[vtkusers] bug in vtkOpenGLVolumeTextureMapper2D + suggestion

[alext at win.tue.nl]

  Hello all,

  I think I've discovered something that looks like a bug in the
texture-based volume renderer (vtkOpenGLVolumeTextureMapper2D).
Basically I tried to add a vtkVolume actor and then to set the transform of
this actor, e.g. by using SetUserMatrix(), to some simple rotation. 
Then I started to interactively rotate the camera, by using the trackball
interactor for example. The rendering looks ok from some angles, but TOTALLY
wrong from other angles!
  I have no clear idea what the problem is. However, I noticed that when the
vtkVolume actor has no user-matrix set, or when this user-matrix is NOT a
rotation, all seems to be ok - the volume rendering looks ok. 
  It seems to me that the problem is that the rotation of the camera and
that of the vtkVolume object seem to interfere when computing the major
viewing direction, or the way the texture-planes are stacked in 3D to
produce the volume rendering. When the vtkVolume is itself rotated, it seems
one can view it from 'bad' angles, i.e. angles from which one doesn't face
the 2D textures anymore but looks almost parallel at them. A very clear
example of the above is when the vtkVolume actor is rotated with e.g.
approximately 180 degrees around some axis, for example due to an:

  volume->RotateWXYZ(170,1,1,1)  //rotate volume around (1,1,1) axis with 170 degrees)

  It is hard to me to find an obvious solution to the above problem. First
of all, is this a problem experienced by other users? Is this something that
the VTK team knows about?

  Regards,

  Alex

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 Dr. Alexandru C. Telea      | Technische Universiteit Eindhoven
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