Wasn't it Andrew C on Mozilla who wrote:
>>>I was wondering... if I (say) take the square root, that makes the high 
>>>values less high, while leaving zero unchanged. (I'm assuming 
>>>threshold=0 here.) Would that make the thing go any faster? Would the 
>>>increase in speed overcome the extra computation needed for the sqrt() call?
>>>
>>>Andrew @ home.
>> 
>> 
>> In the general case, strange things happen when you sqrt() an isosurface
>> function that has threshold=0. That's because POV doesn't like the
>> sqrt() of negative numbers.
>
>Ah... well the function I want POV-Ray to draw *never* goes negative...
>
>Actually, maybe POV-Ray doesn't like that fact in itself! :-S

If the threshold is zero, then the surface that gets rendered is the set
of points where the value changes from positive to negative. I don't
think that POV can find a zero surface that's positive on both sides.
(If it did find such a surface, how would it know which was the inside?)

>> As the POV solver goes looking for a point where a ray intersects an
>> isosurface, it traps the point in an interval where the function
>> evaluates positive on one side and negative on the other, and
>> recursively narrows the interval until it has located the point with the
>> required accuracy.
>
>I was wondering... is there a document anywhere that explains EXACTLY 
>how POV-Ray renders isosurfaces?

As far as I know, only the source code.


-- 
Mike Williams
Gentleman of Leisure

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