[vtkusers] Performance of vtkImplicitModeller

[Marie-Gabrielle Vallet]

The two algorithms included in the vtkImplicitModeller -- the PerCell and
the PerVoxel methods -- perform differently when the input geometry is
scaled.

For my test cases -- more than a thousand of cells -- the PerVoxel algorithm
is faster, as mentioned in the class documentation.

But what is strange is that the speed is greatly influenced by the scale of
the model : using the same data, the CPU time can be reduced by 2 (PerVoxel
: 0.16 sec. instead of 0.38 sec.) or even by 100 (PerCell : 0.32 sec.
instead of 37.11 sec! ) if the geometry and all parameters relative to a
length are scaled to the geometry diameter.

Does someone know how to use the PerVoxel method at its fastest, when
changing the geometry size is not an option ?

(some code lines and output follows)
---
Marie-Gabrielle Vallet

-------------------------------------------------------------------
   vtkPolyData *surf1 = vtkPolyData::New();
     surf1 = surfAlg->GetOutput();
     double* gridBounds = new double[6];
     surf1->ComputeBounds();
     surf1->GetBounds(gridBounds);
     double threshold = .1*(gridBounds[1]-gridBounds[0]);

  vtkImplicitModeller *imp = vtkImplicitModeller::New();
     imp->SetInputConnection( surfAlg->GetOutputPort(0) );
     imp->SetModelBounds( gridBounds );
     imp->SetSampleDimensions( dim );
     imp->SetMaximumDistance( threshold );
     imp->SetCapValue( 2*threshold );
     //     imp->SetProcessModeToPerVoxel ();

   vtkContourFilter *contour = vtkContourFilter::New();
     contour->SetInputConnection( imp->GetOutputPort() );
     contour->SetValue(0, threshold );

     timer->StartTimer();
     contour->Update();
     timer->StopTimer();
     cout << "ElapsedTime "<<timer->GetElapsedTime()<<"  ";
     cout << surf1->GetNumberOfCells() << "  "
          << (dim[0]-1)*(dim[1]-1)*(dim[2]-1) << "  "
          << threshold << "  "
          << imp->GetMaximumDistance()  << "  "
          << imp->GetProcessModeAsString() << " "
          << imp->GetCapValue() << " "
          << imp->GetLocatorMaxLevel() << "  "
          << endl;
--------------------------------------------------------------------
// Using the original geometry, diameter = 28.43
ElapsedTime 0.383428  4676  32760  1.548  1  PerVoxel 3.096 5
ElapsedTime 37.1108  4676  32760  1.548  1  PerCell 3.096 5

// Using the scaled geometry, diameter = 1
ElapsedTime 0.163597  4676  32760  0.0544493  0.0544493  PerVoxel 0.108899 5
ElapsedTime 0.322232  4676  32760  0.0544493  0.0544493  PerCell 0.108899 5
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