>>It's especially odd because other non-viewable objects, such as cylinders
>>and spheres with radii of 0 or even negative numbers, don't even generate
>>warnings.  Perhaps its something with how the cylinder is processed
>>internally, that is, what is the function for a cylinder?  It might be
>>something that just chokes when the two points are the same.
> 
> I think that's the case. My theory is this. A cylinder with both points
> equal is a disc, or to put it another way, the intersection of a sphere
> and a plane. However, pov-ray doesn't (can't) know what the plane would
> be, thus it's ambiguous.

I'd just say skip
- meshes with no triangles
- spheres and cylinders with radii = 0
- cylinders with apex = base
- all the other non-viewable objects

simply generate a warning that the object has been ignored. In the case 
of the cylinder (apex=base) being a disc actually, I think, as this disc 
is not really defined (because it's normal can not be computed), it's as 
senseless as a sphere with radius 0.

Florian

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