->buffer_pkt);returnret;}if(!avci->buffer_frame->buf[0]){ret=decode_receive_frame_internal(avctx,avci->buffer_frame);if(ret<0&&ret!=AVERROR(EAGAIN)&&ret!=AVERROR_EOF)returnret;}return0;}intav_bsf_send_packet(AVBSFContext*ctx,AVPacket*pkt){intret;if(!pkt||(!pkt->data&&!pkt->side_data_elems)){ctx->internal->eof=1;return0;}if(ctx->internal->eof){av_log(ctx,AV_LOG_ERROR,“Anon-NULLpacketsentafteranEOF.\n”);returnAVERROR(EINVAL);}if(ctx->internal->buffer_pkt->data||ctx->internal->buffer_pkt->side_data_elems)returnAVERROR(EAGAIN);ret=av_packet_make_refcounted(pkt);if(ret<0)returnret;av_packet_move_ref(ctx->internal->buffer_pkt,pkt);return0;}

可以看到，返回AVERROR(EAGAIN)表示有数据，这个其实没有送进去，所以这里就比较好理解了，就是缓存一下数据。

学习提升资料包免费分享,资料内容包括《Andoird音视频开发必备手册+音视频最新学习视频+大厂面试真题+2022最新学习路线图》，点击自取FFmpegWebRTCRTMPRTSPHLSRTP播放器-音视频流媒体高级开发