Bug in vtkPolygon::Triangulate

[Will Schroeder]

Hi Horst-

Thanks for the feedback. I'll fix it.

Will

At 02:01 PM 8/3/99 +0000, you wrote:
>Hi!
>
>I already posted a similar message last week, but I think it did not
>reach the mailing list. Anyway, in the meantime I dug a bit deeper into
>the vtk code.
>
>My problem is that I observe that for some really simple polygons
>triangulation completely fails. The triangulation routine returns
>success, yet the list of triangles is empty.
>
>First of all, I think, that the statement in
>vtkPolygon::RecursiveTriangulate (vtkPolygon.c):
>
>    return 1;
>
>should be replaced with
>
>    return this->SuccessfulTriangulation;
>
>Otherwise in many cases the routine incorrectky returns 1 although the
>flag SuccessfulTriangulation is set to zero (i.e. triangulation failed).
>
>Unfortunately, this does not solve my problem, but at least gives me the
>correct return value (i.e no success). After some debugging, I think the
>real problem is somewhere in vtkPolygon::CanSplitLoop(). It seems to
>have problems when several vertices are on a straight line.
>
>Attached to the end of this message is a simple TCL example
>demonstrating the problem. The example is based on the triangulate.tcl
>code of examplesTCL in the vtk distribution using a simple 8 point
>polygon.
>No polygon will be rendered, yet if you increase the y coordinates of
>points 5 & 6 in the code by one, then triangulation suddenly work
>correctly.
>I tested this for vtk2.2 and vtk2.3 on Intel NT and on Alpha Tru64-Unix.
>Unfortunately, the nightly distributions are to large for me to download
>by modem.
>Does anyone know a workaround to this problem ?
>
>vtkPolygon.cxx says:
>// Triangulate polygon. Tries to use the fast triangulation technique
>// first, and if that doesn't work, uses more complex routine that is
>//  guaranteed to work
>However, I cannot find this more complex and reliable routine.
>
>Thanks,
>
>Horst.
>
>----------------------------------------------------------------------
>catch {load vtktcl}
>## Example tests the triangulation method in vtkPolygon
># get the interactor ui
>source ../../examplesTcl/vtkInt.tcl
>
>vtkPoints points
>  points InsertPoint 0 0 2 0
>  points InsertPoint 1 0 1 0
>  points InsertPoint 2 1 1 0
>  points InsertPoint 3 1 0 0
>  points InsertPoint 4 4 0 0
>  points InsertPoint 5 4 4 0
>  points InsertPoint 6 1 4 0
>  points InsertPoint 7 1 2 0
>
># if you change y from 4 to 5 in points 5 and 6,
>#   then polygon is triangulated correctly
>
>vtkCellArray polys
>     polys InsertNextCell 8;#number of points
>     polys InsertCellPoint 0
>     polys InsertCellPoint 1
>     polys InsertCellPoint 2
>     polys InsertCellPoint 3
>     polys InsertCellPoint 4
>     polys InsertCellPoint 5
>     polys InsertCellPoint 6
>     polys InsertCellPoint 7
>
>vtkPolyData polyData
>     polyData SetPoints points
>     polyData SetPolys polys
>
># triangulate the polygons
>vtkTriangleFilter tf
>     tf SetInput polyData
>
>vtkPolyDataMapper map
>     map SetInput [tf GetOutput]
>
>vtkActor tri
>     tri SetMapper map
>
># Create the RenderWindow, Renderer and both Actors
>#
>vtkRenderer ren1
>vtkRenderWindow renWin
>     renWin AddRenderer ren1
>vtkRenderWindowInteractor iren
>     iren SetRenderWindow renWin
>
># Add the actors to the renderer, set the background and size
>#
>ren1 AddActor tri
>ren1 SetBackground 0 0 0
>[ren1 GetActiveCamera] Dolly 1.4
>
>renWin SetSize 500 500
>iren Initialize
>
># render the image
>#
>iren SetUserMethod {wm deiconify .vtkInteract}
>
># prevent the tk window from showing up then start the event loop
>wm withdraw .
>
>
>
>
>
>
>
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