# FindCellIntersections

VTKExamples/Cxx/VisualizationAlgorithms/FindCellIntersections

### Description

This example shows how to use cell locator's FindCellsAlongLine method to visualize the cells that are intersected in an unstructured grid. The example uses the data file: https://github.com/lorensen/VTKWikiExamples/raw/master/Testing/Data/Disc_BiQuadraticQuads_0_0.vtu

### Code

FindCellIntersections.cxx

    #include <vtkSmartPointer.h>

    #include <vtkCellLocator.h>
    #include <vtkXMLUnstructuredGridReader.h>
    #include <vtkDataObject.h>
    #include <vtkUnstructuredGrid.h>
    #include <vtkIdList.h>
    #include <vtkCellArray.h>

    #include <vtkUnsignedCharArray.h>
    #include <vtkCellData.h>
    #include <vtkLineSource.h>

    #include <vtkShrinkFilter.h>

    #include <vtkDataSetSurfaceFilter.h>
    #include <vtkDataSetMapper.h>
    #include <vtkPolyDataMapper.h>
    #include <vtkActor.h>
    #include <vtkRenderer.h>
    #include <vtkRenderWindow.h>
    #include <vtkRenderWindowInteractor.h>
    #include <vtkProperty.h>
    #include <vtkCamera.h>

    int main (int argc, char *argv[])
    {
      if (argc < 2)
      {
        std::cout << "Usage: " << argv[0] << " filename.vtu" << std::endl;
        return EXIT_FAILURE;
      }

      // Get the filename from the command line
      std::string inputFilename = argv[1];

      // Read a xml unstructured grid dataset
      vtkSmartPointer<vtkXMLUnstructuredGridReader> reader =
        vtkSmartPointer<vtkXMLUnstructuredGridReader>::New();
      reader->SetFileName(inputFilename.c_str());
      reader->Update();

      // The line goes through the center of the data and runs along the x axis
      double center[3];
      reader->GetOutput()->GetCenter(center);
      double bounds[6];
      reader->GetOutput()->GetBounds(bounds);

      double startRay[3];
      startRay[0] = bounds[0] - center[0];
      startRay[1] = center[1];
      startRay[2] = center[2];

      double endRay[3];
      endRay[0] = bounds[1] + center[0];
      endRay[1] = center[1];
      endRay[2] = center[2];
      std::cout << "center: "
                << center[0] << ", "
                << center[1] << ", "
                << center[2] << std::endl;
      std::cout << "startRay: "
                << startRay[0] << ", "
                << startRay[1] << ", "
                << startRay[2] << std::endl;
      std::cout << "endRay: "
                << endRay[0] << ", "
                << endRay[1] << ", "
                << endRay[2] << std::endl;

      // Initialize all of the cell data colors
      vtkSmartPointer<vtkUnsignedCharArray> cellData =
        vtkSmartPointer<vtkUnsignedCharArray>::New();
      cellData->SetNumberOfComponents(3);
      cellData->SetNumberOfTuples(reader->GetOutput()->GetNumberOfCells());
      reader->GetOutput()->GetCellData()->SetScalars(cellData);

      for (int i = 0; i < cellData->GetNumberOfTuples(); ++i)
      {
        float rgb[3];
        rgb[0] = 227.0; rgb[1] = 207.0; rgb[2] =  87.0; // banana
        cellData->InsertTuple(i, rgb);
      }

      // Find the cells that intersect the line and color those cells
      vtkSmartPointer<vtkIdList> cellIds =
        vtkSmartPointer<vtkIdList>::New();
      vtkSmartPointer<vtkCellLocator> locator =
        vtkSmartPointer<vtkCellLocator>::New();
      locator->SetDataSet(reader->GetOutput());
      locator->BuildLocator();
      locator->FindCellsAlongLine(startRay, endRay, .001, cellIds);

      for (vtkIdType i = 0; i < cellIds->GetNumberOfIds(); ++i)
      {
        float rgb[3];
        rgb[0] = 255; rgb[1] = 99; rgb[2] = 71; // tomato
        cellData->InsertTuple(cellIds->GetId(i), rgb);
      }

      // Shrink each cell to make them visible
      vtkSmartPointer<vtkShrinkFilter> shrink =
        vtkSmartPointer<vtkShrinkFilter>::New();
      shrink->SetInputConnection(reader->GetOutputPort());
      shrink->SetShrinkFactor(.95);

      // Convert the cells to polydata
      vtkSmartPointer<vtkDataSetSurfaceFilter> surface =
        vtkSmartPointer<vtkDataSetSurfaceFilter>::New();
      surface->SetInputConnection(shrink->GetOutputPort());
      surface->SetNonlinearSubdivisionLevel(2);
      surface->Update();

      // Create a line
      vtkSmartPointer<vtkLineSource> lineSource =
        vtkSmartPointer<vtkLineSource>::New();
      lineSource->SetPoint1(startRay);
      lineSource->SetPoint2(endRay);
      vtkSmartPointer<vtkPolyDataMapper> lineMapper =
        vtkSmartPointer<vtkPolyDataMapper>::New();
      lineMapper->SetInputConnection(lineSource->GetOutputPort());
      vtkSmartPointer<vtkActor> lineActor =
        vtkSmartPointer<vtkActor>::New();
      lineActor->SetMapper(lineMapper);

      // Render the results
      vtkSmartPointer<vtkPolyDataMapper> mapper =
        vtkSmartPointer<vtkPolyDataMapper>::New();
      mapper->SetInputConnection(surface->GetOutputPort());
      mapper->ScalarVisibilityOn();
      mapper->SetScalarModeToUseCellData();

      vtkSmartPointer<vtkActor> actor =
        vtkSmartPointer<vtkActor>::New();
      actor->SetMapper(mapper);

      vtkSmartPointer<vtkRenderer> renderer =
        vtkSmartPointer<vtkRenderer>::New();
      renderer->AddActor(actor);
      renderer->AddActor(lineActor);
      renderer->SetBackground(.2, .3, .4);

      // Make an oblique view
      renderer->GetActiveCamera()->Azimuth(30);
      renderer->GetActiveCamera()->Elevation(30);
      renderer->ResetCamera();

      vtkSmartPointer<vtkRenderWindow> renWin =
        vtkSmartPointer<vtkRenderWindow>::New();
      vtkSmartPointer<vtkRenderWindowInteractor>
        iren = vtkSmartPointer<vtkRenderWindowInteractor>::New();

      iren->SetRenderWindow(renWin);
      renWin->AddRenderer(renderer);
      renWin->SetSize(640, 480);
      renWin->Render();

      iren->Start();

      return EXIT_SUCCESS;
    }

### CMakeLists.txt

    cmake_minimum_required(VERSION 2.8)

    PROJECT(FindCellIntersections)

    find_package(VTK REQUIRED)
    include(${VTK_USE_FILE})

    add_executable(FindCellIntersections MACOSX_BUNDLE FindCellIntersections.cxx )

    target_link_libraries(FindCellIntersections ${VTK_LIBRARIES})

### Download and Build FindCellIntersections

Click here to download FindCellIntersections and its CMakeLists.txt file. Once the tarball FindCellIntersections.tar has been downloaded and extracted,

    cd FindCellIntersections/build

If VTK is installed:

    cmake ..

If VTK is not installed but compiled on your system, you will need to specify the path to your VTK build:

    cmake -DVTK_DIR:PATH=/home/me/vtk_build ..

Build the project:

    make

and run it:

    ./FindCellIntersections

WINDOWS USERS PLEASE NOTE: Be sure to add the VTK bin directory to your path. This will resolve the VTK dll's at run time.