在之前笔记VTK笔记-图形相关-线段平滑-vtkSplineFilter类中，得知
vtkSplineFilter类用于对多线段的输入数据进行细化输出具有更多线段的点集合；
vtkSplineFilter类不仅可以用于插值，也可以进行抽样；
通过vtkSplineFilter类的SetSubdivideToSpecified()接口和SetSubdivideToLength()接口可以设置插值或抽样的方法；
SetSubdivideToSpecified()接口设置细化的个数，SetNumberOfSubdivisions用于设定输出线段的个数；

spline->SetSubdivideToSpecified();
spline->SetNumberOfSubdivisions(3);	

SetSubdivideToLength()接口设置每个线片段的长度；将曲线的长度总和除以每个线段的长度，就可以得到最终线段个数；

vtkNew<vtkSplineFilter> spline;
spline->SetInputDataObject(polyData);
spline->SetSpline(cardinal_spine);
spline->SetSubdivideToLength();
spline->SetLength(1);

示例代码

使用vtkParametricSpline类将四个点插值出一条曲线；

double p0[3] = {
     1.0, 0.0, 0.0 };
double p1[3] = {
     0.0, 1.0, 0.0 };
double p2[3] = {
     0.0, 0.0, 1.0 };
double p3[3] = {
     1.0, 2.0, 3.0 };

vtkNew<vtkPoints> points;
points->InsertNextPoint(p0);
points->InsertNextPoint(p1);
points->InsertNextPoint(p2);
points->InsertNextPoint(p3);

vtkNew<vtkParametricSpline> spline;
spline->SetPoints(points);
	
vtkNew<vtkParametricFunctionSource> functionSource;
functionSource->SetParametricFunction(spline);
functionSource->Update();

vtkNew<vtkPolyDataMapper> splineMapper;
splineMapper->SetInputConnection(functionSource->GetOutputPort());

vtkNew<vtkActor> splineActor;
splineActor->SetMapper(splineMapper);

vtkNew<vtkPolyData> resultPolydata;
resultPolydata->SetPoints(points);

vtkNew<vtkVertexGlyphFilter> resultGlyphFilter;
resultGlyphFilter->AddInputData(resultPolydata);
resultGlyphFilter->Update();

vtkNew<vtkPolyDataMapper> resultMapper;
resultMapper->SetInputConnection(resultGlyphFilter->GetOutputPort());
vtkNew<vtkActor> resultActor;
resultActor->SetMapper(resultMapper);
resultActor->GetProperty()->SetPointSize(5);//定义点的尺寸大小，这样点才能在画布上显示出来
resultActor->GetProperty()->SetColor(1, 0.0, 0.0);
	
vtkNew<vtkRenderer> ren1;
ren1->AddActor(splineActor);
ren1->AddActor(resultActor);
ren1->SetBackground(0.1, 0.2, 0.4);

vtkNew<vtkRenderWindow> renWin;
renWin->AddRenderer(ren1);
renWin->SetSize(300, 300);

vtkNew<vtkRenderWindowInteractor> iren;
iren->SetRenderWindow(renWin);
iren->Initialize();
iren->Start();

或者使用vtkSplineFilter类将四个点插值出一条曲线；

double p0[3] = {
     1.0, 0.0, 0.0 };
double p1[3] = {
     0.0, 1.0, 0.0 };
double p2[3] = {
     0.0, 0.0, 1.0 };
double p3[3] = {
     1.0, 2.0, 3.0 };

vtkNew<vtkPoints> points;
points->InsertNextPoint(p0);
points->InsertNextPoint(p1);
points->InsertNextPoint(p2);
points->InsertNextPoint(p3);

vtkNew<vtkCellArray> lines;
lines->InsertNextCell(4);
for (unsigned long i = 0; i < 4; ++i) {
    
	lines->InsertCellPoint(i);
}
vtkNew<vtkPolyData> polyData;
polyData->SetPoints(points);
polyData->SetLines(lines);

vtkNew<vtkSplineFilter> spline;
vtkNew<vtkCardinalSpline> cardinal_spine;

spline->SetInputDataObject(polyData);
spline->SetSpline(cardinal_spine);
spline->SetSubdivideToLength();
spline->SetLength(0.1);
spline->Update();

vtkNew<vtkPolyDataMapper> splineMapper;
splineMapper->SetInputConnection(spline->GetOutputPort());

vtkNew<vtkActor> splineActor;
splineActor->SetMapper(splineMapper);

vtkNew<vtkPolyData> resultPolydata;
resultPolydata->SetPoints(points);

vtkNew<vtkVertexGlyphFilter> resultGlyphFilter;
resultGlyphFilter->AddInputData(resultPolydata);
resultGlyphFilter->Update();

vtkNew<vtkPolyDataMapper> resultMapper;
resultMapper->SetInputConnection(resultGlyphFilter->GetOutputPort());
vtkNew<vtkActor> resultActor;
resultActor->SetMapper(resultMapper);
resultActor->GetProperty()->SetPointSize(5);//定义点的尺寸大小，这样点才能在画布上显示出来
resultActor->GetProperty()->SetColor(1, 0.0, 0.0);
	
vtkNew<vtkRenderer> ren1;
ren1->AddActor(splineActor);
ren1->AddActor(resultActor);
ren1->SetBackground(0.1, 0.2, 0.4);

vtkNew<vtkRenderWindow> renWin;
renWin->AddRenderer(ren1);
renWin->SetSize(300, 300);

vtkNew<vtkRenderWindowInteractor> iren;
iren->SetRenderWindow(renWin);
iren->Initialize();
iren->Start();
return 0;

使用SetLength(1)的结果：

注意：从上图可以看出vtkSplineFilter类抽点和插值后的线段不一定通过输入点的位置；
设置SetNumberOfSubdivisions的个数为3时；

double p0[3] = {
     1.0, 0.0, 0.0 };
double p1[3] = {
     0.0, 1.0, 0.0 };
double p2[3] = {
     0.0, 0.0, 1.0 };
double p3[3] = {
     1.0, 2.0, 3.0 };

vtkNew<vtkPoints> points;
points->InsertNextPoint(p0);
points->InsertNextPoint(p1);
points->InsertNextPoint(p2);
points->InsertNextPoint(p3);
vtkNew<vtkCellArray> lines;
lines->InsertNextCell(4);
for (unsigned long i = 0; i < 4; ++i) {
    
	lines->InsertCellPoint(i);
}
vtkNew<vtkPolyData> polyData;
polyData->SetPoints(points);
polyData->SetLines(lines);

vtkNew<vtkSplineFilter> spline;
vtkNew<vtkCardinalSpline> cardinal_spine;

spline->SetInputDataObject(polyData);
spline->SetSpline(cardinal_spine);
spline->SetSubdivideToSpecified();
spline->SetNumberOfSubdivisions(3);	
spline->Update();

vtkNew<vtkPolyDataMapper> splineMapper;
splineMapper->SetInputConnection(spline->GetOutputPort());

vtkNew<vtkActor> splineActor;
splineActor->SetMapper(splineMapper);

vtkNew<vtkPolyData> resultPolydata;
resultPolydata->SetPoints(points);

vtkNew<vtkVertexGlyphFilter> resultGlyphFilter;
resultGlyphFilter->AddInputData(resultPolydata);
resultGlyphFilter->Update();

vtkNew<vtkPolyDataMapper> resultMapper;
resultMapper->SetInputConnection(resultGlyphFilter->GetOutputPort());
vtkNew<vtkActor> resultActor;
resultActor->SetMapper(resultMapper);
resultActor->GetProperty()->SetPointSize(5);//定义点的尺寸大小，这样点才能在画布上显示出来
resultActor->GetProperty()->SetColor(1, 0.0, 0.0);

vtkNew<vtkRenderer> ren1;
ren1->AddActor(splineActor);
ren1->AddActor(resultActor);
ren1->SetBackground(0.1, 0.2, 0.4);

vtkNew<vtkRenderWindow> renWin;
renWin->AddRenderer(ren1);
renWin->SetSize(300, 300);

vtkNew<vtkRenderWindowInteractor> iren;
iren->SetRenderWindow(renWin);
iren->Initialize();
iren->Start();
return 0;

设置SetNumberOfSubdivisions的个数为30时；

对曲线数据的进一步处理

如果我们的输入数据是一条携带很多干扰的曲线，曲线上有较多的高频信号；例如下图，

我们就可以先对该数据进行一个抽点操作（其实这里应该操作的是低通滤波），然后再插值操作，就会得到稍微平滑的曲线；
vtkSplineFilter类就可以实现这么一个简单的操作；
当我们对一条平滑曲线移动或者修改某一个点的位置时，就可能会破坏曲线的平滑度。可以使用vtkSplineFilter类修复平滑，不过使用vtkSplineFilter类并不是上上之策，三维曲线的滤波应该是一个比较好的选择。