[vtk-developers] Should vtkClipPolyData keep the normals?

[Berk Geveci]

Hi Mathieu,

Can you provide a full example that I can compile and debug? The
normals should probably not be dropped. It sounds like a bug...

-berk


On Wed, Sep 10, 2008 at 9:47 AM, Mathieu Coursolle
<mcoursolle at rogue-research.com> wrote:
> Hi,
> Here is a more detailed description of the issue:
> I loaded 42400-IDGH.stl from the VTKData folder using vtkSTLReader.
> I then use the following code to compute the normals, as none are read:
> vtkSmartPointer<vtkPolyDataNormals> normalsFilter =
> vtkSmartPointer<vtkPolyDataNormals>::New();
> normalsFilter->SetInput(myPolyData);
> normalsFilter->SetFeatureAngle(60.0);
> normalsFilter->Update();
> myPolyData = normalsFilter->GetOutput();
> Then, if I display this polydata, the result looks just fine (see
> attachment). However, if I apply the vtkClipPolyData filter
> using an implicit function that includes the entire polydata, the normals
> are lost:
> vtkSmartPointer<vtkClipPolyData> clipper =
> vtkSmartPointer<vtkClipPolyData>::New();
> clipper->SetInput(myPolyData);
> clipper->SetClipFunction(aBox);
> clipper->SetGenerateClippedOutput(0);
> clipper->SetInsideOut(1);
> clipper->Update();
> myPolyData = clipper->GetOutput();
> Would it be easy to change the filter to add an option to keep the normals,
> and to compute the missing ones?
> Sure I can call the normals filter after the clip filter, but some of my
> data have specific normals I need to keep, and
> calling a normals filter after the clipping modifies them. Since I have no
> knowledge on how those normals were created,
> I need to keep them for the untouched cells.
> See attachment on the effect of the clipping process.
> Thanks!
> Mathieu
> On 9-Sep-08, at 4:44 PM, Berk Geveci wrote:
>
> Something is fishy there. The line you added is copying the point data
> onto itself.
>
>  vtkPointData *inPD=input->GetPointData(), *outPD = output->GetPointData();
>  ....
>  output->GetPointData()->PassData(outPD);
>
> That PassData() should not do anything at best and may corrupt the
> output data at worst. Can you post a small example demonstrating the
> issue?
>
> -berk
>
>
> 2008/9/9 Mathieu Coursolle <mcoursolle at rogue-research.com>:
>
> Hi VTK developers,
>
> I am using vtkClipPolyData to clip a vtkPolyData with an implicit function
>
> (vtkBox).
>
> The origin vtkPolyData has specific normals I want to keep for the resulting
>
> polydata.
>
> Unfortunately, the normals are not the same in the output polydata.
>
> I looked at the code, and I have a question regarding the creation of the
>
> output polydata:
>
> For the Clipped Output, the PassData() method and the Squeeze() method are
>
> called one
>
> the points, lines, polys, strips are set. This is not true for the standard
>
> output. Is there a reason?
>
> Shouldn't the code be:
>
>  if (newVerts->GetNumberOfCells())
>
>    {
>
>    output->SetVerts(newVerts);
>
>    }
>
>  newVerts->Delete();
>
>  if (newLines->GetNumberOfCells())
>
>    {
>
>    output->SetLines(newLines);
>
>    }
>
>  newLines->Delete();
>
>  if (newPolys->GetNumberOfCells())
>
>    {
>
>    output->SetPolys(newPolys);
>
>    }
>
>  newPolys->Delete();
>
>
>
>  output->GetPointData()->PassData(outPD);      <<<<<<<<<< ADDED CODE
>
> <<<<<<<<<<<<
>
>  output->Squeeze();                            <<<<<<<<<< ADDED CODE
>
> <<<<<<<<<<<<
>
>  if ( this->GenerateClippedOutput )
>
>    {
>
>    this->GetClippedOutput()->SetPoints(newPoints);
>
>    if (clippedVerts->GetNumberOfCells())
>
>      {
>
>      this->GetClippedOutput()->SetVerts(clippedVerts);
>
>      }
>
>    clippedVerts->Delete();
>
>    if (clippedLines->GetNumberOfCells())
>
>      {
>
>      this->GetClippedOutput()->SetLines(clippedLines);
>
>      }
>
>    clippedLines->Delete();
>
>    if (clippedPolys->GetNumberOfCells())
>
>      {
>
>      this->GetClippedOutput()->SetPolys(clippedPolys);
>
>      }
>
>    clippedPolys->Delete();
>
>
>
>    this->GetClippedOutput()->GetPointData()->PassData(outPD);
>
>    this->GetClippedOutput()->Squeeze();
>
>    }
>
> It seems to work alright with that fix, but I am not too sure of what it is
>
> doing...
>
> Thanks!
>
> Mathieu
>
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