较新的WebRTC源代码中已经没有了与VoiceEngine结构相应的VidoeEngine了，取而代之的是MeidaEngine。MediaEngine包括了MediaEngineInterface接口及事实上现CompositeMediaEngine，CompositeMediaEngine本身也是个模板类，两个模板參数各自是音频引擎和视频引擎。CompositeMediaEngine派生类WebRtcMediaEngine依赖的模板參数是WebRtcVoiceEngine和WebRtcVideoEngine2。

        上图中base文件夹中是一些抽象类。engine文件夹中是相应抽象类的实现，使用时直接调用engine文件夹中的接口就可以。WebRtcVoiceEngine实际上是VoiceEngine的再次封装。它使用VoiceEngine进行音频处理。

注意命名，WebRtcVideoEngine2带了个2字，不用想，这肯定是个升级版本号的VideoEngine，还有个WebRtcVideoEngine类。

WebRtcVideoEngine2比WebRtcVideoEngine改进之处在于将视频流一分为二：发送流（WebRtcVideoSendStream）和接收流（WebRtcVideoReceiveStream），从而结构上更合理。源代码更清晰。

       本文的实现主要是使用了WebRtcVideoEngine2中WebRtcVideoCapturer类。

一.环境

參考上篇：

二.实现

        打开WebRtcVideoCapturer的头文件webrtcvideocapture.h。公有的函数基本上都是base文件夹中VideoCapturer类的实现，用于初始化设备和启动捕捉。私有函数OnIncomingCapturedFrame和OnCaptureDelayChanged会在摄像头採集模块VideoCaptureModeule中回调。将採集的图像传给OnIncomingCapturedFrame，并将採集的延时变化传给OnCaptureDelayChanged。

        WebRTC中也实现了类似Qt中的信号和槽机制，详见 。可是就像在该文中提到的，sigslot.h中的emit函数名会和Qt中的emit宏冲突。我将sigslot.h中的emit改成了Emit，当然改完之后，须要又一次编译rtc_baseproject。

       VideoCapturer类有两个信号sigslot::signal2<VideoCapturer*, CaptureState> SignalStateChange和sigslot::signal2<VideoCapturer*, const CapturedFrame*, sigslot::multi_threaded_local> SignalFrameCaptured，从SignalFrameCaptured的參数能够看出我们仅仅要实现相应的槽函数就能获取到CapturedFrame，在槽函数中将 CapturedFrame进行转换显示就可以。SignalStateChange信号的參数CaptureState是个枚举。标识捕捉的状态（停止、開始、正在进行、失败）。

        信号SignalFrameCaptured正是在回调函数OnIncomingCapturedFrame中发射出去的。

OnIncomingCapturedFrame里面用到了函数的异步运行。详见。

mainwindow.h

#ifndef MAINWINDOW_H#define MAINWINDOW_H#include 
     
      #include 
      
       #include 
       
      
     

mainwindow.cpp

#include "mainwindow.h"#include "ui_mainwindow.h"MainWindow::MainWindow(QWidget *parent) :    QMainWindow(parent),    ui(new Ui::MainWindow),    videoCapturer(new cricket::WebRtcVideoCapturer()),    videoFrame(new cricket::WebRtcVideoFrame()){   ui->setupUi(this);   getDeviceList();}MainWindow::~MainWindow(){    delete ui;    videoCapturer->SignalFrameCaptured.disconnect(this);    videoCapturer->SignalStateChange.disconnect(this);    videoCapturer->Stop();}void MainWindow::OnFrameCaptured(cricket::VideoCapturer* capturer,const cricket::CapturedFrame* frame){    videoFrame->Init(frame, frame->width, frame->height,true);    //将视频图像转成RGB格式    videoFrame->ConvertToRgbBuffer(cricket::FOURCC_ARGB,                                  videoImage.get(),                                  videoFrame->width()*videoFrame->height()*32/8,                                  videoFrame->width()*32/8);    QImage image(videoImage.get(), videoFrame->width(), videoFrame->height(), QImage::Format_RGB32);    ui->label->setPixmap(QPixmap::fromImage(image));}void MainWindow::OnStateChange(cricket::VideoCapturer* capturer, cricket::CaptureState state){}void MainWindow::getDeviceList(){    deviceNameList.clear();    deviceIDList.clear();    webrtc::VideoCaptureModule::DeviceInfo *info=webrtc::VideoCaptureFactory::CreateDeviceInfo(0);    int deviceNum=info->NumberOfDevices();    for (int i = 0; i < deviceNum; ++i)    {        const uint32_t kSize = 256;        char name[kSize] = {0};        char id[kSize] = {0};        if (info->GetDeviceName(i, name, kSize, id, kSize) != -1)        {            deviceNameList.append(QString(name));            deviceIDList.append(QString(id));            ui->comboBoxDeviceList->addItem(QString(name));        }    }    if(deviceNum==0)    {        ui->pushButtonOpen->setEnabled(false);    }}void MainWindow::on_pushButtonOpen_clicked(){    static bool flag=true;    if(flag)    {         ui->pushButtonOpen->setText(QStringLiteral("关闭"));        const std::string kDeviceName = ui->comboBoxDeviceList->currentText().toStdString();        const std::string kDeviceId = deviceIDList.at(ui->comboBoxDeviceList->currentIndex()).toStdString();        videoCapturer->Init(cricket::Device(kDeviceName, kDeviceId));        int width=videoCapturer->GetSupportedFormats()->at(0).width;        int height=videoCapturer->GetSupportedFormats()->at(0).height;        cricket::VideoFormat format(videoCapturer->GetSupportedFormats()->at(0));        //開始捕捉        if(cricket::CS_STARTING == videoCapturer->Start(format))        {            qDebug()<<"Capture is started";        }        //连接WebRTC的信号和槽        videoCapturer->SignalFrameCaptured.connect(this,&MainWindow::OnFrameCaptured);        videoCapturer->SignalStateChange.connect(this,&MainWindow::OnStateChange);        if(videoCapturer->IsRunning())        {            qDebug()<<"Capture is running";        }        videoImage.reset(new uint8_t[width*height*32/8]);    }    else    {        ui->pushButtonOpen->setText(QStringLiteral("打开"));        //反复连接会报错，须要先断开，才干再次连接        videoCapturer->SignalFrameCaptured.disconnect(this);        videoCapturer->SignalStateChange.disconnect(this);        videoCapturer->Stop();        if(!videoCapturer->IsRunning())        {            qDebug()<<"Capture is stoped";        }        ui->label->clear();    }    flag=!flag;}

main.cpp

#include "mainwindow.h"#include 
     
      int main(int argc, char *argv[]){    QApplication a(argc, argv);    MainWindow w;    w.show();    while(true)    {        //WebRTC消息循环        rtc::Thread::Current()->ProcessMessages(0);        rtc::Thread::Current()->SleepMs(1);        //Qt消息循环        a.processEvents( );    }}
     

注意main函数中对WebRTC和Qt消息循环的处理，这是用Qt调用WebRTC进行摄像头捕捉和显示的关键。

三.效果