Andrew Coppin wrote:
> 
> 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 there is a big difference between a sphere with radius 0 and a
plane with a zero normal vector.  While the former is perfectly correct
(although quite useless in most situations) the latter is mathematically
undefined (like a cylinder with identical base and end point) and
therefore produces an error.

Christoph

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