# IntersectLine

VTKExamples/Cxx/ImageData/IntersectLine

### Description

This example demonstrates a (rather manual) way of finding which cells of a vtkImageData a finite line intersects. Note that this is not exact - a cell-centric approach is used. That is, a discrete line (DDA-like http://en.wikipedia.org/wiki/Digital_differential_analyzer_%28graphics_algorithm%29) is followed on the cell grid between the cells that contain the intersection points of the line with the bounding box of the image. These are all not necessarily the exact same cells as the line actually intersects, but it gives a reasonable "path" of cells through the image which might be suitable for some applications.

### Code

IntersectLine.cxx

    #include <vtkActor.h>
    #include <vtkBox.h>
    #include <vtkCellData.h>
    #include <vtkDataSetMapper.h>
    #include <vtkIdList.h>
    #include <vtkImageData.h>
    #include <vtkIntArray.h>
    #include <vtkMath.h>
    #include <vtkRenderWindow.h>
    #include <vtkRenderWindowInteractor.h>
    #include <vtkRenderer.h>
    #include <vtkShrinkFilter.h>
    #include <vtkSmartPointer.h>
    #include <vtkThreshold.h>
    #include <vtkUnstructuredGrid.h>

    #include <algorithm>
    #include <cmath>

    static vtkIdType FindCell(vtkImageData* grid, double point[3]);
    static std::vector<vtkIdType> IntersectImage(vtkImageData* image, double p0[3], double p1[3]);

    int main(int, char *[])
    {
      // Create a 3x4x5 grid of points (60), which will define a 2x3x4
      // (24) grid of cubes
      vtkSmartPointer<vtkImageData> grid =
        vtkSmartPointer<vtkImageData>::New();

      // Create the grid data structure
      grid->SetDimensions(3, 4, 5);

      std::cout << "There are "
                << grid->GetNumberOfPoints() << " points before shrinking."
                << std::endl;
      std::cout << "There are "
                << grid->GetNumberOfCells() << " cells before shrinking."
                << std::endl;

      // Define a ray
      double rayOrigin[3] = {-5.0, 0, 0};
      double rayDirection[3] = {1.0, 0, 0};
      double rayEndPoint[3];
      for(size_t i = 0; i < 3; ++i)
      {
        rayEndPoint[i] = rayOrigin[i] + rayDirection[i] * 1000; // 1000 is an arbitrary constant that should be much larger than the size of the scene (to create an "infinite" ray)
      }
      std::vector<vtkIdType> intersectedCells = IntersectImage(grid, rayOrigin, rayEndPoint);

      vtkSmartPointer<vtkShrinkFilter> shrinkFilter =
        vtkSmartPointer<vtkShrinkFilter>::New();
      shrinkFilter->SetInputData(grid);
      shrinkFilter->SetShrinkFactor(.8);
      shrinkFilter->Update();

      // Setup visibility array. Cells with visibility > 1 will be
      // visible, and < 1 will be invisible.
      vtkSmartPointer<vtkIntArray> visibilityArray =
        vtkSmartPointer<vtkIntArray>::New();
      visibilityArray->SetNumberOfComponents(1);
      visibilityArray->SetName("Visibility");

      // Initially, set all cells to visible
      for(vtkIdType cellId = 0; cellId < shrinkFilter->GetOutput()->GetNumberOfCells(); ++cellId)
      {
        visibilityArray->InsertNextValue(10);
      }

      // Set the intersected cells to invisible
      for(size_t i = 0; i < intersectedCells.size(); ++i)
      {
        visibilityArray->SetTuple1(intersectedCells[i], 0);
      }

      shrinkFilter->GetOutput()->GetCellData()->AddArray(visibilityArray);

      // Threshold
      vtkSmartPointer<vtkThreshold> threshold =
        vtkSmartPointer<vtkThreshold>::New();
      threshold->SetInputData(shrinkFilter->GetOutput());
      threshold->ThresholdByUpper(1); // Criterion is cells whose scalars are greater or equal to threshold.

      threshold->SetInputArrayToProcess(0, 0, 0, vtkDataObject::FIELD_ASSOCIATION_CELLS, "Visibility");
      threshold->Update();

      vtkUnstructuredGrid* thresholdedPolydata = threshold->GetOutput();
      std::cout << "There are " << thresholdedPolydata->GetNumberOfCells()
                << " cells after thresholding." << std::endl;

      /////////// Standard visualization setup //////////////
        // Create a mapper and actor
        vtkSmartPointer<vtkDataSetMapper> mapper =
          vtkSmartPointer<vtkDataSetMapper>::New();
        mapper->SetInputConnection(threshold->GetOutputPort());
        vtkSmartPointer<vtkActor> actor =
          vtkSmartPointer<vtkActor>::New();
        actor->SetMapper(mapper);

        // Create a renderer, render window, and interactor
        vtkSmartPointer<vtkRenderer> renderer =
          vtkSmartPointer<vtkRenderer>::New();
        vtkSmartPointer<vtkRenderWindow> renderWindow =
          vtkSmartPointer<vtkRenderWindow>::New();
        renderWindow->SetMultiSamples(0);
        renderWindow->AddRenderer(renderer);
        renderWindow->SetWindowName("Space Carving");
        vtkSmartPointer<vtkRenderWindowInteractor> renderWindowInteractor =
          vtkSmartPointer<vtkRenderWindowInteractor>::New();
        renderWindowInteractor->SetRenderWindow(renderWindow);

        // Add the actor to the scene
        renderer->AddActor(actor);
        renderer->SetBackground(.2, .3, .4);

        // Render and interact
        renderWindow->Render();
        renderWindowInteractor->Start();

        return EXIT_SUCCESS;
    }

    vtkIdType FindCell(vtkImageData* grid, double point[3])
    {
      double pcoords[3];
      double weights[8];

      int subId;

      // The third parameter is an "initial guess" of the cellId
      vtkIdType cellId = grid->FindCell(point, NULL, 0, .1, subId, pcoords, weights);
      return cellId;
    }

    // Intersect a finite line (with end points p0 and p1) with all of the
    // cells of a vtkImageData
    std::vector<vtkIdType> IntersectImage(vtkImageData* image, double p0[3], double p1[3])
    {
      // Intersect a ray with the bounding box of the grid. There should
      // be two points (an entrance and an exit)

      // Define a box
      double bounds[6];
      image->GetBounds(bounds);

      // The two points of intersection of the bounding box with the line
      // will be stored here
      double entrancePoint[3];
      double exitPoint[3];

      // We do not need the results stored in these variables, but they
      // are required for the function call.
      int entryPlane[6];
      int exitPlane[6];

      double t0, t1; // the [0,1] normalized distances to the intersections along the ray

      int hit = vtkBox::IntersectWithLine(bounds,
                                          p0,
                                          p1,
                                          t0,
                                          t1,
                                          entrancePoint,
                                          exitPoint,
                                          *entryPlane,
                                          *exitPlane);

      std::vector<vtkIdType> intersectedCells;

      if(!hit)
      {
        std::cout << "Not hit!" << std::endl;
        return intersectedCells;
      }
      else
      {
        std::cout << "Entry point: " << entrancePoint[0] << " " << entrancePoint[1] << " " << entrancePoint[2] << std::endl;
        std::cout << "Exit point: " << exitPoint[0] << " " << exitPoint[1] << " " << exitPoint[2] << std::endl;
      }

      // This simple algorithm was adapted from
      // http://www.cb.uu.se/~cris/blog/index.php/archives/400

      int dim[3];
      image->GetDimensions(dim);

      // Make sure to include the first cell that the line hits
      vtkIdType startCellId = FindCell(image, entrancePoint);
      intersectedCells.push_back(startCellId);

      double p[3];
      p[0] = entrancePoint[0];
      p[1] = entrancePoint[1];
      p[2] = entrancePoint[2];

      double d[3];
      d[0] = exitPoint[0] - entrancePoint[0];
      d[1] = exitPoint[1] - entrancePoint[1];
      d[2] = exitPoint[2] - entrancePoint[2];

       double N = std::max(std::max(
                  std::abs(d[0]), std::abs(d[1])),std::abs(d[2]));

      double s[3];
      s[0] = d[0]/N;
      s[1] = d[1]/N;
      s[2] = d[2]/N;

      for(size_t i = 0; i < N; ++i)
      {
        int ijk[3];
        ijk[0] = vtkMath::Round(p[0]);
        ijk[1] = vtkMath::Round(p[1]);
        ijk[2] = vtkMath::Round(p[2]);

        for(unsigned int index = 0; index < 3; ++index)
        {
          if(ijk[index] > dim[index] - 2)
          {
            ijk[index] = dim[index] - 2;
          }
        }

        vtkIdType cellId = vtkStructuredData::ComputeCellId (dim, ijk);
        intersectedCells.push_back(cellId);
        p[0] += s[0];
        p[1] += s[1];
        p[2] += s[2];
      }

      // Make sure to include the last cell that the line hits
      vtkIdType endCellId = FindCell(image, exitPoint);
      intersectedCells.push_back(endCellId);

      return intersectedCells;
    }

### CMakeLists.txt

    cmake_minimum_required(VERSION 3.3 FATAL_ERROR)

    project(IntersectLine)

    find_package(VTK COMPONENTS
      vtkCommonCore
      vtkCommonDataModel
      vtkFiltersCore
      vtkFiltersGeneral
      vtkInteractionStyle
      vtkRenderingCore
      vtkRenderingFreeType
      vtkRenderingOpenGL2 QUIET)
    if (NOT VTK_FOUND)
      message("Skipping IntersectLine: ${VTK_NOT_FOUND_MESSAGE}")
      return ()
    endif()
    message (STATUS "VTK_VERSION: ${VTK_VERSION}")
    if (VTK_VERSION VERSION_LESS "8.90.0")
      # old system
      include(${VTK_USE_FILE})
      add_executable(IntersectLine MACOSX_BUNDLE IntersectLine.cxx )
      target_link_libraries(IntersectLine PRIVATE ${VTK_LIBRARIES})
    else ()
      # include all components
      add_executable(IntersectLine MACOSX_BUNDLE IntersectLine.cxx )
      target_link_libraries(IntersectLine PRIVATE ${VTK_LIBRARIES})
      # vtk_module_autoinit is needed
      vtk_module_autoinit(
        TARGETS IntersectLine
        MODULES ${VTK_LIBRARIES}
        )
    endif ()

### Download and Build IntersectLine

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

    cd IntersectLine/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:

    ./IntersectLine

WINDOWS USERS

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