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

[Rasmus Reinhold Paulsen]

Hi Ken,

Thanks for the swift and informative response!

On Wed, 12 Mar 2003, 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.

Hail the mighty ARB! We obviously have a tendency to focus on our own data
and problems. I had not thought about the strips and polylines. These
indeed makes the original code more obscure. Luca is working on a new
implementation.

Currently vtkPolyData has a BuildLinks and BuildCells that builds internal
lookup tables allowing inquires about which cells using a given point etc.

Why not include a vtkEdgeTable in the vtkPolyData and add a method called
BuildEdges? Then letting the internal edgetable handle edge related
queries? For graph-like manipulation; like for eksample when calculating
surface geodesics, a method like GetPointNeighbourPoints that return a
list of points connected to a given point would be nice. The edgetable
could be used for that as well.

I know that another LUT will make it simpler to do topology-queries, but
will be a great pain when performing topology-changing operations on the
mesh.

Regards,
Rasmus