[vtkusers] Possible Bug in vtkPolyDataToImageStencil

David Gobbi dgobbi at atamai.com
Tue May 22 20:09:28 EDT 2007 

Hi Frank,

Sure thing.   Let me know how it works for you.

 - David



Frank Gao wrote:
> Hi David,
>  
> I am using vtkPolyDataToImageStencil, and saw an earlier email thread as below.
>  
> Most of the time I am getting good results but have same issue as Jesus. Would it be possible for you to send me a copy of another vtkPolyDataToImageStencil class you mentioned in your post?
>  
> Thanks in advance,
>  
> Frank Gao
>  
>  
>  
> Hi Jesus,
>  
> The vtkPolyDataToImageStencil does use a scanline approach, and the way 
> it works is that it uses vtkOBBTree to find out where the scanline 
> intersects the polydata.  Let's just say that this isn't the best way to 
> rasterize a polydata, and it doesn't work very well with sharp points.  
> I'm the original author of the class, so I take responsibility for its 
> less-than-robust implementation.
>  
> My company has another vtkPolyDataToImageStencil class that uses a 
> different algorithm, I will send it to you so that you can try it out.  
> It is definitely more robust than the current VTK one.
>  
>  - David
>  
>  
> Jesús Spí­nola wrote:
> >/ Hello,///
> >//
> >/ I'm using the vtkPolyDataToImageStencil class to cut parts of a ///
> >/ dataset. It works fine for several polygons but I have a case with a ///
> >/ star-shaped polygon with a strange result. I've attached several ///
> >/ images with the resuts ( http://hades.udg.es/~chus/images.tar.bz2 <http://hades.udg.es/%7Echus/images.tar.bz2> ///
> >/ <http://hades.udg.es/%7Echus/images.tar.bz2> )///
> >//
> >/ For example, when I make the stencil with an hexahedron the result is ///
> >/ ok ( see noStencilHexahedron.png and stencilHexahedron.png, that's ///
> >/ before cutting and after cutting with the stencil, respectively ).///
> >//
> >/ But when constructing another kind of polygon I get a strange result. ///
> >/ First I thought the coordinates I was giving were wrong, so I created ///
> >/ and actor with the same polydata to see if it was wrong, but the actor ///
> >/ is rendered as I expected, so the polydata is ok. You can see the ///
> >/ original image before cutting is noStencilstarShape.png. After cutting ///
> >/ with the stencil I get stencilOnlyStarShape.png. Then you can see the ///
> >/ polygon created for the stencil in this image ///
> >/ stencilWithPolyDataStarShape.png.///
> >//
> >/ The original dicom images i used for this examples are: ///
> >/ http://www.creatis.insa-lyon.fr/~jpr/PUBLIC/gdcm/gdcmSampleData/imgdisplay_testcases/dish_p05.dcm <http://www.creatis.insa-lyon.fr/%7Ejpr/PUBLIC/gdcm/gdcmSampleData/imgdisplay_testcases/dish_p05.dcm> ///
> >/ <http://www.creatis.insa-lyon.fr/%7Ejpr/PUBLIC/gdcm/gdcmSampleData/imgdisplay_testcases/dish_p05.dcm> ///
> >/ for the hexahedron and ///
> >/ http://www.creatis.insa-lyon.fr/~jpr/PUBLIC/gdcm/gdcmSampleData/imgdisplay_testcases/dish_p09.dcm <http://www.creatis.insa-lyon.fr/%7Ejpr/PUBLIC/gdcm/gdcmSampleData/imgdisplay_testcases/dish_p09.dcm> ///
> >/ <http://www.creatis.insa-lyon.fr/%7Ejpr/PUBLIC/gdcm/gdcmSampleData/imgdisplay_testcases/dish_p09.dcm> ///
> >/ for the star shape.///
> >//
> >/ The coordinates for the star shape ( refering to pixels coordinates, ///
> >/ where 1,1 is the first pixel ) are///
> >/ (133, 257)///
> >/ (199, 233)///
> >/ (169, 169)///
> >/ (233, 199)///
> >/ (257, 133)///
> >/ (281, 199)///
> >/ (345, 169)///
> >/ (315, 233)///
> >/ (381, 257)///
> >/ (315, 281)///
> >/ (345, 345)///
> >/ (281, 315)///
> >/ (257, 381)///
> >/ (233, 315)///
> >/ (169, 345)///
> >/ (199, 281)///
> >//
> >/ My code looks like this ( it's the same for the hexahedron and for the ///
> >/ star shape )///
> >//
> >/ int vertices = getNumberOfVerticesOfThePolygon();///
> >/ vtkPoints *points = vtkPoints::New();///
> >/ vtkCellArray *vertexs = vtkCellArray::New();///
> >/ vertexs->InsertNextCell( vertices + 1 );///
> >/ int polyX, polyY;///
> >/ double x, y;///
> >/ for( int i = 0; i < vertices; i++ )///
> >/ {///
> >/       getPolygonVertex( i, polyX, polyY ) )///
> >/      ///
> >/       // to transform the pixel coordinates to world points coordinates///
> >/       x = origin[0] + (polyX-1.0)*spacing[0];///
> >/       y = origin[1] + ( polyY-1.0)*spacing[1];///
> >//
> >/       points->InsertPoint( i , x, y, 0.0 );///
> >/       vertexs->InsertCellPoint( i );///
> >/ }///
> >/ // to close the polygon///
> >/ vertexs->InsertCellPoint( 0 );///
> >/ vtkPolyData *polyData = vtkPolyData::New();///
> >/ polyData->SetPoints( points );///
> >/ polyData->SetLines( vertexs );///
> >//
> >/ vtkLinearExtrusionFilter *extruder = vtkLinearExtrusionFilter::New();///
> >/ extruder->SetScaleFactor(1);///
> >/ extruder->SetExtrusionTypeToNormalExtrusion();///
> >/ extruder->SetVector( 0 , 0 , 1 );///
> >/ extruder->SetInput( polyData );///
> >//
> >/ m_polygonalPolyDataStencil = vtkPolyDataToImageStencil::New();///
> >/ m_polygonalPolyDataStencil->SetInput( extruder->GetOutput() );///
> >/ extruder->Delete();///
> >//
> >/ stencil = vtkImageStencil::New();///
> >/ stencil->SetInput( m_imageData ); // m_imageData is of type ///
> >/ vtkImageData, contains the original dicom dataset///
> >/ stencil->ReverseStencilOn();///
> >/ stencil->SetBackgroundValue( m_backgroundValue );///
> >/ stencil->SetStencil( m_rectangularPolyDataStencil->GetOutput() );///
> >/ ....///
> >//
> >/ m_imageViewer->SetInput( stencil->GetOutput() ); // m_imageViewer is ///
> >/ of type vtkImageViewer2///
> >//
> >/ I want to know if this is because of an error of my code or if it's ///
> >/ due to some bug in the implicated classes.///
> >//
> >/ As a side note, other workmates that are using this class had also ///
> >/ problems like this on the sharpen vertexs of the polygons. Maybe it's ///
> >/ because the algorithm is a kind of scanline method and when the same ///
> >/ pixel in the polygon is the begin and the end of the region that it ///
> >/ encloses the algorithm fails.///
> >//
> >/ Thanks in advance!///
> >/ ------------------------------------------------------------------------///
> >//
> >/ _______________________________________________///
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