[vtkusers] Cell data in a vtkPolyData object

[Sander Niemeijer]

Yes I have tried this.

If you do this when you've added your mixed typed cells in random order 
you will end up with a reordered list of cells. And if you add your 
cells in the appropriate order (vertex/line/poly/strip) then a 
BuildCells() will have no apparent effect.

If you look at the implementation of the vtkPolyData class you will see 
that vtkPolyData::InsertNextCell() performs the same 
this->Cells->InsertNextCell() calls that vtkPolyData::BuildCells() 
uses. So as long as you call your vtkPolyData::InsertNextCell() in the 
right order you will do the same as vtkPolyData::BuildCells().

Just for clarification, let me restate the problems I'm having:

1) Adding mixed typed cells in random order to a vtkPolyData object can 
have all kinds of nasty side effects because the ordering of cells can 
be changed while scalar data that you associate with these cells 
doesn't get reordered! According to John this is known behavior, but 
the fact that this kind of behavior is not documented anywhere, makes 
this a bug in my opinion (either the class documentation should make 
this behavior explicit, or the vtkPolyData class should allow adding 
cells in a mixed ordering and KEEP that ordering).

2) Even if you add cells in the right order to a vtkPolyData object you 
will still get problems when you use a scalar color lookup table with a 
vtkPolyDataMapper2D (see my other e-mail in this thread). As far as I 
can see this is due to the vtkOpenGLPolyDataMapper2D class handling the 
cell types in a different order. If this is indeed the case then even 
VTK internally does not keep to this fixed ordering of cell types in 
the vtkPolyData object.

So as far as I can see, there IS a problem. And given the complexity, I 
think it will take quite some time before we can see this problem fixed 
properly. The easiest solution now would probably be to 'fix' the 
vtkOpenGLPolyDataMapper2D class and to add the behavior about the 
ordering to the vtkPolyData class documentation.

Best regards,
Sander

On vrijdag, aug 27, 2004, at 14:42 Europe/Amsterdam, Clinton Stimpson 
wrote:

>
> Did you ever try calling vtkPolyData::BuildCells() after you modify the
> cells in the vtkCellArray's that a vtkPolyData references?
>
> Clint