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.  if 
this is true, take the following scene:

camera
{
   location <3,12,-15>
   look_at  <0,0,0>
}

light_source
{
   <5,14,-10>
   color rgb <2,2,2>
   area_light
   <3,0,0>,<0,0,3>,5,5
   adaptive 1
   jitter
}

isosurface
{
   function{-y}
   threshold 0
   contained_by{sphere{0,5}}
   pigment
   {
      color rgb <1,1,1>
   }
}


and help me out here. 

1) Wouldnt just a plane show up?  y equals 0 on only one plane, doesnt 
it?
2) If y is negative, why is the positive part of the sphere shaded?


thanks a lot.

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