Picky mathematical points with vtkCamera

[Ken Martin]

>Using vtkCamera::SetWindowCenter() in conjunction with
>ParallelProjection.  The code used to shear the view volume,
>I changed it so that it translates the view volume instead.
>This seemed much more in line with what happens when
>SetWindowCenter is used with with perspective views.
>
>However, I think that SetWindowCenter was originally meant
>as a counter-intuitive (IMHO) method for creating oblique
>projections.  So what I thought was an error in the VTK code
>probably wasn't.
>
>So, any opinions?  Should the result of using
>vtkCamera::SetWindowCenter() with a parallel projection
>be to shift the view volume, or to shear it?

I never considered WindowCenter with a parallel projection although it is 
perfectly reasonable. It should not perform a shear with a parallel 
projection, only perspective. A common application of window center is for 
rendering to multiple screens such as a power wall. In such cases a 
parallel projection should result in a translation. For perspective, it is 
a shear and an off axis projection.  Window center is essentially the same 
as CW in Foley vanDam. The intent of WindowCenter is to support off axis 
projections required for proper stereo rendering (with a shared view 
plane), power wall, etc.

  - Ken

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