### Description¶

The algorithm used is the one described by Peter Lindstrom in his Siggraph 2000 paper, "Out-of-Core Simplification of Large Polygonal Models." The general approach of the algorithm is to cluster vertices in a uniform binning of space, accumulating the quadric of each triangle (pushed out to the triangles vertices) within each bin, and then determining an optimal position for a single vertex in a bin by using the accumulated quadric.

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See (CSharp)

Question

### Code¶

#include <vtkPolyData.h>
#include <vtkSphereSource.h>
#include <vtkSmartPointer.h>
#include <vtkPolyDataMapper.h>
#include <vtkProperty.h>
#include <vtkRenderWindow.h>
#include <vtkRenderWindowInteractor.h>
#include <vtkRenderer.h>
#include <vtkCamera.h>

#include <vtkNamedColors.h>

int main(int argc, char *argv[])
{
vtkSmartPointer<vtkPolyData> inputPolyData;
if(argc > 1)
{
}
else
{
vtkSmartPointer<vtkSphereSource> sphereSource =
vtkSmartPointer<vtkSphereSource>::New();
sphereSource->SetThetaResolution(30);
sphereSource->SetPhiResolution(15);
sphereSource->Update();
inputPolyData = sphereSource->GetOutput();
}

vtkSmartPointer<vtkNamedColors> colors =
vtkSmartPointer<vtkNamedColors>::New();

std::cout << "Before decimation" << std::endl << "------------" << std::endl;
std::cout << "There are "
<< inputPolyData->GetNumberOfPoints() << " points." << std::endl;
std::cout << "There are "
<< inputPolyData->GetNumberOfPolys() << " polygons." << std::endl;

decimate->SetInputData(inputPolyData);
decimate->UseFeatureEdgesOn();
decimate->Update();

vtkSmartPointer<vtkPolyData> decimated =
vtkSmartPointer<vtkPolyData>::New();
decimated->ShallowCopy(decimate->GetOutput());

std::cout << "After decimation" << std::endl << "------------" << std::endl;
std::cout << "There are "
<< decimated->GetNumberOfPoints() << " points." << std::endl;
std::cout << "There are "
<< decimated->GetNumberOfPolys() << " polygons." << std::endl;
std::cout << "Reduction: " <<
static_cast<double>((inputPolyData->GetNumberOfPolys() - decimated->GetNumberOfPolys())) /
static_cast<double>(inputPolyData->GetNumberOfPolys()) << std::endl;

vtkSmartPointer<vtkPolyDataMapper> inputMapper =
vtkSmartPointer<vtkPolyDataMapper>::New();
inputMapper->SetInputData(inputPolyData);

vtkSmartPointer<vtkProperty> backFace =
vtkSmartPointer<vtkProperty>::New();
backFace->SetColor(colors->GetColor3d("gold").GetData());

vtkSmartPointer<vtkActor> inputActor =
vtkSmartPointer<vtkActor>::New();
inputActor->SetMapper(inputMapper);
inputActor->GetProperty()->SetInterpolationToFlat();
inputActor->GetProperty()->SetColor(colors->GetColor3d("flesh").GetData());
inputActor->SetBackfaceProperty(backFace);

vtkSmartPointer<vtkPolyDataMapper> decimatedMapper =
vtkSmartPointer<vtkPolyDataMapper>::New();
decimatedMapper->SetInputData(decimated);

vtkSmartPointer<vtkActor> decimatedActor =
vtkSmartPointer<vtkActor>::New();
decimatedActor->SetMapper(decimatedMapper);
decimatedActor->GetProperty()->SetColor(colors->GetColor3d("flesh").GetData());
decimatedActor->GetProperty()->SetInterpolationToFlat();
decimatedActor->SetBackfaceProperty(backFace);

// There will be one render window
vtkSmartPointer<vtkRenderWindow> renderWindow =
vtkSmartPointer<vtkRenderWindow>::New();
renderWindow->SetSize(600, 300);

// And one interactor
vtkSmartPointer<vtkRenderWindowInteractor> interactor =
vtkSmartPointer<vtkRenderWindowInteractor>::New();
interactor->SetRenderWindow(renderWindow);

// Define viewport ranges
// (xmin, ymin, xmax, ymax)
double leftViewport[4] = {0.0, 0.0, 0.5, 1.0};
double rightViewport[4] = {0.5, 0.0, 1.0, 1.0};

// Setup both renderers
vtkSmartPointer<vtkRenderer> leftRenderer =
vtkSmartPointer<vtkRenderer>::New();
leftRenderer->SetViewport(leftViewport);
leftRenderer->SetBackground(colors->GetColor3d("burlywood").GetData());

vtkSmartPointer<vtkRenderer> rightRenderer =
vtkSmartPointer<vtkRenderer>::New();
rightRenderer->SetViewport(rightViewport);
rightRenderer->SetBackground(colors->GetColor3d("slate_grey").GetData());

// Add the sphere to the left and the cube to the right

// Shared camera looking down the -y axis
vtkSmartPointer<vtkCamera> camera =
vtkSmartPointer<vtkCamera>::New();
camera->SetPosition (0, -1, 0);
camera->SetFocalPoint (0, 0, 0);
camera->SetViewUp (0, 0, 1);
camera->Elevation(30);
camera->Azimuth(30);

leftRenderer->SetActiveCamera(camera);
rightRenderer->SetActiveCamera(camera);

leftRenderer->ResetCamera();
leftRenderer->ResetCameraClippingRange();

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

return EXIT_SUCCESS;
}


### CMakeLists.txt¶

cmake_minimum_required(VERSION 3.3 FATAL_ERROR)

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


cd QuadricClustering/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:

./QuadricClustering


WINDOWS USERS

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