前言

osgearth_featurequery示例，分析了如何拾取地球上的特征，并对特征信息进行提取与显示。

执行命令：osgearth_featurequeryd.exe earth_image\china-simple.earth

需要注意的是：earth文件，必须有特征内容。否则无法正常进行功能测试。

效果

 代码分析

/**
 * Creates a simple user interface for the demo.
 * 创建一个简单的用户操作界面
 */
Container* createUI()
{
    VBox* vbox = new VBox();
    vbox->setVertAlign( Control::ALIGN_TOP );// 位置在右上角
    vbox->setHorizAlign( Control::ALIGN_RIGHT );
    vbox->addControl( new LabelControl("Feature Query Demo", Color::Yellow) );
    vbox->addControl( new LabelControl("Click on a feature to see its attributes.") );
    return vbox;
}

//-----------------------------------------------------------------------

/**
 * Query Callback that displays the targeted feature's attributes in a
 * user interface grid control.
 * 用户操作接口面板，请求回调，进而展示目标特征属性。
 */
// RTTPicker类：RTT相机的拾取类
// ReadoutCallback重写的方法，均来自 RTTPicker::Callback
class ReadoutCallback : public RTTPicker::Callback
{
public:
    ReadoutCallback(ControlCanvas* container) : _lastFID( ~0 )
    {
        _grid = new Grid();
        _grid->setBackColor( osg::Vec4(0,0,0,0.7f) );
        container->addControl( _grid );
    }
	// 拾取到的特征id
    void onHit(ObjectID id)
    {
        FeatureIndex* index = Registry::objectIndex()->get<FeatureIndex>(id).get();
        Feature* feature = index ? index->getFeature( id ) : 0L;
        if ( feature && feature->getFID() != _lastFID )
        {
            _grid->clearControls();// 清空网格信息
            unsigned r=0;
			
			// 在网格中添加信息
			// 特征id = FID
            _grid->setControl( 0, r, new LabelControl("FID", Color::Red) );
            _grid->setControl( 1, r, new LabelControl(Stringify()<<feature->getFID(), Color::White) );
            ++r;

			// 输出特征信息(earth文件我也不太懂，所以特征信息也看不太明白)
            const AttributeTable& attrs = feature->getAttrs();
            for( AttributeTable::const_iterator i = attrs.begin(); i != attrs.end(); ++i, ++r )
            {
                _grid->setControl( 0, r, new LabelControl(i->first, 14.0f, Color::Yellow) );
                _grid->setControl( 1, r, new LabelControl(i->second.getString(), 14.0f, Color::White) );
            }
            if ( !_grid->visible() )// 如果网格不可见，则设置为可见
                _grid->setVisible( true );
        
            _lastFID = feature->getFID();
        }
    }

	// Called when a query results in nothing
	// 什么也拾取不到时，隐藏网格
    void onMiss()
    {
        _grid->setVisible(false);
        _lastFID = 0u;
    }

	// 接收点击释放事件
    bool accept(const osgGA::GUIEventAdapter& ea, const osgGA::GUIActionAdapter& aa) 
    {
        return ea.getEventType() == ea.RELEASE; // click
    }

    Grid*     _grid;
    FeatureID _lastFID;
};

//------------------------------------------------------------------------

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

    // a basic OSG viewer 基础osg视景器
    osgViewer::Viewer viewer(arguments);

    // install our default manipulator (do this before using MapNodeHelper)
	// 添加操作器
    viewer.setCameraManipulator( new EarthManipulator() );

    // load an earth file, and support all or our example command-line options
    // and earth file <external> tags
	// 添加的earth文件，文件需要有feature这些信息
    osg::Group* root = MapNodeHelper().load( arguments, &viewer, createUI() );
    if ( root )
    {
        viewer.setSceneData( root );

        MapNode* mapNode = MapNode::findMapNode( root );
        if ( mapNode )
        {
            // Install the query tool.安装请求对象
			// RTTPicker 使用RTT摄影机和顶点属性拾取对象。
            RTTPicker* picker = new RTTPicker();
            viewer.addEventHandler( picker );// 添加请求事件
            picker->addChild( mapNode );

            // Install a readout for feature metadata.
            ControlCanvas* canvas = ControlCanvas::getOrCreate(&viewer);
            picker->setDefaultCallback( new ReadoutCallback(canvas) );
        }

        return viewer.run();
    }
    else
    {
        OE_NOTICE 
            << "\nUsage: " << argv[0] << " file.earth" << std::endl
            << MapNodeHelper().usage() << std::endl;
    }
}