[vtkusers] Near plane clipping problem

[Serge Lalonde]

Has anyone ever noticed that it is very easy to cause the near clipping 
plane to cut-off the model when in parallel projection mode?

I ran into this recently when adding a vtkBoxWidget and scaling a simple 
cube a few times after rotating it so that one of the corners points 
towards the viewer.

I tried calling vtkRenderer::ResetCameraClippingRange() to no avail. 
vtkRenderer::ResetCamera() fixed the problem, but would lose my camera 
setting.

After searching fruitlessly in the vtkusers archives, I decided to trace 
into the code and I discovered that there is code in 
vtkRenderer::ResetCameraClippingRange() and 
vtkCamera::SetClippingRange() that modifies the near plane value to 
"fix" potential camera and depth buffer problems. In my opinion, these 
fixes are valid, but only for perspective projections. For parallel 
projections, they are unnecessary and in fact cause the clipping problem 
I was encountering. To test this, I modified the code and rebuilt and lo 
and behold, no more near plane clipping!

I decided to document this in the vtkusers list in case others have run 
into this problem. Attached is my modified version of vtkCamera.cxx and 
vtkRenderer.cxx along with the patch files relative to version 5.2.0 
(only vtkCamera.cxx has changed since then). The changes are small and 
localized. If anyone tries them, let me know if you run into any 
problems (or even if you don't). I will submit them to Kitware for 
consideration also.

Thanks.
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