I'm getting some unexpected (at least by me) results using the "clipped_by"
operator in POVray.

Specifically, when I clip triangle mesh objects, I get empty spaces where
I believe I should be getting objects.  It is as if the ray is terminated
by the clipped off part of the object and background color is rendered.

For instance, I rendered a simple open-ended cylinder using an OpenGL-based
application (screen grab at
ftp://ncmir.ucsd.edu/outgoing/spl/sv_cylinder.gif)
and with POVray (ftp://ncmir.ucsd.edu/outgoing/spl/pov_cylinder.gif).

Then I moved in the front clipping plane and rendered again,

 ftp://ncmir.ucsd.edu/outgoing/spl/sv_clipped_cylinder.gif

and

 ftp://ncmir.ucsd.edu/outgoing/spl/pov_clipped_cylinder.gif

show the OpenGL and POVray results, respectively.

The POVray scripts and the mesh file may be found at

 ftp://ncmir.ucsd.edu/outgoing/spl/pov_cylinder

 ftp://ncmir.ucsd.edu/outgoing/spl/pov_clipped_cylinder

and

 ftp://ncmir.ucsd.edu/outgoing/spl/cyl_synu.pov

The files are a bit ugly since they're machine generated but should be
fairly self explanatory.  The "clipped_by" operators are invoked at about
line 536 in both files.  The __far_clip is outside the bounding box of the
object.

Does "clipped_by" only work with non-mesh type objects?

Any clues or RTFMs welcome.

       spl

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