CardinalSpline

VTKExamples/Cxx/Utilities/CardinalSpline

Description

This example takes two points that have associated scalar values and allows you to interpolate the value at any point on the line between them using cardinal spline interpolation.

Question

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Code

CardinalSpline.cxx

#include <vtkSmartPointer.h>
#include <vtkCardinalSpline.h>
#include <vtkFloatArray.h>

int main(int, char *[])
{
  // Create a scalar array.  The array could be associated with the scalars of a vtkDataSet
  vtkSmartPointer<vtkFloatArray> distances =
      vtkSmartPointer<vtkFloatArray>::New();
  distances->SetNumberOfComponents(1);
  distances->SetName("Distances");
  distances->InsertNextValue(100.0);
  distances->InsertNextValue(300.0);

  // Create a cardinal spline to show how to linearly interpolate between two scalar values
  vtkSmartPointer<vtkCardinalSpline> spline =
      vtkSmartPointer<vtkCardinalSpline>::New();
  spline->ClosedOff();

  // Set the left and right second derivatives to 0 corresponding to linear interpolation
  spline->SetLeftConstraint(2);
  spline->SetLeftValue(0);
  spline->SetRightConstraint(2);
  spline->SetRightValue(0);
  double* r = distances->GetRange();
  double xmin = r[0];
  double xmax = r[1];
  double length = xmax-xmin;
  for( vtkIdType i = 0; i < distances->GetNumberOfTuples(); ++i )
  {
    double x = distances->GetTuple1(i);
    double t = (x - xmin)/length;
    spline->AddPoint( t, x );
  }

  // Evaluate every 50 distance units along the line
  std::cout << "Spline interpolation:" << std::endl;
  double dt = .25;
  for( double t = dt ; t <= 1. - dt; t += dt )
  {
    std::cout << "t: " << t << " value = " << spline->Evaluate(t) << std::endl;
  }

  return EXIT_SUCCESS;
}

CMakeLists.txt

cmake_minimum_required(VERSION 3.3 FATAL_ERROR)

project(CardinalSpline)

find_package(VTK COMPONENTS 
  vtkCommonComputationalGeometry
  vtkCommonCore QUIET)
if (NOT VTK_FOUND)
  message("Skipping CardinalSpline: ${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(CardinalSpline MACOSX_BUNDLE CardinalSpline.cxx )
  target_link_libraries(CardinalSpline PRIVATE ${VTK_LIBRARIES})
else ()
  # include all components
  add_executable(CardinalSpline MACOSX_BUNDLE CardinalSpline.cxx )
  target_link_libraries(CardinalSpline PRIVATE ${VTK_LIBRARIES})
  # vtk_module_autoinit is needed
  vtk_module_autoinit(
    TARGETS CardinalSpline
    MODULES ${VTK_LIBRARIES}
    )
endif () 

Download and Build CardinalSpline

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

cd CardinalSpline/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:

./CardinalSpline

WINDOWS USERS

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