VTK：体绘制——梯度不透明度传输函数

梯度不透明度函数是将梯度模值映射成不透明度乘子，从而增强过渡区域的显示效果。该函数也被使用vtkPiecewiseFunction类别。例如，在不同材料的临界区域，如空气到软组织，或软组织到骨骼的临界区域，梯度值相对较大，材料的内部梯度值相对较小。 vtkVolumeProperty梯度不透明度函数通过以下函数设置获得:

void SetGradientOpacity(vtkPiecewiseFunction* function); vtkPiecewiseFunction* GetGradientOpacity(); 

 vtkSmartPointer<vtkPiecewiseFunction> gradientOpacity =   vtkSmartPointer<vtkPiecewiseFunction>::New();  gradientOpacity->AddPoint(10, 0.0);  gradientOpacity->AddPoint(90, 0.5);  gradientOpacity->AddPoint(100, 1.0);  volumeProperty->SetGradientOpacity(gradientOpacity);  ///上述代码将梯度小于10点的不透明乘子设置为0，即完全透明。梯度大小为10时，不透明度为0；梯度大小为90时，不透明乘客为0.五、梯度大小为10~90时，不透明度乘子通过线性映射至0~0.5之间的值；同样，当梯度大小为90时~不透明度乘子在100之间线性映射到0.5~1.0之间；当梯度值大于100时，不透明度乘子是1吗？.   

传输函数的不同梯度不透明度实验：

#include <vtkAutoInit.h> VTK_MODULE_INIT(vtkRenderingOpenGL2); VTK_MODULE_INIT(vtkRenderingVolumeOpenGL2); VTK_MODULE_INIT(vtkRenderingFreeType); VTK_MODULE_INIT(vtkInteractionStyle);  #include <vtkSmartPointer.h> #include <vtkStructuredPointsReader.h> #include <vtkStructuredPoints.h> #include <vtkGPUVolumeRayCastMapper.h>
#include <vtkVolumeProperty.h>
#include <vtkPiecewiseFunction.h>
#include <vtkColorTransferFunction.h>
#include <vtkVolume.h>
#include <vtkRenderer.h>
#include <vtkRenderWindow.h>
#include <vtkRenderWindowInteractor.h>
#include <vtkCamera.h>

int main()
{ 
        
	vtkSmartPointer<vtkStructuredPointsReader> reader =
		vtkSmartPointer<vtkStructuredPointsReader>::New();
	reader->SetFileName("data/mummy.128.vtk");
	reader->Update();

	vtkSmartPointer<vtkGPUVolumeRayCastMapper> volumeMapper1 =
		vtkSmartPointer<vtkGPUVolumeRayCastMapper>::New();
	volumeMapper1->SetInputData(reader->GetOutput());

	vtkSmartPointer<vtkGPUVolumeRayCastMapper> volumeMapper2 =
		vtkSmartPointer<vtkGPUVolumeRayCastMapper>::New();
	volumeMapper2->SetInputData(reader->GetOutput());
	/****************************************************************/
	vtkSmartPointer<vtkVolumeProperty> volumeProperty1 =
		vtkSmartPointer<vtkVolumeProperty>::New();
	volumeProperty1->SetInterpolationTypeToLinear(); //设置线性插值
	volumeProperty1->ShadeOn();//开启阴影功能 
	volumeProperty1->SetAmbient(0.4);//设置环境温度系数
	volumeProperty1->SetDiffuse(0.6);//设置漫反射系数
	volumeProperty1->SetSpecular(0.2);//设置镜面反射系数

	vtkSmartPointer<vtkVolumeProperty> volumeProperty2 =
		vtkSmartPointer<vtkVolumeProperty>::New();
	volumeProperty2->SetInterpolationTypeToLinear(); //设置线性插值
	volumeProperty2->ShadeOn();//开启阴影功能 
	volumeProperty2->SetAmbient(0.4);//设置环境光强系数
	volumeProperty2->SetDiffuse(0.6);//设置漫反射系数
	volumeProperty2->SetSpecular(0.2);//设置镜面反射系数

	vtkSmartPointer<vtkPiecewiseFunction> compositeOpacity =
		vtkSmartPointer<vtkPiecewiseFunction>::New();
	compositeOpacity->AddPoint(50, 0.0);
	compositeOpacity->AddPoint(120, 0.5);
	compositeOpacity->AddPoint(200, 1.0);
	volumeProperty1->SetScalarOpacity(compositeOpacity);
	volumeProperty2->SetScalarOpacity(compositeOpacity);
	
	//梯度不透明度函数1
	vtkSmartPointer<vtkPiecewiseFunction> gradientOpacity1 =
		vtkSmartPointer<vtkPiecewiseFunction>::New();
	gradientOpacity1->AddPoint(1, 0.0);
	gradientOpacity1->AddPoint(70, 0.5);
	gradientOpacity1->AddPoint(130, 1.0);
	volumeProperty1->SetGradientOpacity(gradientOpacity1);

	//梯度不透明度函数2
	vtkSmartPointer<vtkPiecewiseFunction> gradientOpacity2 =
		vtkSmartPointer<vtkPiecewiseFunction>::New();
	gradientOpacity2->AddPoint(60, 0.0);
	gradientOpacity2->AddPoint(90, 0.2);
	gradientOpacity2->AddPoint(150, 0.4);
	volumeProperty2->SetGradientOpacity(gradientOpacity2);

	vtkSmartPointer<vtkColorTransferFunction> color =
		vtkSmartPointer<vtkColorTransferFunction>::New();
	color->AddRGBPoint(0, 0, 0, 0);
	color->AddRGBPoint(64, 1.0, 0.52, 0.3);
	color->AddRGBPoint(190.0, 1.00, 1.00, 1.00);
	color->AddRGBPoint(220.0, 0.20, 0.20, 0.20);
	volumeProperty1->SetColor(color);
	volumeProperty2->SetColor(color);
	/****************************************************************/
	vtkSmartPointer<vtkVolume> volume1 =
		vtkSmartPointer<vtkVolume>::New();
	volume1->SetMapper(volumeMapper1);
	volume1->SetProperty(volumeProperty1);

	vtkSmartPointer<vtkVolume> volume2 =
		vtkSmartPointer<vtkVolume>::New();
	volume2->SetMapper(volumeMapper2);
	volume2->SetProperty(volumeProperty2);
	//
	double View1[4] = { 
         0, 0, 0.5, 1 };
	double View2[4] = { 
         0.5, 0, 1, 1 };
	vtkSmartPointer<vtkRenderer> render1 =
		vtkSmartPointer<vtkRenderer>::New();
	render1->AddVolume(volume1);
	render1->SetViewport(View1);
	render1->SetBackground(1, 1, 0);

	vtkSmartPointer<vtkRenderer> render2 =
		vtkSmartPointer<vtkRenderer>::New();
	render2->AddVolume(volume2);
	render2->SetViewport(View2);
	render2->SetBackground(0, 1, 0);

	vtkSmartPointer<vtkRenderWindow> rw =
		vtkSmartPointer<vtkRenderWindow>::New();
	rw->AddRenderer(render2);
	rw->AddRenderer(render1);
	rw->SetSize(640, 400);
	rw->SetWindowName("Differ Gradient Opacity Function");

	vtkSmartPointer<vtkRenderWindowInteractor> rwi =
		vtkSmartPointer<vtkRenderWindowInteractor>::New();
	rwi->SetRenderWindow(rw);

	render1->GetActiveCamera()->SetPosition(0, -1, 0);
	render1->GetActiveCamera()->SetFocalPoint(0, 0, 0);
	render1->GetActiveCamera()->SetViewUp(0, 0, 1);
	render1->GetActiveCamera()->Azimuth(30);
	render1->GetActiveCamera()->Elevation(30);
	render1->ResetCamera();
	
	render2->SetActiveCamera(render1->GetActiveCamera());

	rw->Render();
	rwi->Start();
	return 0;
}