[vtkusers] Completing / extending ExtractTensorComponents

[Jose Paulo Moitinho de Almeida]

Hello 

In vtkExtractTensorComponents the code to compute the determinant is empty. 
One possible expression is:

 
      else //VTK_EXTRACT_EFFECTIVE_DETERMINANT
        {
        sx = tensor[0];
        sy = tensor[4];
        sz = tensor[8];
        txy = tensor[3];
        tyz = tensor[7];
        txz = tensor[6];
 
        s = sx*(sy*sz-tyz*tyz) + txy*(txz*tyz-txy*sz) + txz*(txy*tyz-sy*txz);
        }                                                                     

I have several questions regarding this expression:

a) It implies (as well as the expression used for the effective stress) that 
the tensor is symmetric. It is perfect for me, but perhaps it isn't for 
everyone. Is it preferable to use the more generic expression?

b) Why the name effective determinant? It was copied from effective stress or 
is there some function (a cubic root?) in mind?

c) I would like to have access to the eigenvalues within vtk. They can be 
inserted as scalars/vectors, but there is also the possibility of extracting 
a vector which has the three ordered eigenvalues. It would replicate the code 
for the scalars, ie the vectors can be the columns of the tensor (components) 
or the eigenvalues.

If it is ok I can try to prepare a patch.

Regards

Ze Paulo