前言

本示例主要演示osgEarth的事件处理的用法，内容比较多，这部分功能也很重要。

输入命令依然采用china-simple.earth的示例，加上了模型，但是模型并没有看到，可能是因为模型没有放大太小的原因。在代码中设置了不加载模型的话，会绘制两个半圆。

// 加载模型，但是看不到，估计是模型太小，需要放大
osgearth_manipd.exe earth_image/china-simple.earth --model axes.osgt

// 并没有加载模型
osgearth_manipd.exe earth_image/china-simple.earth --model modelToLoad

// 不加载模型，会默认创建一个半球，距离太远的话，半球也看不到
osgearth_manipd.exe earth_image/china-simple.earth

执行效果

按下W键，开启坐标，也就是黄色的两条线，交点始终跟随 Thing 1 节点。

按键说明

[注] 必须在英文下输入，如果没有注意，在中文状态下输入字符，地球会被卡住。此时只能退出重新运行程序。

thing1 thing 2,这两个文本是移动的。

代码分析

#include <string>

#include <osg/Notify>
#include <osg/Timer>
#include <osg/ShapeDrawable>
#include <osg/Depth>
#include <osg/PositionAttitudeTransform>
#include <osgGA/StateSetManipulator>
#include <osgGA/GUIEventHandler>
#include <osgViewer/Viewer>
#include <osgViewer/ViewerEventHandlers>
#include <osgEarth/GeoMath>
#include <osgEarth/GeoTransform>
#include <osgEarth/MapNode>
#include <osgEarth/TerrainEngineNode>
#include <osgEarth/Viewpoint>
#include <osgEarthUtil/EarthManipulator>
#include <osgEarthUtil/Controls>
#include <osgEarthUtil/ExampleResources>
#include <osgEarthUtil/LogarithmicDepthBuffer>
#include <osgEarthUtil/ViewFitter>
#include <osgEarthAnnotation/AnnotationUtils>
#include <osgEarthAnnotation/LabelNode>
#include <osgEarthSymbology/Style>
#include <osgEarth/ScreenSpaceLayout>

using namespace osgEarth::Util;
using namespace osgEarth::Util::Controls;
using namespace osgEarth::Annotation;

#define D2R (osg::PI/180.0)
#define R2D (180.0/osg::PI)

namespace
{
    /**
     * Tether callback test.是否绑定
     */
    struct TetherCB : public EarthManipulator::TetherCallback
    {
        void operator()(osg::Node* node)
        {
            if ( node ) {
                OE_WARN << "Tether on\n";
            }
            else {
                OE_WARN << "Tether off\n";
            }
        }
    };

    /**
     * Builds our help menu UI.创建左上角的说明文档
     */
    Container* createHelp( osgViewer::View* view )
    {
        const char* text[] =
        {
            "left mouse :",        "pan",
            "middle mouse :",      "rotate",
            "right mouse :",       "continuous zoom",
            "double-click :",      "zoom to point",
            "scroll wheel :",      "zoom in/out",
            "arrows :",            "pan",
            //"1-6 :",               "fly to preset viewpoints",
            "shift-left-mouse :",  "locked pan",
            "u :",                 "toggle azimuth lock",
            "o :",                 "toggle perspective/ortho",
            "8 :",                 "Tether to thing 1",
            "9 :",                 "Tether to thing 2",
            "t :",                 "cycle tethermode",
            "b :",                 "break tether",
            "a :",                 "toggle viewpoint arcing",
            "q :",                 "toggle throwing",
            "k :",                 "toggle collision",
            "L :",                 "toggle log depth buffer"
			//"W :",				   "坐标系",		// 源代码并没有这两个提示
			//"j :",				   "定位到指定点组的视点",
        };

        Grid* g = new Grid();// 创建网格
        unsigned i, c, r;
		std::cout << sizeof(text) << std::endl;		// 存储所有指针占位多少
		std::cout << sizeof(text[0]) << std::endl;	// 每个指针占位多少
        for( i=0; i<sizeof(text)/sizeof(text[0]); ++i )
        {
            c = i % 2;
            r = i / 2;
            g->setControl( c, r, new LabelControl(text[i]) );// 两列c，i行r,显示label控件
        }

        VBox* v = new VBox();
        v->addControl( g );// 将网格控件放入VBox控件中

        return v;
    }


    /**
     * Some preset viewpoints to show off the setViewpoint function.
     */
    static Viewpoint VPs[] = {
        Viewpoint( "Africa",            0.0,   0.0, 0.0, 0.0, -90.0, 10e6 ),
        Viewpoint( "California",     -121.0,  34.0, 0.0, 0.0, -90.0, 6e6 ),
        Viewpoint( "Europe",            0.0,  45.0, 0.0, 0.0, -90.0, 4e6 ),
        Viewpoint( "Washington DC",   -77.0,  38.0, 0.0, 0.0, -90.0, 1e6 ),
        Viewpoint( "Australia",       135.0, -20.0, 0.0, 0.0, -90.0, 2e6 ),
        Viewpoint( "Boston",         -71.096936, 42.332771, 0, 0.0, -90, 1e5 )
    };


    /**
     * Handler that demonstrates the "viewpoint" functionality in 
     *  osgEarthUtil::EarthManipulator. Press a number key to fly to a viewpoint.
     */
    struct FlyToViewpointHandler : public osgGA::GUIEventHandler 
    {
        FlyToViewpointHandler( EarthManipulator* manip ) : _manip(manip) { }

        bool handle( const osgGA::GUIEventAdapter& ea, osgGA::GUIActionAdapter& aa )
        {
            if ( ea.getEventType() == ea.KEYDOWN && ea.getKey() >= '1' && ea.getKey() <= '6' )
            {
                _manip->setViewpoint( VPs[ea.getKey()-'1'], 4.0 );// 根据设置好的位置，123456定位不同的地点
                aa.requestRedraw();
            }
            return false;
        }

        osg::observer_ptr<EarthManipulator> _manip;
    };
    

    /**
     * Toggles the logarithmic depth buffer切换对数深度缓冲区
     */
    struct ToggleLDB : public osgGA::GUIEventHandler
    {
        ToggleLDB(char key) : _key(key), _installed(false) { }// key=L

        bool handle(const osgGA::GUIEventAdapter& ea, osgGA::GUIActionAdapter& aa)
        {
            if (ea.getEventType() == ea.KEYDOWN && ea.getKey() == _key)
            {
                if ( !_installed )
                {
                    _nfratio = aa.asView()->getCamera()->getNearFarRatio();// 获取到近/远率
                    _ldb.install(aa.asView()->getCamera());
                    aa.asView()->getCamera()->setNearFarRatio(0.00001);
                }
                else
                {
                    _ldb.uninstall(aa.asView()->getCamera());
                    aa.asView()->getCamera()->setNearFarRatio(_nfratio);
                }

                _installed = !_installed;// 每次切换
                return true;
            }
            return false;
        }

        void getUsage(osg::ApplicationUsage& usage) const
        {
            using namespace std;
            usage.addKeyboardMouseBinding(string(1, _key), string("Toggle LDB"));
        }

        char _key;
        float _nfratio;
        bool _installed;
        osgEarth::Util::LogarithmicDepthBuffer _ldb;
    };

    /**
     * Toggles screen space layout on the sismulated objects 切换模拟对象的屏幕空间布局
     */
    struct ToggleSSL : public osgGA::GUIEventHandler
    {
        ToggleSSL(osg::Group* g, char key) : _group(g), _key(key), _installed(false) { }// key=)

        bool handle(const osgGA::GUIEventAdapter& ea, osgGA::GUIActionAdapter& aa)
        {
            if (ea.getEventType() == ea.KEYDOWN && ea.getKey() == _key)
            {
                _installed = !_installed;
                ScreenSpaceLayout::setDeclutteringEnabled(_installed);// 启动或禁用清理
                return true;
            }
            return false;
        }

        void getUsage(osg::ApplicationUsage& usage) const
        {
            using namespace std;
            usage.addKeyboardMouseBinding(string(1, _key), string("Toggle SSL"));
        }

        char _key;
        osg::Group* _group;
        bool _installed;
    };

    /**
     * Handler to toggle "azimuth locking", which locks the camera's relative Azimuth
     * while panning. For example, it can maintain "north-up" as you pan around. The
     * caveat is that when azimuth is locked you cannot cross the poles.
     */
    struct LockAzimuthHandler : public osgGA::GUIEventHandler // 锁定/解锁 方位角
    {
        LockAzimuthHandler(char key, EarthManipulator* manip) // 传入key = u
            : _key(key), _manip(manip) { }

        bool handle(const osgGA::GUIEventAdapter& ea, osgGA::GUIActionAdapter& aa)
        {
            if (ea.getEventType() == ea.KEYDOWN && ea.getKey() == _key)
            {
                bool lockAzimuth = _manip->getSettings()->getLockAzimuthWhilePanning();
				std::cout << "按下 u" << std::endl;// 确实收到消息，但是在操作上并没有看到所定方位角
                _manip->getSettings()->setLockAzimuthWhilePanning(!lockAzimuth);// 设置为当前状态相反的状态
                aa.requestRedraw();
                return true;
            }
            return false;
        }

        void getUsage(osg::ApplicationUsage& usage) const
        {
            using namespace std;
            usage.addKeyboardMouseBinding(string(1, _key), string("Toggle azimuth locking"));
        }

        char _key;
        osg::ref_ptr<EarthManipulator> _manip;
    };


    /**
     * Handler to toggle "viewpoint transition arcing", which causes the camera to "arc"
     * as it travels from one viewpoint to another.
     */
    struct ToggleArcViewpointTransitionsHandler : public osgGA::GUIEventHandler
    {
        ToggleArcViewpointTransitionsHandler(char key, EarthManipulator* manip)
            : _key(key), _manip(manip) { }// key=a

        bool handle(const osgGA::GUIEventAdapter& ea, osgGA::GUIActionAdapter& aa)
        {
            if (ea.getEventType() == ea.KEYDOWN && ea.getKey() == _key)
            {
                bool arc = _manip->getSettings()->getArcViewpointTransitions();
				std::cout << "按下 a" << std::endl;// 确实收到消息，但是没看出什么变化
                _manip->getSettings()->setArcViewpointTransitions(!arc);
                aa.requestRedraw();
                return true;
            }
            return false;
        }

        void getUsage(osg::ApplicationUsage& usage) const
        {
            using namespace std;
            usage.addKeyboardMouseBinding(string(1, _key), string("Arc viewpoint transitions"));
        }

        char _key;
        osg::ref_ptr<EarthManipulator> _manip;
    };


    /**
     * Toggles the throwing feature.切换投掷功能。
     */
    struct ToggleThrowingHandler : public osgGA::GUIEventHandler
    {
        ToggleThrowingHandler(char key, EarthManipulator* manip)
            : _key(key), _manip(manip)// key=q
        {
        }

        bool handle(const osgGA::GUIEventAdapter& ea, osgGA::GUIActionAdapter& aa)
        {
            if (ea.getEventType() == ea.KEYDOWN && ea.getKey() == _key)
            {
                bool throwing = _manip->getSettings()->getThrowingEnabled();
				std::cout << "按下 q" << std::endl;// 确实收到消息，但是没看出什么变化
                _manip->getSettings()->setThrowingEnabled( !throwing );
                aa.requestRedraw();
                return true;
            }
            return false;
        }

        void getUsage(osg::ApplicationUsage& usage) const
        {
            using namespace std;
            usage.addKeyboardMouseBinding(string(1, _key), string("Toggle throwing"));
        }

        char _key;
        osg::ref_ptr<EarthManipulator> _manip;
    };

    /**
     * Toggles the collision feature.切换碰撞功能。
     */
    struct ToggleCollisionHandler : public osgGA::GUIEventHandler
    {
        ToggleCollisionHandler(char key, EarthManipulator* manip)
            : _key(key), _manip(manip)// key=k
        {
        }

        bool handle(const osgGA::GUIEventAdapter& ea, osgGA::GUIActionAdapter& aa)
        {
            if (ea.getEventType() == ea.KEYDOWN && ea.getKey() == _key)
            {
                bool value = _manip->getSettings()->getTerrainAvoidanceEnabled();
				std::cout << "按下 k" << std::endl;// 确实收到消息，但是没看出什么变化
                _manip->getSettings()->setTerrainAvoidanceEnabled( !value );
                aa.requestRedraw();
                return true;
            }
            return false;
        }

        void getUsage(osg::ApplicationUsage& usage) const
        {
            using namespace std;
            usage.addKeyboardMouseBinding(string(1, _key), string("Toggle terrain avoidance"));
        }

        char _key;
        osg::ref_ptr<EarthManipulator> _manip;
    };

    /**
     * Breaks a tether.三种状态的切换
     */
    struct CycleTetherMode : public osgGA::GUIEventHandler
    {
        CycleTetherMode(char key, EarthManipulator* manip)
            : _key(key), _manip(manip) { }// key=t

        bool handle(const osgGA::GUIEventAdapter& ea, osgGA::GUIActionAdapter& aa)
        {
            if (ea.getEventType() == ea.KEYDOWN && ea.getKey() == _key)
            {
                EarthManipulator::TetherMode mode = _manip->getSettings()->getTetherMode();
                if ( mode == _manip->TETHER_CENTER ) { 
                    _manip->getSettings()->setTetherMode( _manip->TETHER_CENTER_AND_HEADING );// 相机将跟随节点，仅跟随方向旋转
                    OE_NOTICE << "Tether mode = TETHER_CENTER_AND_HEADING\n";
                }
                else if ( mode == _manip->TETHER_CENTER_AND_HEADING ) {
                    _manip->getSettings()->setTetherMode( _manip->TETHER_CENTER_AND_ROTATION ); // 摄影机将跟随节点和节点所做的所有旋转
                    OE_NOTICE << "Tether mode = TETHER_CENTER_AND_ROTATION\n";
                }
                else {
                    _manip->getSettings()->setTetherMode( _manip->TETHER_CENTER );// 摄影机将跟随节点的中心。
                    OE_NOTICE << "Tether mode = CENTER\n";
                }

                aa.requestRedraw();
                return true;
            }
            return false;
        }

        void getUsage(osg::ApplicationUsage& usage) const
        {
            using namespace std;
            usage.addKeyboardMouseBinding(string(1, _key), string("Cycle Tether Mode"));
        }

        char _key;
        osg::ref_ptr<EarthManipulator> _manip;
    };

    /**
     * Breaks a tether.解绑
     */
    struct BreakTetherHandler : public osgGA::GUIEventHandler
    {
        BreakTetherHandler(char key, EarthManipulator* manip)
            : _key(key), _manip(manip) { }//key=b

        bool handle(const osgGA::GUIEventAdapter& ea, osgGA::GUIActionAdapter& aa)
        {
            if (ea.getEventType() == ea.KEYDOWN && ea.getKey() == _key)
            {
                _manip->clearViewpoint();// 解除所有操作器绑定，恢复自由状态。thing1和 thing2 的状态改变明显
                aa.requestRedraw();
                return true;
            }
            return false;
        }

        void getUsage(osg::ApplicationUsage& usage) const
        {
            using namespace std;
            usage.addKeyboardMouseBinding(string(1, _key), string("Break a tether"));
        }

        char _key;
        osg::ref_ptr<EarthManipulator> _manip;
    };
    

    /**
     * Adjusts the position offset.调整位置偏移。
     */
    struct SetPositionOffset : public osgGA::GUIEventHandler
    {
        SetPositionOffset(EarthManipulator* manip)
            : _manip(manip) { }

        bool handle(const osgGA::GUIEventAdapter& ea, osgGA::GUIActionAdapter& aa)
        {
            if (ea.getEventType() == ea.KEYDOWN && (ea.getModKeyMask() & ea.MODKEY_SHIFT) )
            {
                Viewpoint oldvp = _manip->getViewpoint();

                double seconds = 0.5; // 时间秒。每次切换都需要0.5秒

                if ( ea.getKey() == ea.KEY_Left )// 左键
                {
                    Viewpoint vp;
                    vp.positionOffset() = oldvp.positionOffset().get() + osg::Vec3f(-1000,0,0);
                    _manip->setViewpoint( vp, seconds );
                }
                else if ( ea.getKey() == ea.KEY_Right )// 右键
                {
                    Viewpoint vp;
                    vp.positionOffset() = oldvp.positionOffset().get() + osg::Vec3f(1000,0,0);
                    _manip->setViewpoint( vp, seconds );
                }
                else if ( ea.getKey() == ea.KEY_Up )// 上键
                {
                    Viewpoint vp;
                    vp.positionOffset() = oldvp.positionOffset().get() + osg::Vec3f(0,0,1000);
                    _manip->setViewpoint( vp, seconds );
                }
                else if ( ea.getKey() == ea.KEY_Down )// 下键
                {
                    Viewpoint vp;
                    vp.positionOffset() = oldvp.positionOffset().get() + osg::Vec3f(0,0,-1000);
                    _manip->setViewpoint( vp, seconds );
                }
                aa.requestRedraw();
                return true;
            }
            return false;
        }

        osg::ref_ptr<EarthManipulator> _manip;
    };


    /**
     * Toggles perspective/ortho projection matrix. 切换投影矩阵，透视投影矩阵和正射投影矩阵
     */
    struct ToggleProjMatrix : public osgGA::GUIEventHandler
    {
        ToggleProjMatrix(char key, EarthManipulator* manip)
            : _key(key), _manip(manip)// ley=o
        {
        }

        bool handle(const osgGA::GUIEventAdapter& ea, osgGA::GUIActionAdapter& aa)
        {
            if (ea.getEventType() == ea.KEYDOWN && ea.getKey() == _key)
            {
                osg::Matrix proj = aa.asView()->getCamera()->getProjectionMatrix();
                if ( proj(3,3) == 0 )
                {
                    OE_NOTICE << "Switching to orthographc.\n";
                    proj.getPerspective(_vfov, _ar, _zn, _zf); // 获取值
                    aa.asView()->getCamera()->setProjectionMatrixAsOrtho(-1, 1, -1, 1, _zn, _zf);// 正射投影矩阵
                }
                else
                {
                    OE_NOTICE << "Switching to perspective.\n";
                    aa.asView()->getCamera()->setProjectionMatrixAsPerspective(_vfov, _ar, _zn, _zf);// 透视矩阵
                }
                aa.requestRedraw();
                return true;
            }
            return false;
        }

        void getUsage(osg::ApplicationUsage& usage) const
        {
            using namespace std;
            usage.addKeyboardMouseBinding(string(1, _key), string("Toggle projection matrix type"));
        }

        char _key;
        osg::ref_ptr<EarthManipulator> _manip;
        double _vfov, _ar, _zn, _zf;
    };
    // 定位到某组点的视点，一组点都可以看到
    struct FitViewToPoints : public osgGA::GUIEventHandler
    {
        std::vector<GeoPoint> _points;
        const SpatialReference* _mapSRS;

        FitViewToPoints(char key, EarthManipulator* manip, const SpatialReference* mapSRS)
            : _key(key), _manip(manip), _mapSRS(mapSRS)// key=j
        {
            // Set up a list of control points
            const SpatialReference* srs = SpatialReference::get("wgs84");
            _points.push_back(GeoPoint(srs, -120, 30, 0));
            _points.push_back(GeoPoint(srs, -100, 45, 0));
        }

        bool handle(const osgGA::GUIEventAdapter& ea, osgGA::GUIActionAdapter& aa)
        {
            if (ea.getEventType() == ea.KEYDOWN && ea.getKey() == _key)
            {
				// fitter 创建适合摄影机视图平截头体的视点，以尽可能紧密地包围一组地理空间点。
                ViewFitter fitter(_mapSRS, aa.asView()->getCamera());
                fitter.setBuffer( 100000.0 );
                Viewpoint vp;
                if (fitter.createViewpoint(_points, vp))// 根据_points重新生成一个vp视点
                {
                    _manip->setViewpoint(vp);// 定位到vp视点
                    aa.requestRedraw();
                }
                return true;
            }
            return false;
        }

        void getUsage(osg::ApplicationUsage& usage) const
        {
            using namespace std;
            usage.addKeyboardMouseBinding(string(1, _key), string("FitViewToPoints"));
        }

        char _key;
        osg::ref_ptr<EarthManipulator> _manip;
    };
        

    /**
     * A simple simulator that moves an object around the Earth. We use this to
     * demonstrate/test tethering.
     */
	// 一个简单的模拟器，可以使物体绕地球运动。用来演示跟踪thing
    struct Simulator : public osgGA::GUIEventHandler
    {
        Simulator( osg::Group* root, EarthManipulator* manip, MapNode* mapnode, osg::Node* model, const char* name, char key)
            : _manip(manip), _mapnode(mapnode), _model(model), _name(name), _key(key) // key=8 或 9
        {
            if ( !model )
            { 
				// 模型不存在，则创建一个半球，半径和颜色
                _model = AnnotationUtils::createHemisphere(250.0, osg::Vec4(1,.7,.4,1));
				std::cout << "no model" << std::endl;
            }

            _geo = new GeoPositionNode();
            _geo->getPositionAttitudeTransform()->addChild(_model);

            Style style;
            TextSymbol* text = style.getOrCreate<TextSymbol>();
            text->size() = 32.0f;
            text->declutter() = false;
            text->pixelOffset()->set(50, 50);// 文本偏移像素点
            
            _label = new LabelNode(_name, style);// _name 即要显示的名称
            _label->setDynamic( true );// 因为文本移动，所以此处设置为true
            _label->setHorizonCulling(false);// 不需要裁剪

            _geo->getPositionAttitudeTransform()->addChild(_label);

            mapnode->addChild(_geo.get());
        }

        bool handle(const osgGA::GUIEventAdapter& ea, osgGA::GUIActionAdapter& aa)
        {
            if ( ea.getEventType() == ea.FRAME )// 此语段为标签添加了动画
            {
                double t0 = osg::Timer::instance()->time_s();// 儒略日时间
                double t = fmod( t0, 6000.0 ) / 6000.0;// t0%6000/6000, 将数字控制在[0,1)范围内
                GeoPoint p = _start.interpolate(_end, t);// 差值，在_end和t之间
				// GeoMath::bearing 以弧度计算从一个点到下一个点的初始方位
                double bearing = GeoMath::bearing(_start.y(), _start.x(), p.y(), p.x());

                float a = sin(t0*0.2);// a值没有被用
                float pitch = 0.0;

                _geo->setPosition(p);

                _geo->setLocalRotation(
                    osg::Quat(pitch, osg::Vec3d(1, 0, 0)) *
                    osg::Quat(bearing, osg::Vec3d(0, 0, -1)));
            }
            else if ( ea.getEventType() == ea.KEYDOWN )
            {
                if ( ea.getKey() == _key )// 8 或 9
                {                                
                    Viewpoint vp = _manip->getViewpoint();
                    //vp.setNode( _pat.get() );
                    vp.setNode(_model);
                    vp.range() = 25000.0;
                    vp.pitch() = -45.0;
                    _manip->setViewpoint(vp, 2.0);// 2秒内飞到指定位置
                }
                return true;
            }
            return false;
        }

        std::string                        _name;
        char                               _key;
        MapNode*                           _mapnode;
        EarthManipulator*                  _manip;
        GeoPoint                           _start, _end;
        LabelNode*                         _label;
        osg::Node*                         _model;
        float                              _heading;
        float                              _pitch;

        osg::ref_ptr<GeoPositionNode>      _geo;
    };

    /**
     * Place an X at the sim entity position, in screen space.
     * The point of this is to test the EarthManipulator::UpdateCameraCallback
     * which provides a frame-synched camera matrix (post-update traversal)
     */
	// 创建一个十字坐标
    struct CalculateWindowCoords : public osgGA::GUIEventHandler
                                   
    {
        CalculateWindowCoords(char key, EarthManipulator* manip, Simulator* sim)
            : _key(key), _active(false), _sim(sim), _xform(0L)
        {
            //nop
        }

        void onUpdateCamera(const osg::Camera* cam)
        {
            if (_active)
            {
                if (!_xform)
                {
                    osg::Geometry* geom = new osg::Geometry();
                    osg::Vec3Array* verts = new osg::Vec3Array();
                    verts->push_back(osg::Vec3(-10000, 0, 0));
                    verts->push_back(osg::Vec3( 10000, 0, 0));
                    verts->push_back(osg::Vec3( 0, -10000, 0));
                    verts->push_back(osg::Vec3( 0,  10000, 0));
                    verts->push_back(osg::Vec3( 0, 0, -10000));
                    verts->push_back(osg::Vec3( 0, 0,  10000));
                    geom->setVertexArray(verts);
                    osg::Vec4Array* colors = new osg::Vec4Array();
                    colors->push_back(osg::Vec4(1, 1, 0, 1));// 黄色线
                    colors->setBinding(colors->BIND_OVERALL);
                    geom->setColorArray(colors);
                    geom->addPrimitiveSet(new osg::DrawArrays(GL_LINES, 0, 6));// 6个点，绘制3条线，但屏幕上仅能看出2条线，水平和垂直
                    geom->setCullingActive(false);// 不裁剪
                    geom->getOrCreateStateSet()->setAttributeAndModes(new osg::Depth(osg::Depth::ALWAYS, 0, 1, true), 1);//深度设置

                    _xform = new osg::MatrixTransform();
                    _xform->addChild(geom);

                    osg::View* view = const_cast<osg::View*>(cam->getView());
                    ControlCanvas::getOrCreate(view)->addChild(_xform);
                }
				/// 坐标变换 V_clip=M_projection⋅M_view⋅M_model⋅V_local  裁剪坐标=透视矩阵*观察矩阵*模型矩阵*本地坐标——来自OpenGL
                GeoPoint p = _sim->_geo->getPosition();// 获取thing1的坐标点

                osg::Vec3d world;
                p.toWorld(world);// 转化为世界坐标

                osg::Matrix worldToWindow =
                    cam->getViewMatrix() *
                    cam->getProjectionMatrix() *
                    cam->getViewport()->computeWindowMatrix();// 世界坐标转到窗口坐标

                osg::Vec3d win = world * worldToWindow;// 得到窗口坐标

                _xform->setMatrix(osg::Matrix::translate(win));
            }
        }

        bool handle(const osgGA::GUIEventAdapter& ea, osgGA::GUIActionAdapter& aa)
        {
            if (ea.getEventType() == ea.KEYDOWN && ea.getKey() == _key)
            {
                _active = !_active;
                aa.requestRedraw();
                return true;
            }
            
            return false;
        }

        void getUsage(osg::ApplicationUsage& usage) const
        {
            using namespace std;
            usage.addKeyboardMouseBinding(string(1, _key), string("Show Window Coords"));
        }

        osg::MatrixTransform* _xform;
        Simulator* _sim;
        bool _active;
        char _key;
    };
	// 更新相机回调
    struct CameraUpdater : public EarthManipulator::UpdateCameraCallback
    {
        CalculateWindowCoords* _calc;

        CameraUpdater(CalculateWindowCoords* calc) : _calc(calc) { }
        
        void onUpdateCamera(const osg::Camera* cam)
        {
            _calc->onUpdateCamera(cam);// 当物体运动时，需要让十字线跟随物体运动
        }
    };
}


int main(int argc, char** argv)
{
    osg::ArgumentParser arguments(&argc,argv);

    if (arguments.read("--help") || argc==1)
    {
        OE_WARN << "Usage: " << argv[0] << " [earthFile] [--model modelToLoad]"
            << std::endl;
        return 0;
    }

    osgViewer::Viewer viewer(arguments);

    // install the programmable manipulator.安装操作器
    EarthManipulator* manip = new EarthManipulator();
    viewer.setCameraManipulator( manip );

    // UI: 创建帮助说明控件，在左上角的位置。当鼠标移动到控件上之后，是无法点击到地球上的，没有鼠标穿透功能。
    Container* help = createHelp(&viewer);
	// MapNodeHelper 显示帮助
	// load() 加载earth映射文件并处理所有内置示例命令行参数和XML外部变量(help)。
    osg::Node* earthNode = MapNodeHelper().load( arguments, &viewer, help );
    if (!earthNode)
    {
        OE_WARN << "Unable to load earth model." << std::endl;
        return -1;
    }

    osg::Group* root = new osg::Group();
    root->addChild( earthNode );
	// 获取mapNode
    osgEarth::MapNode* mapNode = osgEarth::MapNode::findMapNode( earthNode );

    // user model?
	// 加载模型,这里传入osg osgt格式的模型，并没有显示
    osg::ref_ptr<osg::Node> model;
    std::string modelFile;
    if (arguments.read("--model", modelFile))
        model = osgDB::readRefNodeFile(modelFile + ".osgearth_shadergen");

    osg::Group* sims = new osg::Group();
    root->addChild( sims );

    const SpatialReference* wgs84 = SpatialReference::get("wgs84");// 坐标系

    // Simulator for tethering: 系留模拟器
	// thing 1 和 thing 2 会随着时间移动
    Simulator* sim1 = new Simulator(sims, manip, mapNode, model.get(), "Thing 1", '8');// 8：绑住第一个物体
	sim1->_name = "Thing 1";
    sim1->_start = GeoPoint(wgs84, 45.0, 55.0, 10000);// 开始坐标点
    sim1->_end = GeoPoint(wgs84, -45, -55.0, 10000);  // 结束坐标点
    viewer.addEventHandler(sim1);	// 加入到操作器

    Simulator* sim2 = new Simulator(sims, manip, mapNode, model.get(), "Thing 2", '9');// 9：绑住第二个物体
    sim2->_name = "Thing 2";
    sim2->_start = GeoPoint(wgs84, 45.0, 54.0, 10000);
    sim2->_end = GeoPoint(wgs84, -44.0, -54.0, 10000);
    viewer.addEventHandler(sim2);

    manip->getSettings()->getBreakTetherActions().push_back( EarthManipulator::ACTION_GOTO );  // 操作器动作，直接过去  

    // Set the minimum distance to something larger than the default
	// 距离物体小于最小距离时，物体会变大
    manip->getSettings()->setMinMaxDistance(10.0, manip->getSettings()->getMaxDistance());

    // Sets the maximum focal point offsets (usually for tethering)
    manip->getSettings()->setMaxOffset(5000.0, 5000.0);
    
    // Pitch limits. 俯仰角范围
    manip->getSettings()->setMinMaxPitch(-90, 90);


    viewer.setSceneData( root );
	//为 鼠标左键按下、修改键、拖动鼠标的动作指定行为。
    manip->getSettings()->bindMouse(
        EarthManipulator::ACTION_EARTH_DRAG,
        osgGA::GUIEventAdapter::LEFT_MOUSE_BUTTON,
        osgGA::GUIEventAdapter::MODKEY_SHIFT );

    manip->getSettings()->bindMouseClick(
        EarthManipulator::ACTION_GOTO,
        osgGA::GUIEventAdapter::LEFT_MOUSE_BUTTON,
        osgGA::GUIEventAdapter::MODKEY_SHIFT);

    manip->getSettings()->setArcViewpointTransitions( true );    
	// 设置tether连接和断裂时的动作回调
    manip->setTetherCallback( new TetherCB() );
    
    //viewer.addEventHandler(new FlyToViewpointHandler( manip ));
	// 添加 自定义的各个事件处理
    viewer.addEventHandler(new LockAzimuthHandler('u', manip));
    viewer.addEventHandler(new ToggleArcViewpointTransitionsHandler('a', manip));
    viewer.addEventHandler(new ToggleThrowingHandler('q', manip));
    viewer.addEventHandler(new ToggleCollisionHandler('k', manip));
    viewer.addEventHandler(new ToggleProjMatrix('o', manip));
    viewer.addEventHandler(new BreakTetherHandler('b', manip));
    viewer.addEventHandler(new CycleTetherMode('t', manip));
    viewer.addEventHandler(new SetPositionOffset(manip));// 设置偏移
    viewer.addEventHandler(new ToggleLDB('L'));
    viewer.addEventHandler(new ToggleSSL(sims, ')'));
    viewer.addEventHandler(new FitViewToPoints('j', manip, mapNode->getMapSRS()));
    
	// 根据 thing 1 的位置，更新相机
    CalculateWindowCoords* calc = new CalculateWindowCoords('W', manip, sim1);// 根据thing 1计算窗口坐标
    viewer.addEventHandler(calc);
    manip->setUpdateCameraCallback(new CameraUpdater(calc));// 相机回调

    viewer.getCamera()->setSmallFeatureCullingPixelSize(-1.0f);

    while(!viewer.done())
    {
        viewer.frame();

        // simulate slow frame rate
        //OpenThreads::Thread::microSleep(1000*1000);
    }
    return 0;
}