1.拾取
选择拾取是人机交互过程的一个重要功能。 一个最经典的例子就是，在玩3D游戏时，场景中可能会存在多个角色，有时需要用鼠标来选择所要控制的角色，这就要用到拾取功能。 另外，在某些三维图形的编辑软件中，经常需要编辑其中的一个点、一个面片或者一个局部区域，这也需要通过拾取功能来完成。 VTK中定义了多个拾取功能的类，具体的继承关系如下：VTK中所有的拾取类都继承自vtkAbstractPicker类，在这些类的基础之上可以实现非常复杂的功能。
2.点拾取
从上图中能够知晓，完成点拾取功能的类是vtkPointPicker。 vtk中的消息是通过vtkRenderWindowInteractor类处理的，在类vtkRenderWindowInteractor中，定义如下函数：virtual void SetPicker(vtkAbstractPicker* );该函数用来设置具体的VTKAbstractPicker对象，并执行相应的拾取操作。因此对于点拾取，实际就是设置VTKPointPicker的过程。 之前，曾经细致的研究过，vtkRenderWindowInteractor内部定义了一个vtkInteractorStyle对象。vtkInteractorStyle类是一个虚基类，其子类定义了多种鼠标和键盘消息的处理方法，在实现拾取操作是，需要定制相应的鼠标消息处理函数。比如拾取某个点时，应该响应鼠标的左键按下消息，并在响应该消息的函数中根据鼠标的当前窗口坐标来完成拾取操作。 点拾取的示例代码如下：
#include <vtkAutoInit.h>VTK_MODULE_INIT(vtkRenderingOpenGL)VTK_MODULE_INIT(vtkRenderingFreeType)VTK_MODULE_INIT(vtkInteractionStyle)#include <vtkSmartPointer.h>#include <vtkSphereSource.h>#include <vtkPolyDataMapper.h>#include <vtkActor.h>#include <vtkRenderer.h>#include <vtkRenderWindow.h>#include <vtkRenderWindowInteractor.h>#include <vtkPointPicker.h>//this->Interactor->GetRenderWindow()->GetRenderers()->GetFirstRenderer()#include <vtkRendererCollection.h> #include <vtkInteractorStyleTrackballCamera.h>#include <vtkObjectFactory.h> //vtkStandardNewMacro();#include <vtkProperty.h>#include <vtkAxesActor.h>#include <vtkOrientationMarkerWidget.h>/**************************************************************************************************/class PointPickerInteractorStyle : public vtkInteractorStyleTrackballCamera{public:static PointPickerInteractorStyle* New();vtkTypeMacro(PointPickerInteractorStyle, vtkInteractorStyleTrackballCamera);virtual void OnLeftButtonDown(){//打印鼠标左键像素位置std::cout << "Picking pixel: " << this->Interactor->GetEventPosition()[0] << " " << this->Interactor->GetEventPosition()[1] << std::endl;//注册拾取点函数this->Interactor->GetPicker()->Pick(this->Interactor->GetEventPosition()[0],this->Interactor->GetEventPosition()[1], 0, // always zero.this->Interactor->GetRenderWindow()->GetRenderers()->GetFirstRenderer());//打印拾取点空间位置double picked[3];this->Interactor->GetPicker()->GetPickPosition(picked);std::cout << "Picked value: " << picked[0] << " " << picked[1] << " " << picked[2] << std::endl;//对拾取点进行标记vtkSmartPointer<vtkSphereSource> sphereSource =vtkSmartPointer<vtkSphereSource>::New();sphereSource->Update();vtkSmartPointer<vtkPolyDataMapper> mapper =vtkSmartPointer<vtkPolyDataMapper>::New();mapper->SetInputConnection(sphereSource->GetOutputPort());vtkSmartPointer<vtkActor> actor = vtkSmartPointer<vtkActor>::New();actor->SetMapper(mapper);actor->SetPosition(picked);actor->SetScale(0.05);actor->GetProperty()->SetColor(1.0, 0.0, 0.0);this->Interactor->GetRenderWindow()->GetRenderers()->GetFirstRenderer()->AddActor(actor);vtkInteractorStyleTrackballCamera::OnLeftButtonDown();}};/**************************************************************************************************/vtkStandardNewMacro(PointPickerInteractorStyle);int main(){vtkSmartPointer<vtkSphereSource> sphereSource =vtkSmartPointer<vtkSphereSource>::New();sphereSource->Update();vtkSmartPointer<vtkPolyDataMapper> mapper =vtkSmartPointer<vtkPolyDataMapper>::New();mapper->SetInputConnection(sphereSource->GetOutputPort());vtkSmartPointer<vtkActor> actor =vtkSmartPointer<vtkActor>::New();actor->SetMapper(mapper);vtkSmartPointer<vtkRenderer> renderer =vtkSmartPointer<vtkRenderer>::New();renderer->AddActor(actor);renderer->SetBackground(1, 1, 1);vtkSmartPointer<vtkRenderWindow> renderWindow =vtkSmartPointer<vtkRenderWindow>::New();renderWindow->Render();renderWindow->SetWindowName("PointPicker");renderWindow->AddRenderer(renderer);vtkSmartPointer<vtkPointPicker> pointPicker =vtkSmartPointer<vtkPointPicker>::New();vtkSmartPointer<vtkRenderWindowInteractor> renderWindowInteractor =vtkSmartPointer<vtkRenderWindowInteractor>::New();renderWindowInteractor->SetPicker(pointPicker);renderWindowInteractor->SetRenderWindow(renderWindow);vtkSmartPointer<PointPickerInteractorStyle> style =vtkSmartPointer<PointPickerInteractorStyle>::New();renderWindowInteractor->SetInteractorStyle(style);/vtkSmartPointer<vtkAxesActor> Axes = vtkSmartPointer<vtkAxesActor>::New();vtkSmartPointer<vtkOrientationMarkerWidget> widget =vtkSmartPointer<vtkOrientationMarkerWidget>::New();widget->SetInteractor(renderWindowInteractor);widget->SetOrientationMarker(Axes);widget->SetOutlineColor(1, 1, 1);widget->SetViewport(0, 0, 0.2, 0.2);widget->SetEnabled(1);widget->InteractiveOn();renderWindow->Render();renderWindowInteractor->Start();return 0;}实际操作细节分析：vtkInteractorStyleTrackballCemera派生类设计PointPickerInteractorStyle类从vtkInteractorStyleTrackballCemera派生，并覆盖了该类OnLeftButtonDown()函数。在该函数中，调用了vtkRenderWindowInteractor的GetEventPosition()函数输出鼠标点击的屏幕坐标。拾取函数Pick()设计int Pick(double selectionX, double selectionY, double selectionZ, vtkRender* renderer); 该函数需要接受四个参数，前三个为（selectionX，selectionY，selectionZ），即鼠标的当前窗口坐标，其中selectionZ通常为零。最后一个是vtkRenderer对象。 GetPackPosition()是指世界坐标系下拾取点的坐标mian()函数中设计拾取调用流程
vtkSmartPointer<vtkPointPicker> pointPicker =vtkSmartPointer<vtkPointPicker>::New();vtkSmartPointer<vtkRenderWindowInteractor> renderWindowInteractor =vtkSmartPointer<vtkRenderWindowInteractor>::New();renderWindowInteractor->SetPicker(pointPicker);renderWindowInteractor->SetRenderWindow(renderWindow);vtkSmartPointer<PointPickerInteractorStyle> style =vtkSmartPointer<PointPickerInteractorStyle>::New();renderWindowInteractor->SetInteractorStyle(style);实例化vtkPointPicker对象以后，调用vtkRenderWindowInteractor::SetPicker()函数将其设置到渲染窗口交互器中。PointPickerInteractorStyle类与vtkInteractorStyleImage等交互器样式使用方法一致。
2.参看资料
1.《C++ primer》2.《The VTK User’s Guide – 11thEdition》
3. 张晓东, 罗火灵. VTK图形图像开发进阶[M]. 机械工业出版社, .
