"JEofVA" <jce### [at] attglobalnet> wrote in message 
news:web.486ada99de595f198a187d850@news.povray.org...
> "Chris B" <nom### [at] nomailcom> wrote:
>> "JEofVA" <jce### [at] attglobalnet> wrote in message
> I guess I'm not fully understanding
> this coincident surface thing.
>

Yes. It's one of those things where, if you read enough different 
explanations it can suddenly all make sense.

If you have two objects (or in this case two components of the 
'NestedSpheres' object) where some of the overlapping surfaces align 
perfectly, then, when a ray that POV-Ray traces out from the camera hits 
this overlapping surface it sometimes hits one of them first and sometimes 
the other, giving an unpredictable mixture of their colours. In this 
instance the box shaped cutout caused the exposed internal surfaces of your 
spheres to align perfectly. Indeed the entire cutout surface of the 
innermost sphere aligned with the cut surfaces of all of the other 3 
spheres.

In real life this wouldn't happen because you'd have to carve a hollow out 
from inside the first sphere to be able to get the second one in, so by the 
time you sliced a corner out you'd only be cutting down through one of the 
two spheres at a time. Otherwise it's as if the crust could continue all the 
way to the centre of the earth with the inner layers also occupying the same 
3D space which would be impossible.

By moving the corner of the box that's doing the cutting slightly when 
cutting away the different spheres, you avoid the coincident surfaces. In 
this instance you need to move it away from the origin towards the opposite 
corner of the box as you cut into the progressively smaller spheres so that 
the cut surfaces of the smaller spheres stand proud of the cut surfaces of 
all larger spheres. If you expand the corner in the opposite direction you'd 
also avoid coincident surfaces, but the cut surfaces of the smaller spheres 
would be burried inside the larger one so you'd only see the crust.

I hope that helps, otherwise a google on "povray coincident surfaces": 
http://www.google.com/search?q=povray+coincident+surfaces gives links to 
various slightly different explanations and discussions that may help.

Regards,
Chris B.

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