ENH: Add VTK C++ conversion example

There may be more efficient approaches.

PointData and CellData are not transferred.

examples/CMakeLists.txt

@@ -0,0 +1,20 @@
+cmake_minimum_required(VERSION 3.16)
+project(ITKMeshToPolyDataExamples)
+
+find_package(ITK REQUIRED
+ COMPONENTS
+   MeshToPolyData
+   ITKMeshIO
+   )
+find_package(VTK REQUIRED)
+
+include(${ITK_USE_FILE})
+
+add_executable(convert-itk-poly-data-to-vtk-poly-data convert-itk-poly-data-to-vtk-poly-data.cxx)
+target_link_libraries(convert-itk-poly-data-to-vtk-poly-data PUBLIC ${ITK_LIBRARIES} ${VTK_LIBRARIES})
+
+enable_testing()
+add_test(NAME convert-itk-poly-data-to-vtk-poly-data
+ COMMAND convert-itk-poly-data-to-vtk-poly-data
+  ${CMAKE_CURRENT_SOURCE_DIR}/cow.vtk
+  ${CMAKE_CURRENT_BINARY_DIR}/cow-vtk.vtk)

examples/convert-itk-poly-data-to-vtk-poly-data.cxx

@@ -0,0 +1,146 @@
+#include "itkMesh.h"
+#include "itkMeshFileReader.h"
+#include "itkMeshToPolyDataFilter.h"
+#include "vtkPolyData.h"
+#include "vtkSmartPointer.h"
+#include "vtkPolyDataWriter.h"
+#include "vtkNew.h"
+
+using MeshType = itk::Mesh<float, 3>;
+
+// Function to convert itk::Mesh to vtkPolyData
+vtkSmartPointer<vtkPolyData> ConvertITKMeshToVTKPolyData(MeshType::Pointer itkMesh)
+{
+    typedef itk::MeshToPolyDataFilter<MeshType> MeshToPolyDataType;
+    MeshToPolyDataType::Pointer meshToPolyData = MeshToPolyDataType::New();
+    meshToPolyData->SetInput(itkMesh);
+    meshToPolyData->Update();
+
+    const auto polyDataItk = meshToPolyData->GetOutput();
+
+    vtkSmartPointer<vtkPolyData> polyDataVtk = vtkSmartPointer<vtkPolyData>::New();
+
+    // Transfer the points from polyDataItk to polyDataVtk
+    vtkSmartPointer<vtkPoints> points = vtkSmartPointer<vtkPoints>::New();
+    const auto itkPoints = polyDataItk->GetPoints();
+    for (itk::IdentifierType i = 0; i < itkPoints->size(); ++i)
+    {
+        const auto point = itkPoints->GetElement(i);
+        points->InsertNextPoint(point[0], point[1], point[2]);
+    }
+    polyDataVtk->SetPoints(points);
+
+    // Transfer the vertices from polyDataItk to polyDataVtk
+    const auto verticesItk = polyDataItk->GetVertices();
+    vtkNew<vtkCellArray> verticesVtk;
+    for (itk::IdentifierType i = 0; i < verticesItk->size() / 2; ++i)
+    {
+        const vtkIdType vertex = static_cast<vtkIdType>(verticesItk->GetElement(i*2+1));
+        verticesVtk->InsertNextCell(1, &vertex);
+    }
+    polyDataVtk->SetVerts(verticesVtk);
+
+    // Transfer the lines from polyDataItk to polyDataVtk
+    const auto linesItk = polyDataItk->GetLines();
+    vtkNew<vtkCellArray> linesVtk;
+    auto linesIt = linesItk->begin();
+    while(linesIt != linesItk->end())
+    {
+        const auto numberOfPoints = *linesIt;
+        ++linesIt;
+        vtkNew<vtkIdList> line;
+        for (itk::IdentifierType i = 0; i < numberOfPoints; ++i)
+        {
+            const vtkIdType pointId = static_cast<vtkIdType>(*linesIt);
+            line->InsertNextId(pointId);
+            ++linesIt;
+        }
+        linesVtk->InsertNextCell(line);
+    }
+    polyDataVtk->SetLines(linesVtk);
+
+    // Transfer the polygons from polyDataItk to polyDataVtk
+    const auto polygonsItk = polyDataItk->GetPolygons();
+    vtkNew<vtkCellArray> polygonsVtk;
+    auto polygonsIt = polygonsItk->begin();
+    while (polygonsIt != polygonsItk->end())
+    {
+        const auto numberOfPoints = *polygonsIt;
+        ++polygonsIt;
+        vtkNew<vtkIdList> polygon;
+        for (itk::IdentifierType i = 0; i < numberOfPoints; ++i)
+        {
+            const vtkIdType pointId = static_cast<vtkIdType>(*polygonsIt);
+            polygon->InsertNextId(pointId);
+            ++polygonsIt;
+        }
+        polygonsVtk->InsertNextCell(polygon);
+    }
+    polyDataVtk->SetPolys(polygonsVtk);
+
+    // Transfer the triangle strips from polyDataItk to polyDataVtk
+    const auto triangleStripsItk = polyDataItk->GetTriangleStrips();
+    vtkNew<vtkCellArray> triangleStripsVtk;
+    auto triangleStripsIt = triangleStripsItk->begin();
+    while (triangleStripsIt != triangleStripsItk->end())
+    {
+        const auto numberOfPoints = *triangleStripsIt;
+        ++triangleStripsIt;
+        vtkNew<vtkIdList> triangleStrip;
+        for (itk::IdentifierType i = 0; i < numberOfPoints; ++i)
+        {
+            const vtkIdType pointId = static_cast<vtkIdType>(*triangleStripsIt);
+            triangleStrip->InsertNextId(pointId);
+            ++triangleStripsIt;
+        }
+        triangleStripsVtk->InsertNextCell(triangleStrip);
+    }
+    polyDataVtk->SetStrips(triangleStripsVtk);
+
+    // PointData and CellData could also be transferred, if needed.
+
+    return polyDataVtk;
+}
+
+int main(int argc, char *argv[])
+{
+    if (argc < 3)
+    {
+        std::cerr << "Usage: " << argv[0] << " <input ITK mesh file> <output VTK file>" << std::endl;
+        return EXIT_FAILURE;
+    }
+
+    const char *inputFilename = argv[1];
+    const char *outputFilename = argv[2];
+
+    // Read the input ITK mesh
+    typedef itk::MeshFileReader<MeshType> ReaderType;
+    ReaderType::Pointer reader = ReaderType::New();
+    reader->SetFileName(inputFilename);
+
+    try
+    {
+        reader->Update();
+    }
+    catch (itk::ExceptionObject &error)
+    {
+        std::cerr << "Error reading ITK mesh: " << error << std::endl;
+        return EXIT_FAILURE;
+    }
+
+    MeshType::Pointer itkMesh = reader->GetOutput();
+
+    // Convert ITK mesh to VTK PolyData
+    vtkSmartPointer<vtkPolyData> vtkPolyData = ConvertITKMeshToVTKPolyData(itkMesh);
+
+    // Write the VTK PolyData to file
+    vtkSmartPointer<vtkPolyDataWriter> writer = vtkSmartPointer<vtkPolyDataWriter>::New();
+    writer->SetFileName(outputFilename);
+    writer->SetInputData(vtkPolyData);
+
+    writer->Write();
+
+    std::cout << "Successfully converted ITK mesh to VTK PolyData and saved to " << outputFilename << std::endl;
+
+    return EXIT_SUCCESS;
+}