1.梯度不透明度函數

梯度不透明度函數是將梯度模值映射為一個不透明度乘子，從而增強過渡區域的顯示效果。 該函數也是使用vtkPiecewiseFunction類。例如，在不同材料的臨界區域，如空氣到軟組織，或者軟組織到骨頭的臨界區，梯度值會比較大，而材料的內部梯度值則會相對比較小。 vtkVolumeProperty中通過如下函數設置和獲取梯度不透明度函數： void SetGradientOpacity(vtkPiecewiseFunction* function); vtkPiecewiseFunction* GetGradientOpacity();

1.1 添加梯度不透明度函數標準代碼片段及理解

vtkSmartPointer<vtkPiecewiseFunction> gradientOpacity =vtkSmartPointer<vtkPiecewiseFunction>::New();gradientOpacity->AddPoint(10, 0.0);gradientOpacity->AddPoint(90, 0.5);gradientOpacity->AddPoint(100, 1.0);volumeProperty->SetGradientOpacity(gradientOpacity);上述代碼中將梯度小于10的點的不透明度乘子設為0，即完全透明。當梯度大小為10時，不透明度為0；梯度大小為90時，不透明度乘子為0.5；梯度大小在10~90時，不透明度乘子通過線性映射至0~0.5之間的數值；同理，當梯度大小在90~100之間時，不透明度乘子通過線性映射至0.5~1.0之間；梯度值大于100時，豈不透明度乘子為1.

1.2 不同梯度不透明度傳輸函數實驗

示例代碼如下： #include <vtkAutoInit.h> VTK_MODULE_INIT(vtkRenderingOpenGL); VTK_MODULE_INIT(vtkRenderingVolumeOpenGL); 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("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(); //設置線性插值volumeProperty1->ShadeOn();//開啟陰影功能 volumeProperty1->SetAmbient(0.4);//設置環境溫度系數volumeProperty1->SetDiffuse(0.6);//設置漫反射系數volumeProperty1->SetSpecular(0.2);//設置鏡面反射系數vtkSmartPointer<vtkVolumeProperty> volumeProperty2 =vtkSmartPointer<vtkVolumeProperty>::New();volumeProperty2->SetInterpolationTypeToLinear(); //設置線性插值volumeProperty2->ShadeOn();//開啟陰影功能 volumeProperty2->SetAmbient(0.4);//設置環境光強系數volumeProperty2->SetDiffuse(0.6);//設置漫反射系數volumeProperty2->SetSpecular(0.2);//設置鏡面反射系數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);////梯度不透明度函數1vtkSmartPointer<vtkPiecewiseFunction> gradientOpacity1 =vtkSmartPointer<vtkPiecewiseFunction>::New();gradientOpacity1->AddPoint(1, 0.0);gradientOpacity1->AddPoint(70, 0.5);gradientOpacity1->AddPoint(130, 1.0);volumeProperty1->SetGradientOpacity(gradientOpacity1);//梯度不透明度函數2vtkSmartPointer<vtkPiecewiseFunction> gradientOpacity2 =vtkSmartPointer<vtkPiecewiseFunction>::New();gradientOpacity2->AddPoint(120, 0.0);gradientOpacity2->AddPoint(150, 0.2);gradientOpacity2->AddPoint(180, 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(render1);rw->AddRenderer(render2);rw->SetSize(640, 320);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; }輸出結果為：

2.參看資料

1.《C++ primer》
2.《The VTK User’s Guide – 11thEdition》
3. ?張曉東, 羅火靈. VTK圖形圖像開發進階[M]. 機械工業出版社, 2015.

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