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Here's an interesting one then... how about a zero-radius sphere? If
you ask for a plane with normal <0, 0, 0> you get an error...
Shouldn't a zero-radius sphere do the same? (It doesn't.)

Actually... (Oh God here we go!) A zero-radius sphere is kinda useful
in a way; it produces an object such that all of space is 'outside'.
If you add the inverse keyword, you now have an object such that all
of space in 'inside'. I'm currently working on a 4D visualisation
system written with POV-Ray, and these 'all-inside' and 'all-outside'
objects are usefull for CSG intersection operations. (In particular,
an object ax + by + cd + dw - k = 0 only has a surface in the 3-space
defined by w = m if <a, b, c> != <0, 0, 0>. Otherwise, if dm - k > 0,
all of 3-space is 'outside', or if dm - k < 0 then all of 3-space is
'inside'. If you want to build a hypercube out of the CSG
intersection of 8 hyperplanes, having a macro which can return
'all-inside' or 'all-outside' objects is rather useful ;-)

OK, if you don't follow that last part... don't worry!

Andrew.

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