Transforming Vectors and Tensors

[David Gobbi]

Hi All,

Is there out there who either uses vtkTransformPolyData() on vectors, 
or who simply uses vtkVectors for one thing or another?

The reason I am asking is that, currently, support for Vectors (and
for Tensors) in VTK is incomplete and using vtkVectors to represent
the wrong 'type' of vector can lead to problems with respect to 
the vtkTransform classes.

There are two distinctly different types of vectors that must be treated
in different ways, unless you are applying strictly rigid-body or
rigid-body plus a single scale factor transformations.  These are

- Contravariant Vectors e.g. tangents to surfaces or streamlines etc.
- Covariant Vectors e.g. normals to surfaces or gradients etc.

Likewise, there are four different kinds of 3x3 tensors which 
must all be transformed using different semantics.  I don't have
any strong desire to add support to vtkTransform for these.

Currently, the vtkTransform classes assume that all vtkVectors are
contravariant vectors, and treats them as such.  My intention is to
add a flag, e.g. vtkVectors::SetVectorTypeToCovariant(), to allow
the vtkTransform classes to recognize what type of vector they
are dealing with and behave accordingly.

Is there anyone out there who uses VTK to model vector fields who
would like to share an opinion?  I don't have any experience in
vector fields whatsoever, except for my undergrad courses (which
did cover some tensor algebra).

 - David
 
--
  David Gobbi, MSc                    dgobbi at irus.rri.on.ca
  Advanced Imaging Research Group
  Robarts Research Institute, University of Western Ontario

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