[vtkusers] Simple connectivity question

[Paul Cochrane]

Hi,

I'm trying to understand how vtk orders points when one comes to describing
the connectivity between them.  This is so that I can automatically generate
vtk xml files from an application being developed at my centre.  For
instance, if I want to describe an UnstructuredGrid of 4 points, and 1 cell,
with data defined on each point, and on the cell, one can come up with the
vtk xml file something like the following:

<?xml version="1.0"?>
<VTKFile type="UnstructuredGrid" version="0.1">
<UnstructuredGrid>
<Piece NumberOfPoints="4" NumberOfCells="1">
<PointData Scalars="scalars">
<DataArray Name="scalars" type="Float32" NumberOfComponents="1"
format="ascii">
    0.1 
    0.2   
    0.3 
    0.4 
</DataArray>
</PointData>
<Points>
<DataArray NumberOfComponents="2" type="Float32" format="ascii">
    0 0
    1 0     
    0 1
    1 1     
</DataArray>
</Points>   
<CellData Scalars="cell_scalars">
<DataArray Name="cell_scalars" type="Float32" NumberOfComponents="1"
format="ascii">  
    10  
</DataArray>
</CellData> 
<Cells> 
<DataArray Name="connectivity" type="Int32" format="ascii">
    0
    1
    2
    3
</DataArray>
<DataArray Name="offsets" type="Int32" format="ascii">
    4
</DataArray>
<DataArray Name="types" type="UInt8" format="ascii">
    9
</DataArray>
</Cells>
</Piece>
</UnstructuredGrid>
</VTKFile>

Unfortunately, this doesn't load into mayavi, and so leads me to believe
that I've stuffed something up.  I believe I've narrowed my problem down to
how I've described the connectivity within the Cells element (hence my
post).

My understanding from the way a VTK_QUAD object for example has its points
ordered, then one would think that the cell and points would be something
like this:

 3 _____ 2
   |   |
   |   |
 0 ----- 1

Where I've tried my best with ascii art the labels I would expect on the
object.

My question is then: what's wrong with having the connectivity written as
0 -> 1 -> 2 -> 3, (as I understand I've written it in the xml file above)?

Any help clearing this up would be greatly appreciated.

Thanks in advance,

Paul

-- 
Paul Cochrane
Computational Scientist/Software Developer
Earth Systems Science Computational Centre
University of Queensland
Brisbane
Queensland 4072
Australia

cochrane at esscc dot uq dot edu dot au