VTK修炼之道68：体绘制讨论_梯度不透明度传输函数

1.梯度不透明度函数

梯度不透明度函数是将梯度模值映射为一个不透明度乘子，从而增强过渡区域的显示效果。

该函数也是使用vtkPiecewiseFunction类。例如，在不同材料的临界区域，如空气到软组织，或者软组织到骨头的临界区，梯度值会比较大，而材料的内部梯度值则会相对比较小。

vtkVolumeProperty中通过如下函数设置和获取梯度不透明度函数：

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

1.1 添加梯度不透明度函数标准代码片段及理解

vtkSmartPointer<vtkPiecewiseFunction> gradientOpacity =vtkSmartPointer<vtkPiecewiseFunction>::New();gradientOpacity->AddPoint(10, 0.0);gradientOpacity->AddPoint(90, 0.5);gradientOpacity->AddPoint(100, 1.0);volumeProperty->SetGradientOpacity(gradientOpacity);

上述代码中将梯度小于10的点的不透明度乘子设为0，即完全透明。当梯度大小为10时，不透明度为0；梯度大小为90时，不透明度乘子为0.5；梯度大小在10~90时，不透明度乘子通过线性映射至0~0.5之间的数值；同理，当梯度大小在90~100之间时，不透明度乘子通过线性映射至0.5~1.0之间；梯度值大于100时，岂不透明度乘子为1.

1.2 不同梯度不透明度传输函数实验

示例代码如下：

#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(); //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);//////////////////////////////////////////////////////////梯度不透明度函数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;}

输出结果为：

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, 2015.