Wasn't it Carl Hoff who wrote:
>> I don't think that scaling ever slows down any ray tests. What
>> happens is that all the scaling, rotations and translations for an
>> object are combined during parsing into a single transformation
>> matrix. Applying the resulting matrix goes at the same speed
>> whatever type of transformation is being applied.
>
>Interesting... But is this case you are then fitting a scaled sphere
>inside a scaled sphere bounding shape and telling me its faster
>when its bound by a box.  What sort of shape would render
>faster being bound by a sphere if its not a sphere?  It doesn't
>sound like using a sphere as a bounding shape would be all that
>useful.  I assume I'm missing something.

Here's a rather artificial case where a bounding sphere is faster

#include "functions.inc"
camera { location  <0, 0, -4> look_at <0, 0, 0> angle 50}
light_source {<-100,200,-100> colour rgb 1}

#declare Thing =
isosurface {
  function { x*x + y*y + z*z - 1 + f_noise3d(x*10, y*10, z*10)*0.3}
        max_gradient 5
        contained_by{sphere{0,2}}
        pigment {rgb 1}
  bounded_by {box {-1,1}}        
  //bounded_by {sphere {0,1}}
}

object {Thing}

-- 
Mike Williams
Gentleman of Leisure

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