VTK：交互与拾取——点拾取

1.拾取

拾取最经典的例子就是，在玩3D游戏时，场景中可能会存在多个角色，有时候需要用鼠标来选择所要控制的角色，这时候要用到拾取功能。
另外，在某些三维图形图形的编译软件中，经常需要编辑其中的一个点，一个片面或者一个局部区域，这也需要通过拾取功能来完成。
多个拾取功能：
VTK中所有的拾取类都继承自vtkAbstractPicker类，在这些类的基础上可以实现非常复杂的功能。

2.点拾取

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。
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。

#include 
VTK_MODULE_INIT(vtkRenderingOpenGL2)
VTK_MODULE_INIT(vtkRenderingFreeType)
VTK_MODULE_INIT(vtkInteractionStyle)

#include 
#include 
#include 
#include 
#include 
#include 
#include 

#include 
//this->Interactor->GetRenderWindow()->GetRenderers()->GetFirstRenderer()
#include  
#include 
#include   //vtkStandardNewMacro();
#include 

#include 
#include 
/**************************************************************************************************/
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;
}

• vtkInteractorStyleTrackballCemera派生类设计；
  PointPickerInteractorStyleTrackllCemera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 pointPicker =
  vtkSmartPointer::New();

vtkSmartPointer renderWindowInteractor =
vtkSmartPointer::New();
renderWindowInteractor->SetPicker(pointPicker);
renderWindowInteractor->SetRenderWindow(renderWindow);

vtkSmartPointer style =
vtkSmartPointer::New();
renderWindowInteractor->SetInteractorStyle(style);`