Direction of Projection

[David Gobbi]

On Wed, 3 May 2000, Ken Martin wrote:

> At 07:21 AM 5/3/00 -0400, you wrote:
> >Folks,
> >         I talked to Jim Miller about the Direction of Projection in the 
> > camera. His first reaction
> >was" DOP is only used for parallel projections." Sure enough, I checked 
> >Foley and vanDam (pg 231, top
> >paragraph).
> >
> >Should we revisit this?
> 
> Again I don't have Foley vanDam here.  But I don't see how this changes 
> anything. We don't use DOP in perspective projections, we use VPN, but... 
> -DOP still is not the same as VPN. If you want to keep using VPN that is 
> fine, but it is not the vector between the camera position (PRP or COP I 
> believe) and the focal point (VRP). Maybe what we want is a boolean 
> indicating (assume VPN is -DOP) which is was what the (3 years or so ago) 
> old code did (but without the boolean). But that is confusing because the 
> shears being introduced are there to handle the case where VPN != -DOP. 
> With the shear angles set, a call to GetVPN that returns -DOP will be 
> misleading.
> 
> - Ken

One easy solution is to remove SetDirectionOfProjection(), I don't think
it has any real value anyway.  GetDirectionOfProjection() would be left
in.

Having both GetViewPlaneNormal() or GetDirectionOfProjection() is a
good thing.  However, I think that it is best if the only way to
set the DirectionOfProjection is via SetPosition(), SetFocalPoint().
And that the only way to create a VPN which is not -DOP is by explicitly
creating a shear by setting the angle between the view plane and the
position->focalpoint vector.

 - David

P.S. I've committed a fix for vtkCamera, the Zoom was broken by a typo.
     Next time I'll test things a bit more before committing.  Does 
     either the vtkbaseline or the main vtk module have the facilities 
     for running all the regression test automatically, like the 
     dashboard does?