# IntersectLine

Repository source: 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.

Question

### Code¶

IntersectLine.cxx

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

#include <algorithm>
#include <cmath>

namespace {
vtkIdType FindCell(vtkImageData* grid, double point[3]);

std::vector<vtkIdType> IntersectImage(vtkImageData* image, double p0[3],
double p1[3]);
} // namespace

int main(int, char*[])
{
vtkNew<vtkNamedColors> colors;

// Create a 3x4x5 grid of points (60), which will define a 2x3x4
// (24) grid of cubes
vtkNew<vtkImageData> grid;

// 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);

vtkNew<vtkShrinkFilter> shrinkFilter;
shrinkFilter->SetInputData(grid);
shrinkFilter->SetShrinkFactor(.8);
shrinkFilter->Update();

// Setup visibility array. Cells with visibility > 1 will be
// visible, and < 1 will be invisible.
vtkNew<vtkIntArray> visibilityArray;
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);
}

// Threshold
vtkNew<vtkThreshold> threshold;
threshold->SetInputData(shrinkFilter->GetOutput());
// Criterion is cells whose scalars are greater or equal to threshold.
threshold->SetUpperThreshold(1);
threshold->SetThresholdFunction(vtkThreshold::THRESHOLD_UPPER);

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
vtkNew<vtkDataSetMapper> mapper;
mapper->SetInputConnection(threshold->GetOutputPort());
vtkNew<vtkActor> actor;
actor->SetMapper(mapper);
actor->GetProperty()->SetColor(colors->GetColor3d("BurlyWood").GetData());

// Create a renderer, render window, and interactor
vtkNew<vtkRenderer> renderer;
vtkNew<vtkRenderWindow> renderWindow;
renderWindow->SetMultiSamples(0);
renderWindow->SetSize(640, 480);
vtkNew<vtkRenderWindowInteractor> renderWindowInteractor;
renderWindowInteractor->SetRenderWindow(renderWindow);

// Add the actor to the scene
renderer->SetBackground(colors->GetColor3d("SteelBlue").GetData());

renderer->GetActiveCamera()->SetPosition(-6, 7, 6);
renderer->GetActiveCamera()->SetFocalPoint(1, 1.5, 2);
renderer->GetActiveCamera()->SetViewUp(0.4, 8, -0.4);
renderer->ResetCameraClippingRange();

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

return EXIT_SUCCESS;
}

namespace {

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]{0, 0, 0, 0, 0, 0};
int exitPlane[6]{0, 0, 0, 0, 0, 0};

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;
}

} // namespace


### CMakeLists.txt¶

cmake_minimum_required(VERSION 3.12 FATAL_ERROR)

project(IntersectLine)

find_package(VTK COMPONENTS
CommonColor
CommonCore
CommonDataModel
FiltersCore
FiltersGeneral
InteractionStyle
RenderingContextOpenGL2
RenderingCore
RenderingFreeType
RenderingGL2PSOpenGL2
RenderingOpenGL2
)

if (NOT VTK_FOUND)
message(FATAL_ERROR "IntersectLine: Unable to find the VTK build folder.")
endif()

# Prevent a "command line is too long" failure in Windows.
set(CMAKE_NINJA_FORCE_RESPONSE_FILE "ON" CACHE BOOL "Force Ninja to use response files.")
target_link_libraries(IntersectLine PRIVATE ${VTK_LIBRARIES} ) # vtk_module_autoinit is needed vtk_module_autoinit( TARGETS IntersectLine MODULES${VTK_LIBRARIES}
)


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.