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

[David Gobbi]

Why not use vtkTriangleFilter?  It can tesselate concave polygons.

   David


On Thu, May 6, 2010 at 9:06 AM, Xiaofeng Z <xf10036 at hotmail.com> wrote:
> 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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> ________________________________
> The New Busy think 9 to 5 is a cute idea. Combine multiple calendars with
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> _______________________________________________
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