title: vtk value at x,y,z point
body: <p>I have a <code>vtk</code> file which maps temperature in 3 dimensions. I would like to determine the temperature at a given x, y, z point. I will use the following code in order to load the <code>vtk</code> file (<a href="https://stackoverflow.com/questions/12300310/reading-vtk-file">Reading .vtk file</a>):</p>
  
  <pre><code>int main(int argc, char *argv[]) 
  {
    // simply set filename here (oh static joy)
    std::string inputFilename = "setYourPathToVtkFileHere";
  
    // Get all data from the file
    vtkSmartPointer&lt;vtkGenericDataObjectReader&gt; reader =
          vtkSmartPointer&lt;vtkGenericDataObjectReader&gt;::New();
    reader-&gt;SetFileName(inputFilename.c_str());
    reader-&gt;Update();
  
    // All of the standard data types can be checked and obtained like this:
    if (reader-&gt;IsFilePolyData()) 
    {
      std::cout &lt;&lt; "output is a polydata" &lt;&lt; std::endl;
      vtkPolyData* output = reader-&gt;GetPolyDataOutput();
      std::cout &lt;&lt; "output has " &lt;&lt; output-&gt;GetNumberOfPoints() &lt;&lt; "     points." &lt;&lt; std::endl;
    }
  
    return EXIT_SUCCESS;
  }
  </code></pre>
  
  <p>However, when searching through the extensive list of methods in the <code>vtk</code> library I cant find the appropriate function to extract a value at a specific location. Any suggestions?</p>