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

[David E DeMarle]

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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