Wasn't it Matthew Pace who wrote:
>Last time I asked a isosurface question, it was a stupid error on my 
>part, and thats what I get for coding too late at nite...  This time, 
>though, I am really confused.  IF (IF) I understand isosurfaced 
>correctly here, (tell me if Im wrong), POV checks to see which parts of 
>the containing box fit into the function to equal the threshold. 
>
>
>1) Wouldnt just a plane show up?  y equals 0 on only one plane, doesnt 
>it?

Because you missed out the "open" keyword.

When the contained_by object is "closed", you see the parts of the
container that intersect with the inside of the isosurface. It's like
the difference between intersection and clipped_by in CSG.

So the shape you see is the intersection of the inside of the sphere and
the inside of the isosurface.

>2) If y is negative, why is the positive part of the sphere shaded?

The inside of an isosurface is considered to be the places where the
function has a negative value. (The function is positive outside and
zero on the surface). The logic is exactly the same as the following CSG
operation. The inside of the {-y,0} plane is in the positive y
direction.

intersection {
  plane {-y,0}
  sphere {0,5}
  pigment {rgb 1}
}

-- 
Mike Williams
Gentleman of Leisure

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