Wasn't it Ken who wrote:
>
>
>Jettero Heller wrote:
>
>> I have a very round object made up of the intersection/union of
>> several toruses and spheres.  I wish to find it's maximum
>> diameter in the XZ plane...
>
>max_extent and min_extent should get you close -
>http://www.povray.org/documentation/view/138/

That only gives you the size of the bounding box that POV generates. In
the case of intersections, I believe that POV uses the intersection of
the bounding boxes of the components. This can be a very loose fit in
some circumstances. 

For example, consider this:

camera {location  <0.45,2,0.45> look_at <0.45,0,0.45>}
background {rgb 1}
light_source {<-30, 100, -30> color rgb 1}

#declare THING = 
intersection {
  torus {1,0.22 translate <0.9,0,0>}
  torus {1,0.22 translate <0,0,0.9>}
  torus {1,0.22}
  pigment {rgb 1}
}

object {THING}

box {min_extent(THING),max_extent(THING)
  pigment {rgbt <1,0,0,0.9>}
}

The pink box shows the min_extent and max_extent size, but the actual
intersection object is very much smaller. 

It would be tricky to use trace() on this object unless you already had
a clue about where the tori intersect. If you just trace() rays through
the centre of the bounding box, then most such rays miss the object
completely.

I tried writing some code that did a horizontal scan, using trace to
find the greatest diameter in the x direction, then rotated around the
centre point of that diameter, tracing for the greatest diameter in any
direction. In this particular case, such a scan miserably fails to find
the overall maximum diameter.

-- 
Mike Williams
Gentleman of Leisure

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