WebRTC学习之九：摄像头的捕捉和显示

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