[vtkusers] Updating scalars without recalculating vtkClipDataSet

[Mirek]

Hi,

I'm new to VTK and have a problem with updating scalar values on a clipped
mesh efficiently (without recalculating the whole mesh geometry). The
problem is described below and any advice is welcome. Thank you.

Mirek

// I have an Unstructured grid and array of values at mesh nodes
vtkSmartPointer<vtkUnstructuredGrid> pMyUGrid = ... my 3D unstructured grid  
vtkSmartPointer<vtkDoubleArray> pMyValues = ... my values at mesh nodes
vtkSmartPointer<vtkPlane> pMyPlane = ... my plane used for clipping pMyUGrid  

// Set pMyValues as Scalars in pMyUGrid 
pMyUGrid->GetPointData()->SetScalars(pMyValues);

// Prepare a clipper 
vtkSmartPointer<vtkClipDataSet> pClipper =
vtkSmartPointer<vtkClipDataSet>::New();
pClipper->SetClipFunction(pMyPlane); 
pClipper->SetInputData(pMyUGrid); 

// Surface filter (extracts surface mesh of pMyUGrid after clipping)
vtkSmartPointer<vtkDataSetSurfaceFilter> pSurfFilter =
vtkSmartPointer<vtkDataSetSurfaceFilter>::New();
pSurfFilter->SetInputConnection(pClipper->GetOutputPort());  

// Clip the 3D mesh and extract its surface mesh 
pSurfFilter->Update();
vtkSmartPointer<vtkPolyData> pSurfMesh = pSurfFilter->GetOutput();

// Get "MyValues" at nodes of the surface mesh (some values will be
interpolated by the clipper)
vtkDoubleArray* pMyValuesOnSurface =
vtkDoubleArray::SafeDownCast(pSurfMesh->GetPointData()->GetScalars());

// This all works fine. Now I need to change pMyValues and get new scalar
values on the clipped surface mesh 
vtkSmartPointer<vtkDoubleArray> pMyNewValues = ... some new values
pMyUGrid->GetPointData()->SetScalars(pMyNewValues);
pSurfFilter->Update();
pSurfMesh = pSurfFilter->GetOutput();
vtkDoubleArray* pMyNewValuesOnSurface =
vtkDoubleArray::SafeDownCast(pSurfMesh->GetPointData()->GetScalars());

This also works, however, it forces pClipper and pSurfFilter to recalculate 
everything (clipping and the surface mesh), which is time-consuming and
useless.  How to get updated pMyNewValuesOnSurface without recalculating 
pClipper and pSurfFilter? I was trying to use vtkMergeFilter but I was not
able to figure out how to use it correctly (I just started to learn VTK), so
it did not work. I also noticed the DATA_GEOMETRY_UNMODIFIED
InformationKeyMacro but I don't know how to use it. Is there any example
explaining how to correctly solve situations like this one? I'm using VTK
6.1.0. Thank you. 




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