Remco de Korte wrote:

> >     You might also try a difference instead of an
> > intersection.
> >
> > Ken Tyler
> >
> I'm a relative newcomer with POV but I've had some experiences with CSG
> and found that especially difference took a very long time to render. I
> didn't expect that, nor did I expect blobs to be quite fast in
> comparison. I must add that I'm working on some images using differences
> with non-primitive forms. I guess that makes it even more complicated.
>
> Is there any 'rule of thumb' in this?
>
> Remco

  Be careful about which way the surface normal is
pointing. It took me long time to get the hang of
it and I still find myself taking a look at the
different shapes inc files on occasion for ideas
and to figure out why something isn't working.
  Differences don't necessarily have to take a long
time to render.

Take the following as an example:

difference{
sphere{<0,0, 0.75>,1}
sphere{<0,0,-0.75>,1}}

  This will produce a lens shaped object. Add
a glass texture and an ior value and it will
work just like a magnifying glass. Try rendering
it with a regular pigment attached and you will
find it no slower than a box. I was working with
this shape last night so I can verify this.
  Also the pyramid objects in the includes render
with no speed penalty that I can detect.
  I believe whenever there is a severe time increase
from a CSG it has something to do with not having your
inside and outside properly oriented. Similar to the
error: warning camera is inside a non hollow object
you get if you place your camera on the wrong side of
a plane's surface normal, or you put your camera in a
big sphere, that is your sky, and you forget to use
the inverse/hollow keywords, that generate the same
warning.

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