Ken <tyl### [at] pacbellnet> wrote:
: It is important to remember that a HF object is much like a sheet of
: paper. It is considered an infinately thin object.

  Well, strictly speaking this is not so.
  A triangle or a bicubic patch are infinitely thin objects (they have
nointerior). A plane is not considered an infinitely thin object since it
has an interior (a plane is considered by povray as an infinitely large object
which size is a half of the universe).
  In the same way a heighfield has an interior and thus it's not considered
an infinitely thin object, but a "solid" object. The limits of the object
go from 0 to 1 in the x and z directions and from -infinity to the surface
of the HF in the y direction.
  Of course the fact that a HF looks just like a thin surface is confusing,
but we have to remember that although povray only handles surfaces it has
a notion of "interior" and "exterior" of most objects (including the HF).
When an object has "interior" in terms of povray, it is considered "solid",
not just a thin surface without interior (like a triangle).
  The feature of an object having interior and exterior is very important
in CSG.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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