InterpolateTerrain

Description¶

This example samples a "terrain map" using two approaches.

Creates an image from the x,y points of the terrain and then uses vtkProbeFilter to interpolate the heights.
Uses vtkCellLocator to directly interpolate the terrain polydata.

Note that the results differ when the point is not one of the original terrain points. This is because the image has quadrilateral elements and the polydata has triangles. As the resolution of the grid increases, the two results converge.

Question

If you have a question about this example, please use the VTK Discourse Forum

Code¶

InterpolateTerrain.cxx

#include <vtkCellLocator.h>
#include <vtkDelaunay2D.h>
#include <vtkDoubleArray.h>
#include <vtkImageData.h>
#include <vtkMinimalStandardRandomSequence.h>
#include <vtkNew.h>
#include <vtkPointData.h>
#include <vtkPoints.h>
#include <vtkPolyData.h>
#include <vtkProbeFilter.h>
#include <vtkXMLPolyDataWriter.h>

#include <iostream>
#include <string>

int main(int, char*[])
{
  vtkNew<vtkImageData> image;
  image->SetExtent(0, 9, 0, 9, 0, 0);
  image->AllocateScalars(VTK_DOUBLE, 1);

  // Create a random set of heights on a grid. This is often called a
  //"terrain map".
  vtkNew<vtkPoints> points;

  vtkNew<vtkMinimalStandardRandomSequence> randomSequence;
  randomSequence->SetSeed(8775070);
  unsigned int GridSize = 10;
  for (unsigned int x = 0; x < GridSize; x++)
  {
    for (unsigned int y = 0; y < GridSize; y++)
    {
      double val = randomSequence->GetRangeValue(-1, 1);
      randomSequence->Next();
      points->InsertNextPoint(x, y, val);
      image->SetScalarComponentFromDouble(x, y, 0, 0, val);
    }
  }

  // Add the grid points to a polydata object.
  vtkNew<vtkPolyData> polydata;
  polydata->SetPoints(points);

  // Triangulate the grid points.
  vtkNew<vtkDelaunay2D> delaunay;
  delaunay->SetInputData(polydata);
  delaunay->Update();

  vtkNew<vtkXMLPolyDataWriter> writer;
  writer->SetFileName("surface.vtp");
  writer->SetInputConnection(delaunay->GetOutputPort());
  writer->Write();

  // Add some points to interpolate.
  vtkNew<vtkPoints> probePoints;
  probePoints->InsertNextPoint(5.2, 3.2, 0);
  probePoints->InsertNextPoint(5.0, 3.0, 0);
  probePoints->InsertNextPoint(0.0, 0.0, 0);

  vtkNew<vtkPolyData> probePolyData;
  probePolyData->SetPoints(probePoints);

  vtkNew<vtkProbeFilter> probe;
  probe->SetSourceData(image);
  probe->SetInputData(probePolyData);
  probe->Update();

  vtkDataArray* data = probe->GetOutput()->GetPointData()->GetScalars();
  vtkDoubleArray* doubleData = dynamic_cast<vtkDoubleArray*>(data);

  for (int i = 0; i < doubleData->GetNumberOfTuples(); i++)
  {
    double val = doubleData->GetValue(i);
    cout << "Interpolation using ProbeFilter ";
    cout << "doubleData->GetValue(" << i << "): " << val << endl;
  }

  // Now find the elevation with a CellLocator.
  vtkNew<vtkCellLocator> cellLocator;
  cellLocator->SetDataSet(delaunay->GetOutput());
  cellLocator->BuildLocator();

  for (int i = 0; i < doubleData->GetNumberOfTuples(); i++)
  {
    int subId;
    double t, xyz[3], pcoords[3];
    double rayStart[3], rayEnd[3];
    probePoints->GetPoint(i, rayStart);
    rayStart[2] += 1000.0;
    probePoints->GetPoint(i, rayEnd);
    rayEnd[2] -= 1000.0;

    if (cellLocator->IntersectWithLine(rayStart, rayEnd, 0.0001, t, xyz,
                                       pcoords, subId))
    {
      cout << "Interpolation using CellLocator ";
      cout << "Elevation at " << rayStart[0] << ", " << rayStart[1] << " is "
           << xyz[2] << endl;
    }
  }
  return EXIT_SUCCESS;
}

CMakeLists.txt¶

cmake_minimum_required(VERSION 3.12 FATAL_ERROR)

project(InterpolateTerrain)

find_package(VTK COMPONENTS 
  CommonCore
  CommonDataModel
  FiltersCore
  IOXML
)

if (NOT VTK_FOUND)
  message(FATAL_ERROR "InterpolateTerrain: 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.")
add_executable(InterpolateTerrain MACOSX_BUNDLE InterpolateTerrain.cxx )
  target_link_libraries(InterpolateTerrain PRIVATE ${VTK_LIBRARIES}
)
# vtk_module_autoinit is needed
vtk_module_autoinit(
  TARGETS InterpolateTerrain
  MODULES ${VTK_LIBRARIES}
)

Download and Build InterpolateTerrain¶

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

cd InterpolateTerrain/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:

./InterpolateTerrain

WINDOWS USERS

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