> I thought that if you take any point inside the isosurface's 
container, replace
 > the x, y and z values in the function for the values of the point, 
and the
 > result of the operation is equal to the threshold, the isosurface 
exists in that
 > point...
 > Is that correct?

Yes, but in practice, POV-Ray can only find the point where the function 
is equal to zero by evaluating it at various points along a ray, and 
investigating where the function crosses from positive to negative. If 
POV-Ray finds that the function is equal to 1 at point A, and 2 at point 
B, it assumes that the function does not cross 0 between point A and B. 
You need to have negative values on the inside of the object, so that 
POV-Ray finds 1 at A and -2 at B, and investigates further between A and 
B to find the surface.

What are you trying to accomplish by using abs()?

You might find that if you use {abs(sqrt(x*x+z*z)-1) - .1}, thus forcing 
the function to be negative near the surface of the original sphere, 
that you'll get a hollow sphere with a thickness of .2.

  - Slime

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