[vtk-developers] Rationale for vtkWedge point order

[Mark Olesen]

I ran into this (again). 

For the majority of the primitive 3d cell shapes (vtkTetra, vtkPyramid,
vtkHexahedron, vtkHexagonalPrism, vtkPentagonalPrism), the base forms a
face whose normal points in the direction of the next face (or point, in
the case of vtkTetra).

But vtkWedge is defined the opposite way around - the face normals point
in opposite directions!!

This is presumably historical and probably couldn't be changed without
breaking a huge amount of code, but what is the rationale for it?

Or is actually immaterial if the specified node order results in cells
with a negative volume?

The reason for my question is not only that I stumbled on this again,
but also that other VTK readers seem to be using vtkWedge while assuming
a different point order.

For example, according to the EnSight documents, their "penta6" is
defined with a base (0,1,2) that points towards the opposite face
(3,4,5). The rest of the EnSight shapes are consistent with the node
order used by VTK. Doesn't this cause inverted cells?


/mark


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