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

Tue May 11 19:50:51 EDT 2010

Hi David,

Thanks for your suggestion.  I tried vtkCleanPolyData but it does not make markable improvements.

>From some example I saw of OpenGL tessellation, it appears the process of tessellation and rendering must be interwoven.  I wonder if that is to ensure no pixel overlapping in the neibghouring triangles.

Xiaofeng

> From: david.gobbi at gmail.com
> Date: Tue, 11 May 2010 17:05:20 -0600
> Subject: Re: [vtkusers] Bug or Design: Rendering concave polygons with vtkActor2D
> To: xf10036 at hotmail.com
> CC: dave.demarle at kitware.com; vtkusers at vtk.org
> 
> Hi Xiaofeng,
> 
> These are because vtkPolygon::Triangulate() has trouble when points
> are nearly coincident or when points are co-linear. I had to deal
> with this when I was writing vtkClipClosedSurface and almost decided
> to write my own triangulation code... until I found out how hard it is
> to write robust triangulation code.
> 
> The easiest thing is to use vtkCleanPolyData to merge points that are
> very close together. You'll probably still see the triangulation fail
> sometimes, but it should fail less often.
> 
> David
> 
> 
> On Tue, May 11, 2010 at 4:39 PM, Xiaofeng Z <xf10036 at hotmail.com> wrote:
> > Hello,
> >
> > As suggested by some, I tried vtkTriangleFilter to tessellate concave
> > polygons.  It almost works, except some bright pixels appearing within the
> > polygon forming streak like lines as well as some at the boundary.  Attached
> > is a typical example of such a streak.  This streaks appear or disappear
> > as the magnification changes.  I think they come from overlapped pixels from
> > neighouring triangles.  I need suggestions in how to remove them.
> >
> > Thanks!
> >
> > Xiaofeng
> >
> >
> >
> >> From: dave.demarle at kitware.com
> >> Date: Thu, 6 May 2010 11:15:23 -0400
> >> Subject: Re: [vtkusers] Bug or Design: Rendering concave polygons with
> >> vtkActor2D
> >> To: xf10036 at hotmail.com
> >> CC: vtkusers at vtk.org
> >>
> >> Try vtkTriangleFilter.
> >>
> >> If there is a better alternative hopefully someone on list with more
> >> mesh experience than I will fill in the details.
> >>
> >> David E DeMarle
> >> Kitware, Inc.
> >> R&D Engineer
> >> 28 Corporate Drive
> >> Clifton Park, NY 12065-8662
> >> Phone: 518-371-3971 x109
> >>
> >>
> >>
> >> On Thu, May 6, 2010 at 11:06 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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