[vtkusers] bug in vtkVolume's user matrix vs zbuffer resolution

[alext at win.tue.nl]

  Hello all,

  I have stumbled on a very unexpected behavior of the vtkVolume's transform
with respect to the vtkVolumeRayCastMapper. Basically, I took one of VTK's
example (e.g. volProt) and added a user-matrix to the vtkVolume that did
nothing but scaled the volume uniformly to e.g. 0.1 times the initial size. 
Then I used the vtkVolumeRayCastMapper to volume render the volume.
Surprisingly, the z resolution of the image was extremely bad. The noticed
effect is very similar to subsampling the volume along the rays or setting
the camera's clipping range incorrectly such that not all the zbuffer's
precision is used. 
  It looks to me that this is a bug in VTK. I tried other transforms on the
vtkVolume, such as rotations and translations, and all went ok. But scaling
the volume, namely with a scale factor < 1, seems to destroy the sampling
precision along the rays of the raycaster.

  I thought first that I had to adjust the SampleDistance member of the 
vtkVolumeRayCastMapper, i.e. to make it smaller because the volume is now
smaller. However, it didn't help mcuh to improve the rendering quality -
moreover, the vtkVolumeRayCastMapper.h file states that this distance is in
volume coordinates, so it should be insensitive to scaling of the volume.
Then I thought I should adjust the camera clipping range differently.
However, when printing the bounding-box based computation of the camera
clipping range of vtkRenderer::ResetCameraClippingRange(), I saw that the
clipping range is correctly computed to take into account the smaller
volume.

  Has anyone notices these rendering artifacts when scaling a vtkVolume with
a factor < 1? Is there any way to cope with such volumes? I can't increase
the scaling factor of vtkVolume really, since my application passes it
together with other transform info in a SetUserMatrix() call - so I have to
stick with a sclaed vtkVolume.

  Any help is very welcome,

  Alex Telea

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 Dr. Alexandru C. Telea      | Technische Universiteit Eindhoven
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 E-mail   : alext at win.tue.nl | Dept. of Mathematics and 
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