System.cc是整个系统的主接口

通过构造函数system()对SLAM系统初始化，传入ORB字典文件路径、配置文件路径、传感器类型

System::System(const string &strVocFile, const string &strSettingsFile, const eSensor sensor,
               const bool bUseViewer):mSensor(sensor), mpViewer(static_cast<Viewer*>(NULL)), mbReset(false),mbActivateLocalizationMode(false),
        mbDeactivateLocalizationMode(false)

//首先判断传感器类型是单目、双目还是RGB-D
    if(mSensor==MONOCULAR)
        cout << "Monocular" << endl;
    else if(mSensor==STEREO)
        cout << "Stereo" << endl;
    else if(mSensor==RGBD)
        cout << "RGB-D" << endl;

//Check settings file
        // 通过cv::FileStorage fsSetting()传入配置文件路径的读取配置文件，并由isOpened()判断文件是否存在。
    cv::FileStorage fsSettings(strSettingsFile.c_str(), cv::FileStorage::READ);
    if(!fsSettings.isOpened())
    {
       cerr << "Failed to open settings file at: " << strSettingsFile << endl;
       exit(-1);
    }

//若存在，则通过ORBVocabulary()加载ORB字典到mpVocabulary变量，然后通过loadFromTextFile()判断加载是否成功
    //Load ORB Vocabulary
    cout << endl << "Loading ORB Vocabulary. This could take a while..." << endl;

    mpVocabulary = new ORBVocabulary();
    bool bVocLoad = mpVocabulary->loadFromTextFile(strVocFile);
    if(!bVocLoad)
    {
        cerr << "Wrong path to vocabulary. " << endl;
        cerr << "Falied to open at: " << strVocFile << endl;
        exit(-1);
    }
    cout << "Vocabulary loaded!" << endl << endl;

        //创建关键帧数据库
    //Create KeyFrame Database
    mpKeyFrameDatabase = new KeyFrameDatabase(*mpVocabulary);

        //创建地图
    //Create the Map
    mpMap = new Map();

        //创建两个显示窗口
    //Create Drawers. These are used by the Viewer
    mpFrameDrawer = new FrameDrawer(mpMap);
    mpMapDrawer = new MapDrawer(mpMap, strSettingsFile);

        //初始化tracking对象
        //Initialize the Tracking thread
    //(it will live in the main thread of execution, the one that called this constructor)
    mpTracker = new Tracking(this, mpVocabulary, mpFrameDrawer, mpMapDrawer,
                             mpMap, mpKeyFrameDatabase, strSettingsFile, mSensor);


        //初始化local mapping对象，并开启线程运行
    //Initialize the Local Mapping thread and launch
    mpLocalMapper = new LocalMapping(mpMap, mSensor==MONOCULAR);
    mptLocalMapping = new thread(&ORB_SLAM2::LocalMapping::Run,mpLocalMapper);


        //初始化loop closing对象，并开启线程运行
    //Initialize the Loop Closing thread and launch
    mpLoopCloser = new LoopClosing(mpMap, mpKeyFrameDatabase, mpVocabulary, mSensor!=MONOCULAR);
    mptLoopClosing = new thread(&ORB_SLAM2::LoopClosing::Run, mpLoopCloser);


        //初始化显示线程Viewer()，开启线程显示图像和地图点
    //Initialize the Viewer thread and launch
    if(bUseViewer)
    {
        mpViewer = new Viewer(this, mpFrameDrawer,mpMapDrawer,mpTracker,strSettingsFile);
        mptViewer = new thread(&Viewer::Run, mpViewer);
        mpTracker->SetViewer(mpViewer);
    }


        //设置线程之间的指针
    //Set pointers between threads
    mpTracker->SetLocalMapper(mpLocalMapper);
    mpTracker->SetLoopClosing(mpLoopCloser);

    mpLocalMapper->SetTracker(mpTracker);
    mpLocalMapper->SetLoopCloser(mpLoopCloser);

    mpLoopCloser->SetTracker(mpTracker);
    mpLoopCloser->SetLocalMapper(mpLocalMapper);
}

通过TrackStereo()，TrackRGBD()，TrackMonocular()开启对应的跟踪线程

以TrackMonocular()为例：

//track 单目
cv::Mat System::TrackMonocular(const cv::Mat &im, const double ×tamp)
{
    //判断输入传感器类型是否对应正确
    if(mSensor!=MONOCULAR)
    {
        cerr << "ERROR: you called TrackMonocular but input sensor was not set to Monocular." << endl;
        exit(-1);
    }

    //检查模型
    // Check mode change
    {
        unique_lock<mutex> lock(mMutexMode);

        //如果是mbActivateLocalizationMode则休眠1000ms直到停止局部建图
        if(mbActivateLocalizationMode)
        {
            mpLocalMapper->RequestStop();

            // Wait until Local Mapping has effectively stopped
            while(!mpLocalMapper->isStopped())
            {
                usleep(1000);
            }

            mpTracker->InformOnlyTracking(true);
            mbActivateLocalizationMode = false;
        }

        //如果是mbDeactivateLocalizationMode则重新开启局部建图的线程
        if(mbDeactivateLocalizationMode)
        {
            mpTracker->InformOnlyTracking(false);
            mpLocalMapper->Release();
            mbDeactivateLocalizationMode = false;
        }
    }

    // Check reset
    {
    unique_lock<mutex> lock(mMutexReset);
    if(mbReset)
    {
        mpTracker->Reset();
        mbReset = false;
    }
    }

    //开启tracking线程
    cv::Mat Tcw = mpTracker->GrabImageMonocular(im,timestamp);

    unique_lock<mutex> lock2(mMutexState);
    mTrackingState = mpTracker->mState;
    mTrackedMapPoints = mpTracker->mCurrentFrame.mvpMapPoints;
    mTrackedKeyPointsUn = mpTracker->mCurrentFrame.mvKeysUn;

    return Tcw;
}