[vtkusers] Bug in vtkStripper on a very simple case ?

[David Gobbi]

Then where would the "9" go?

The stripper uses a simple algorithm, it just chooses a starting
point, goes as far as it can in each direction to create the first
polyline, and then repeats this procedure until all the line segments
are accounted for.  It doesn't exhaustively search though all
permutations to see which one gives the smallest number of output
polylines... that would cause the algorithm to run many times slower.




On Sun, Dec 9, 2012 at 1:12 PM, pof <jd379252 at gmail.com> wrote:
> Hi David
>
> the first polyline being:
>
>   9 0 1 2 3 4 5 19 6 7
> and the third polyline (actually it is a segment) being:
>   2 18 0
> I don't see why they shoud not merge together as (unless I misunderstand
> something):
>   10 18 0 1 2 3 4 5 19 6 7
>
> Am I wrong?
>
>
> Le 09/12/2012 21:05, David Gobbi a écrit :
>
>> Hi JD,
>>
>> It could only merge those two lines if it joined them with a "T".
>> A polyline cannot have any branches.
>>
>>   - David
>>
>>
>> On Sun, Dec 9, 2012 at 12:16 PM, pof <jd379252 at gmail.com> wrote:
>>>
>>> Hi all,
>>>
>>> I get a strange result (which may be a bug) using vtk stripper, i.e. it
>>> does
>>> not merge LINES as polylines as it is expected to.
>>> In this example (see below an extract of the polydata produced by
>>> vtkStripper), only 2 polylines should be obtained, but vtkStripper forgot
>>> to
>>> merge the first and the third line together, though they obviously share
>>> point with Id=0 (btw, the fact that only Id 0 remains unmerged might not
>>> be
>>> pure luck).
>>>
>>> LINES 3 24
>>> 9 0 1 2 3 4 5 19 6 7
>>> 10 9 8 17 13 12 15 14 11 10 16
>>> 2 18 0
>>> Does anybody have an idea or an explanation?
>>> Thanks
>>> JD
>>>
>>> PS: Here below is the complete source file that generates these data and
>>> in
>>> which vtkStripper is used.
>>>
>>> //////////////////////////////////////////////////////
>>> // Example modified from IntersectionPolyDataFilter.cxx
>>> //////////////////////////////////////////////////////
>>> #include <vtkActor.h>
>>> #include <vtkIntersectionPolyDataFilter.h>
>>> #include <vtkPolyDataMapper.h>
>>> #include <vtkPolyDataWriter.h>
>>> #include <vtkProperty.h>
>>> #include <vtkRenderer.h>
>>> #include <vtkRenderWindow.h>
>>> #include <vtkRenderWindowInteractor.h>
>>> #include <vtkSmartPointer.h>
>>> #include <vtkSphereSource.h>
>>> #include <vtkCellArray.h>
>>> #include <vtkStripper.h>
>>>
>>> int main(int argc, char *argv[])
>>> {
>>>    vtkSmartPointer<vtkSphereSource> sphereSource1 =
>>> vtkSmartPointer<vtkSphereSource>::New();
>>>    sphereSource1->SetCenter(0.0, 0.0, 0.0);
>>>    sphereSource1->SetRadius(2.0f);
>>>    sphereSource1->SetPhiResolution(20);
>>>    sphereSource1->SetThetaResolution(20);
>>>    sphereSource1->Update();
>>>    vtkSmartPointer<vtkPolyDataMapper> sphere1Mapper =
>>> vtkSmartPointer<vtkPolyDataMapper>::New();
>>>    sphere1Mapper->SetInputConnection( sphereSource1->GetOutputPort() );
>>>    sphere1Mapper->ScalarVisibilityOff();
>>>    vtkSmartPointer<vtkActor> sphere1Actor =
>>> vtkSmartPointer<vtkActor>::New();
>>>    sphere1Actor->SetMapper( sphere1Mapper );
>>>    sphere1Actor->GetProperty()->SetOpacity(.3);
>>>    sphere1Actor->GetProperty()->SetColor(1,0,0);
>>>
>>>    /////////////////////////
>>>    // BEGIN MODIFICATIONS //
>>>    /////////////////////////
>>>    // Create a planar object
>>>    // Define 4 vertices
>>>    vtkSmartPointer<vtkPoints> PlanePoints =
>>> vtkSmartPointer<vtkPoints>::New();
>>>    vtkSmartPointer<vtkCellArray> PlaneCells =
>>> vtkSmartPointer<vtkCellArray>::New();
>>>    PlanePoints->InsertNextPoint(-3, -1, 0);
>>>    PlanePoints->InsertNextPoint(3, -1, 0);
>>>    PlanePoints->InsertNextPoint(-3, 1, 0);
>>>    PlanePoints->InsertNextPoint(3, 1, 0);
>>>    // Create 2 trianglar faces
>>>    PlaneCells->InsertNextCell(3);
>>>    PlaneCells->InsertCellPoint(0);
>>>    PlaneCells->InsertCellPoint(1);
>>>    PlaneCells->InsertCellPoint(2);
>>>    PlaneCells->InsertNextCell(3);
>>>    PlaneCells->InsertCellPoint(1);
>>>    PlaneCells->InsertCellPoint(3);
>>>    PlaneCells->InsertCellPoint(2);
>>>    // Create the polydata from points and faces
>>>    vtkSmartPointer<vtkPolyData> ProtectionWall =
>>> vtkSmartPointer<vtkPolyData>::New();
>>>    ProtectionWall->SetPoints(PlanePoints);
>>>    ProtectionWall->SetPolys(PlaneCells);
>>>    /////////////////////////
>>>    // END MODIFICATIONS //
>>>    /////////////////////////
>>>
>>>    vtkSmartPointer<vtkPolyDataMapper> planeMapper =
>>> vtkSmartPointer<vtkPolyDataMapper>::New();
>>>    planeMapper->SetInputConnection( ProtectionWall->GetProducerPort() );
>>>    planeMapper->ScalarVisibilityOff();
>>>
>>>    vtkSmartPointer<vtkActor> planeActor =
>>> vtkSmartPointer<vtkActor>::New();
>>>    planeActor->SetMapper( planeMapper );
>>>    planeActor->GetProperty()->SetOpacity(.3);
>>>    planeActor->GetProperty()->SetColor(0,1,0);
>>>
>>>    // Get intersection segments (LINES) using the
>>> vtkIntersectionPolyDataFilter
>>>    vtkSmartPointer<vtkIntersectionPolyDataFilter>
>>> intersectionPolyDataFilter
>>> = vtkSmartPointer<vtkIntersectionPolyDataFilter>::New();
>>>    intersectionPolyDataFilter->SplitFirstOutputOff();
>>>    intersectionPolyDataFilter->SplitSecondOutputOff();
>>>    intersectionPolyDataFilter->SetInputConnection( 0,
>>> sphereSource1->GetOutputPort() );
>>>    intersectionPolyDataFilter->SetInputConnection( 1,
>>> ProtectionWall->GetProducerPort() );
>>>    intersectionPolyDataFilter->Update();
>>>
>>>    /////////////////////////
>>>    // BEGIN MODIFICATIONS //
>>>    /////////////////////////
>>>    // Try to get the polylines (there should be 2) from the segments
>>> (LINES)
>>>    vtkSmartPointer<vtkStripper> stripper =
>>> vtkSmartPointer<vtkStripper>::New();
>>>
>>> stripper->SetInputConnection(intersectionPolyDataFilter->GetOutputPort());
>>>    stripper->Update();
>>>    // Save resulting polydata on a file, in which we see that there are 3
>>> polylines (actually the second polyline is ok,
>>>    // but the first and third polylines should have been further merged
>>>    vtkSmartPointer<vtkPolyDataWriter> wr =
>>> vtkSmartPointer<vtkPolyDataWriter>::New();
>>>    wr->SetFileName("polylines.txt");
>>>    wr->SetInputConnection(stripper->GetOutputPort());
>>>    wr->Write();
>>>    ///////////////////////
>>>    // END MODIFICATIONS //
>>>    ///////////////////////
>>>
>>>    vtkSmartPointer<vtkPolyDataMapper> intersectionMapper =
>>> vtkSmartPointer<vtkPolyDataMapper>::New();
>>>    intersectionMapper->SetInputConnection( stripper->GetOutputPort() );
>>>    intersectionMapper->ScalarVisibilityOff();
>>>
>>>    vtkSmartPointer<vtkActor> intersectionActor =
>>> vtkSmartPointer<vtkActor>::New();
>>>    intersectionActor->SetMapper( intersectionMapper );
>>>
>>>    vtkSmartPointer<vtkRenderer> renderer =
>>> vtkSmartPointer<vtkRenderer>::New();
>>>    renderer->AddViewProp(sphere1Actor);
>>>    renderer->AddViewProp(planeActor);
>>>    renderer->AddViewProp(intersectionActor);
>>>
>>>    vtkSmartPointer<vtkRenderWindow> renderWindow =
>>> vtkSmartPointer<vtkRenderWindow>::New();
>>>    renderWindow->AddRenderer( renderer );
>>>
>>>    vtkSmartPointer<vtkRenderWindowInteractor> renWinInteractor =
>>>      vtkSmartPointer<vtkRenderWindowInteractor>::New();
>>>    renWinInteractor->SetRenderWindow( renderWindow );
>>>
>>>    renderWindow->Render();
>>>    renderWindow->SetSize(600, 600);
>>>    renderer->SetBackground(0.1, 0.2, 0.4);
>>>    renWinInteractor->Start();
>>>
>>>    return EXIT_SUCCESS;
>>> }
>
>