VTK中对模型进行封闭性检测

由于受原始数据、重建方法的限制，得到的网格模型并不是封闭的。有时为了显示或者处理某些要求，需要网格必须是封闭的。比如一个球面网格就是封闭性网格。

如果一条边只被一个多边形包含，那么这条边就是边界边。是否存在边界边是检测一个网格模型是否封闭的重要特征。vtkFeatureEdges是一个非常重要的类，该类能够提取多边形网格模型中四种类型的边：

1. 边界边：即只被一个多边形或者一条边包含的边。
2. 非流形边：被3个或者3个以上的多边形包含的边。
3. 特征边：需设置一个特征角的阈值，当包含同一条边的两个三角形的法向量的夹角大于该阈值时，即为一个特征边。
4. 流形边：只被两个多边形包含的边。

可以使用该类来检测是否存在边界边，并以此来判断网格模型是否封闭，代码如下：

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void GenerateData(vtkSmartPointer input)
{
	// 以下代码是创建一个球面网格，并去除两个三角面片
    vtkSmartPointer sphereSource =
		vtkSmartPointer::New();
	sphereSource->Update();
    
    // 根据索引选择要被去除两个三角面片
	vtkSmartPointer ids =
		vtkSmartPointer::New();
	ids->SetNumberOfComponents(1);
	ids->InsertNextValue(2);
	ids->InsertNextValue(10);

    // 选择点，注意Set(vtkSelectionNode::INVERSE(), 1)中的INVERSE(),这是反选
	vtkSmartPointer selectionNode =
		vtkSmartPointer::New();
	selectionNode->SetFieldType(vtkSelectionNode::CELL);
	selectionNode->SetContentType(vtkSelectionNode::INDICES);
	selectionNode->SetSelectionList(ids);
	selectionNode->GetProperties()->Set(vtkSelectionNode::INVERSE(), 1);

	vtkSmartPointer selection =
		vtkSmartPointer::New();
	selection->AddNode(selectionNode);

	vtkSmartPointer extractSelection =
		vtkSmartPointer::New();
	extractSelection->SetInputData(0, sphereSource->GetOutput());
	extractSelection->SetInputData(1, selection);
	extractSelection->Update();

	vtkSmartPointer surfaceFilter =
		vtkSmartPointer::New();
	surfaceFilter->SetInputConnection(extractSelection->GetOutputPort());
	surfaceFilter->Update();

	input->ShallowCopy(surfaceFilter->GetOutput());
}

int main(int argc, char *argv[])
{
	vtkSmartPointer input =
		vtkSmartPointer::New();
	GenerateData(input);

    // 特征边提取
	vtkSmartPointer featureEdges =
		vtkSmartPointer::New();
	featureEdges->SetInputData(input);
	featureEdges->BoundaryEdgesOn();
	featureEdges->FeatureEdgesOff();
	featureEdges->ManifoldEdgesOff();
	featureEdges->NonManifoldEdgesOff();
	featureEdges->Update();

    // 根据特征边的数量判断是否封闭
	int numberOfOpenEdges = featureEdges->GetOutput()->GetNumberOfCells();
	if(numberOfOpenEdges) 
	{
		std::cout<<"该网格模型不是封闭的..."< fillHolesFilter =
		vtkSmartPointer::New();
	fillHolesFilter->SetInputData(input);
	fillHolesFilter->Update();

    // 经过漏洞填充，模型所有点的顺序并不一致，因此需要vtkPolyDataNormals类中
    // 的ConsistencyOn()使点的顺序一致，只有这样才能得到正确的法向量。法向量是
    // 与光照和阴影计算相关的。
	vtkSmartPointer normals =
		vtkSmartPointer::New();
	normals->SetInputConnection(fillHolesFilter->GetOutputPort());
	normals->ConsistencyOn();
	normals->SplittingOff();
	normals->Update();

	//////////////////////////////////////////////////////////////////////////
	double leftViewport[4] = {0.0, 0.0, 0.5, 1.0};
	double rightViewport[4] = {0.5, 0.0, 1.0, 1.0};

	vtkSmartPointer originalMapper =
		vtkSmartPointer::New();
	originalMapper->SetInputData(input);

	vtkSmartPointer backfaceProp =
		vtkSmartPointer::New();
	backfaceProp->SetDiffuseColor(0.89,0.81,0.34);

	vtkSmartPointer originalActor =
		vtkSmartPointer::New();
	originalActor->SetMapper(originalMapper);
	originalActor->SetBackfaceProperty(backfaceProp);
	originalActor->GetProperty()->SetDiffuseColor(1.0, 0.3882, 0.2784);

	vtkSmartPointer edgeMapper =
		vtkSmartPointer::New();
	edgeMapper->SetInputData(featureEdges->GetOutput());
	vtkSmartPointer edgeActor =
		vtkSmartPointer::New();
	edgeActor->SetMapper(edgeMapper);
	edgeActor->GetProperty()->SetEdgeColor(0.,0.,1.0);
	edgeActor->GetProperty()->SetEdgeVisibility(1);
	edgeActor->GetProperty()->SetLineWidth(5);

	vtkSmartPointer filledMapper =
		vtkSmartPointer::New();
	filledMapper->SetInputData(normals->GetOutput());

	vtkSmartPointer filledActor =
		vtkSmartPointer::New();
	filledActor->SetMapper(filledMapper);
	filledActor->GetProperty()->SetDiffuseColor(1.0, 0.3882, 0.2784);

	vtkSmartPointer leftRenderer =
		vtkSmartPointer::New();
	leftRenderer->SetViewport(leftViewport);
	leftRenderer->AddActor(originalActor);
	leftRenderer->AddActor(edgeActor);
	leftRenderer->SetBackground(1.0, 1.0, 1.0);

	vtkSmartPointer rightRenderer =
		vtkSmartPointer::New();
	rightRenderer->SetViewport(rightViewport);
	rightRenderer->AddActor(filledActor);
	rightRenderer->SetBackground(1.0, 1.0, 1.0);

	vtkSmartPointer renderWindow =
		vtkSmartPointer::New();
	renderWindow->AddRenderer(leftRenderer);
	renderWindow->AddRenderer(rightRenderer);
	renderWindow->SetSize(640, 320);
	renderWindow->Render();
	renderWindow->SetWindowName("PolyDataClosed");

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

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

	renderWindow->Render();
	renderWindowInteractor->Start();

	return EXIT_SUCCESS;
}

下图为运行结果，左图为原始模型，并将检测的边界用红色显示，右侧为漏洞填补后的结果。