[Paraview] sources: why redundant number of points needed?

[Cory Quammen]

Bertwim,

You are right that there is a redundancy in points. That stems from
the need for defining different normals at the corner points for each
face. In theory, you could define more than one normal for the same
point and associate it with a face, but that complicates the data
structure. It is much simpler to duplicate points and associate
exactly one normal per point.

In most polygonal data, you won't see duplicated points at most
places. But for anything that should be displayed with a sharp edge,
duplicate points will be present.

- Cory

On Sat, Oct 11, 2014 at 10:23 AM, B.W.H. van Beest <bwvb at xs4all.nl> wrote:
> Hi,
>
> When I look into the source code of vtkCubeSource.cpp,
> I noticed that, although  a (rectangular) 3D box can be described by
> 8 3D points and 6 2D faces (hence also 6 3D normal vectors),
> in the actual code of vtkCubeBoxSource:RequestData() there are 24 3D
> points generated,
> with for every point a normal (???), and indeed 6 faces (polys).
> Why is there this redundancy?
>
> I bumped on this when I applied the "HiddenLineRemoval" representation to
> my own handcrafted box (with 8 points, 6 faces and 6 normals). It yielded
> an incorrect view whereas the box with the redundant numbers of data
> gave the correct one. But I don't get the logic.
>
> Regards,
> Bertwim
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