[vtkusers] Access violation clipping quadratic hex's + fix

[John Platt]

Hi,

 

Clipping an unstructured grid containing (theoretically) more than 8
quadratic hex's with a plane may fail with an access violation copying
the cell data. In practice, at least 24 cells are needed on Win XP and
probably many more for nix.

 

I think the problem is in vtkQuadraticHexahedron::Clip() with the call 

 

            this->Hex->Clip( , , , , , , this->CellData, cellId, , );

 

this->CellData stores 8 identical copies of the quadratic hex's cell
data for each subdivided linear hex but cellId has a maximum value of
the number of cells in the grid - 1. The access violation occurs in
vtkTetra::Clip() in the calls outCD->CopyData() where an attempt is made
to copy cell data for cellId.

 

I think the call should be 

 

            this->Hex->Clip( , , , , , , this->CellData, i, , );

 

where 'i' is the index of the linear hex.

 

BUT .

 

I don't understand why it is necessary to copy the cell data in the
first place. I would have thought that all subdivided cells have the
same cell data as the parent. In this case, dispense with this->CellData
and just use

 

            this->Hex->Clip( , , , , , , inCd, cellId, , );

 

Note that this is a problem in VTK 5.0 but I have patched
vtkQuadraticHexahedron to the latest CVS version and the problem still
repeats. A similar problem may also exist in
vtkQuadraticHexahedron::Contour().

 

John.

 

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