[vtkusers] Bug or Design: Rendering concave polygons with vtkActor2D

[Xiaofeng Z]

David,

 

I am thinking of using GLU to perform Tessellation.  I wonder how involved it would be if I try to do it inside VTK.

 

I would appreciate it if you can provide some insight.

 

Thanks! 

Xiaofeng



 
> From: dave.demarle at kitware.com
> Date: Wed, 5 May 2010 07:51:40 -0400
> Subject: Re: [vtkusers] Bug or Design: Rendering concave polygons with vtkActor2D
> To: xf10036 at hotmail.com
> CC: vtkusers at vtk.org
> 
> I suspect that is because OpenGL doesn't render concave polygons
> properly, thus VTK, which uses GL to render, doesn't either. So
> unfortunately you have to tesselate that into a set convex polygons in
> order to have it drawn properly.
> 
> David E DeMarle
> Kitware, Inc.
> R&D Engineer
> 28 Corporate Drive
> Clifton Park, NY 12065-8662
> Phone: 518-371-3971 x109
> 
> 
> 
> On Tue, May 4, 2010 at 11:38 AM, Xiaofeng Z <xf10036 at hotmail.com> wrote:
> > Concave polygons are not rendered as expected with vtkPolyDataMapper and
> > vtkActor2D (see below).  Is it by design or is it a bug?  If it's by design,
> > is there any flag can be set to alter the rendering behaviour?
> >
> > Thanks!
> >
> > Xiaofeng
> >
> > Hi all,
> >
> > How do I make uniform transparency in a concave polygon, instead of getting
> > multiple shades, e.g. in the attached graph?
> >
> > The polygon are created with:
> >
> > vtkPoints* points = vtkPoints::New();
> > vtkCellArray* cells = vtkCellArray::New();
> > points->SetNumberOfPoints(4);
> >
> > // create a concave polygon (equilateral with a cutout at the bottom)
> >
> > points->SetPoint(0, 10.0, 10.0, 0.0);
> > points->SetPoint(1, 110.0, 183.2, 0.0);
> > points->SetPoint(2, 210.0, 10.0, 0.0);
> > points->SetPoint(3, 110.0, 110.0, 0.0);
> > cells->Allocate(1, 5);
> > cells->InsertNextCell(5);
> > cells->InsertCellPoint(0);
> > cells->InsertCellPoint(1);
> > cells->InsertCellPoint(2);
> > cells->InsertCellPoint(3);
> > cells->InsertCellPoint(0);
> > vtkPolyData* poly = vtkPolyData::New();
> > poly->Initialize();
> > poly->SetGlobalWarningDisplay(1);
> > poly->SetPoints(points);
> > poly->SetPolys(cells);
> >
> > A vtkActor2D and a vtkPolyDataMapper2D was used for the rendering.
> >
> > Many thanks!
> > Xiaofeng
> >
> >
> >
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