[vtk-developers] ARB-Request: Blessing of vtkPolyData->IsEdge fix

[Will Schroeder]

A historical note and problems with suggested code:

According to documentation (VTK Textbook, etc.), by definition PolyVertex 
and PolyLine have no edges (1-dimensional edge is only found on cells 2D 
and higher). Edges are defined as 1-D BOUNDARY entities to cells. For 
example, the vtkLine::GetNumberOfEdges() returns 0, as does vtkPolyVertex 
and vtkPolyLine. Hence the IsEdge method should always return false when 
any type of cell dimension 1 or less is involved. If the meaning of what an 
edge is changed, then these other methods must be changed for consistency. 
I would not recommend going down this snake hole.

The reason the code was written this way was for speed. It is used heavily 
in inner loops for obtaining boundary information and assumes a lot about 
how it is used. I'm betting the new implementation is much slower although 
I have not tested it. I think the problem is that the intended meaning of 
the method has been interpreted differently. One meaning is "do these two 
points form an edge"; the other meaning is "given two points that form an 
edge on one cell, is it an edge in another cell?" Also I suspect that the 
code was originally written for triangle meshes (where it works just fine) 
and evolved into something more general.

Here's what I think should be done:

1. All cells of dimension 1 or less should return false.
2. The separate cases for the various types are good (triangle strip and 
default), however,
3. I would create two new cases, one for VTK_TRIANGLE (the old code works 
just fine for a triangle, no modulo required) and another for VTK_QUAD. 
Adding these two cases should regain most of the old performance.

Will

At 09:16 AM 3/12/2003 -0500, Ken Martin wrote:
>I think there are a couple problems with the suggested code.
>Specifically three cases come to mind that I think will break it.
>
>1) For a triangle strip with points p0, p1, p2, p3 ... the points p0 and
>p2 do form an edge but would not be correctly reported by the code
>below.
>
>2) for a poly vertex there are no edges but the function could say that
>there was one
>
>3) for poly lines if someone checked the first and last point of a poly
>line (in either order) it would match.
>
>And a minor formatting issue in that the "if" statements do not have {}
>under them. Having said that, the original code was clearly screwed up.
>It looks like a valid solution will have to know what type of cell is
>being considered. Perhaps a switch (this->GetCellType(cell[i])) with the
>current code in the default case but special cases for Tstrips,
>polyvert, and polyline. The whole thing should probably be moved into
>the cxx file as well, getting a bit long and complicated for an inline.
>
>Thanks
>Ken
>
> > Luca Antiga and I would like to apply the following fix to
> > vtkPolyData->IsEdge :
> >
> > inline int vtkPolyData::IsEdge(int p1, int p2)
> > {
> >   unsigned short int ncells;
> >   int npts;
> >   int i, j;
> >   vtkIdType *cells, *pts;
> >
> >   this->GetPointCells(p1,ncells,cells);
> >   for (i=0; i<ncells; i++)
> >   {
> >     this->GetCellPoints(cells[i],npts,pts);
> >     for (j=0; j<npts; j++)
> >     {
> >       if (p1==pts[j])
> >         if ((pts[(j-1+npts)%npts]==p2)||(pts[(j+1)%npts]==p2))
> >           return 1;
> >     }
> >   }
> >
> >   return 0;
> > }
>
>
>
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