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：WebRTC学习之三：录音和播放
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WebRTC学习之七：精炼的信号和槽机制 。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WebRTC学习之八：函数的异步运行。
mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H #include <QMainWindow>
#include <QDebug> #include <map>
#include <memory>
#include <string> #include "webrtc/base/sigslot.h"
#include "webrtc/modules/video_capture/video_capture.h"
#include "webrtc/modules/video_capture/video_capture_factory.h"
#include "webrtc/media/base/videocapturer.h"
#include "webrtc/media/engine/webrtcvideocapturer.h"
#include "webrtc/media/engine/webrtcvideoframe.h" namespace Ui {
class MainWindow;
} class MainWindow : public QMainWindow,public sigslot::has_slots<>
{
Q_OBJECT public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
void OnFrameCaptured(cricket::VideoCapturer* capturer, const cricket::CapturedFrame* frame);
void OnStateChange(cricket::VideoCapturer* capturer, cricket::CaptureState state); private slots:
void on_pushButtonOpen_clicked(); private:
void getDeviceList(); private:
Ui::MainWindow *ui;
cricket::WebRtcVideoCapturer *videoCapturer;
cricket::WebRtcVideoFrame *videoFrame;
std::unique_ptr<uint8_t[]> videoImage;
QStringList deviceNameList;
QStringList deviceIDList;
}; #endif // MAINWINDOW_H
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 <QApplication> 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