VTK: 拾取方式的三种实现

      拾取操作是可视化应用程序中常见的一种功能。拾取主要是用于选择数据和Actor或者获取底层的数据值。在显示位置(以像素为坐标值)中拾取时，就会调用vtkAbstractPicker的Pick()方法。依赖于所用的拾取类不同，拾取时返回的信息也不同，最简单的是返回一个x-y-z的全局坐标值，或者是单元（cell）的ID值，点的ID值，单元参数坐标(CellParametric Coordinates)，所拾取的vtkProp实例，以及Assemblypath。与拾取相关的VTK类的 继承图如下：

       本文内容主要介绍一下vtk拾取的实现方式，并不讲解每种拾取器的差别。本文拾取过程以vtkCellPicker为例子讲解。拾取的实现按照vtk交互方式的不同分为两大类：1)重写vtkInteractorStyle完成拾取  2)重写观察者命令模式完成拾取

      1) 重写vtkInteractorStyle 

class MouseInteractorStyle : public vtkInteractorStyleTrackballCamera
{
public:
    static MouseInteractorStyle* New();

    MouseInteractorStyle()
    {
        selectedMapper = vtkSmartPointer::New();
        selectedActor = vtkSmartPointer::New();
    }

    virtual void OnLeftButtonDown()
    {
        // Get the location of the click (in window coordinates)
        int* pos = this->GetInteractor()->GetEventPosition();

        vtkSmartPointer picker =
                vtkSmartPointer::New();
        picker->SetTolerance(0.0005);

        // Pick from this location.
        picker->Pick(pos[0], pos[1], 0, this->GetDefaultRenderer());

        double* worldPosition = picker->GetPickPosition();
        std::cout << "Cell id is: " << picker->GetCellId() << std::endl;

        if(picker->GetCellId() != -1)
        {

            std::cout << "Pick position is: " << worldPosition[0] << " " << worldPosition[1]
                      << " " << worldPosition[2] << endl;

            vtkSmartPointer ids =
                    vtkSmartPointer::New();
            ids->SetNumberOfComponents(1);
            ids->InsertNextValue(picker->GetCellId());

            vtkSmartPointer selectionNode =
                    vtkSmartPointer::New();
            selectionNode->SetFieldType(vtkSelectionNode::CELL);
            selectionNode->SetContentType(vtkSelectionNode::INDICES);
            selectionNode->SetSelectionList(ids);

            vtkSmartPointer selection =
                    vtkSmartPointer::New();
            selection->AddNode(selectionNode);

            vtkSmartPointer extractSelection =
                    vtkSmartPointer::New();
#if VTK_MAJOR_VERSION <= 5
            extractSelection->SetInput(0, this->Data);
            extractSelection->SetInput(1, selection);
#else
            extractSelection->SetInputData(0, this->Data);
            extractSelection->SetInputData(1, selection);
#endif
            extractSelection->Update();

            // In selection
            vtkSmartPointer selected =
                    vtkSmartPointer::New();
            selected->ShallowCopy(extractSelection->GetOutput());

            std::cout << "There are " << selected->GetNumberOfPoints()
                      << " points in the selection." << std::endl;
            std::cout << "There are " << selected->GetNumberOfCells()
                      << " cells in the selection." << std::endl;


#if VTK_MAJOR_VERSION <= 5
            selectedMapper->SetInputConnection(
                        selected->GetProducerPort());
#else
            selectedMapper->SetInputData(selected);
#endif

            selectedActor->SetMapper(selectedMapper);
            selectedActor->GetProperty()->EdgeVisibilityOn();
            selectedActor->GetProperty()->SetEdgeColor(1,0,0);
            selectedActor->GetProperty()->SetLineWidth(3);

            this->Interactor->GetRenderWindow()->GetRenderers()->GetFirstRenderer()->AddActor(selectedActor);

        }
        // Forward events
        vtkInteractorStyleTrackballCamera::OnLeftButtonDown();
    }

    vtkSmartPointer Data;
    vtkSmartPointer selectedMapper;
    vtkSmartPointer selectedActor;

};

       2)重写观察者命令模式完成拾取

      根据观察者的不同分为两种实现方法：1)观察者为vtkRenderWindowInteractor   2）观察者为vtkcellpicker

      1)给vtkRenderWindowInteractor添加回调类，响应的vtk事件是

LeftButtonPressEvent

        拾取方式是鼠标左键点击

     回调类代码如下：    

class PickerCallBack : public vtkCommand
{
public:
    static PickerCallBack *New()
    {
        return new PickerCallBack;
    }

    virtual void Execute(vtkObject *caller, unsigned long eventId, void *callData)
    {
        //相应拾取结果

        if(eventId == vtkCommand::LeftButtonPressEvent){
            QVTKInteractor *interactor = QVTKInteractor::SafeDownCast( caller );
            vtkRenderer *renderer = interactor->GetRenderWindow()->GetRenderers()->GetFirstRenderer();
			 int event_pos[3] = {0, 0, 0};
             interactor->GetEventPosition(event_pos);
             interactor->GetPicker()->Pick(event_pos[0], event_pos[1], 0, renderer);
		
            if(picker->GetCellId() != -1){
                double *pos = picker->GetPickPosition();
                qDebug() <<"pick coord:" << pos[0] << pos[1] << pos[2];
            }
        }
    }

    bool m_enable = false;
};

main函数中 为 vtkRenderWindowInteractor添加该回调类：

vtkSmartPointer callBack =
              vtkSmartPointer::New();
interactor->AddObserver(vtkCommand::LeftButtonPressEvent,callBack);

 

   2)给 vtkCellPicker添加回调类  响应的vtk事件是：

vtkCommand::EndPickEvent

  拾取方式是键盘字母P + 移动鼠标  

 回调类代码如下：

class PickerCallBack : public vtkCommand
{
public:
    static PickerCallBack *New()
    {
        return new PickerCallBack;
    }

    virtual void Execute(vtkObject *caller, unsigned long eventId, void *callData)
    {
        if(!m_enable)
            return;

        if(eventId == vtkCommand::EndPickEvent){
            vtkCellPicker *picker = vtkCellPicker::SafeDownCast(caller);
            //坐标拾取
            if(picker->GetCellId() != -1){
                double *pos = picker->GetPickPosition();
                qDebug() <<"pick coord:" << pos[0] << pos[1] << pos[2];
            }

            picker->SetPath(nullptr);
        }
    }

    bool m_enable = false;
};

main函数中 为 vtkCellPicker添加该回调类：

vtkSmartPointer picker =
              vtkSmartPointer::New();
interactor->SetPicker(picker);		
	  
vtkSmartPointer callBack =
              vtkSmartPointer::New();
interactor->GetPicker->AddObserver(vtkCommand::EndPickEvent,callBack);