"Bald Eagle" <cre### [at] netscapenet> wrote:
> "Kenneth" <kdw### [at] gmailcom> wrote:
> >
> > Personally, I have always wondered why a z-plane doesn't have it's 'outside'
> > facing the -z direction by default, rather than the other way around. I
> > suppose that there are old and deep reasons for this.

Hmm, not so deep as I thought. In the documentation for a plane:

"...most planes are defined with surface normals in the direction of an axis
[meaning the positive direction, which is not specifically mentioned.]"

"By definition the normal vector points to the outside of the plane."

So all of the three (simple) x-y-z planes have their 'outsides' in the positive
directions of the axes. That's logically consistent-- but has to be taken into
account when media is used *inside* the planes. It's a subtle "Gotcha!"

>
> I also didn't quite know that any overlapping container needed to be hollow as
> well.
>

Yeah, this situation has tripped me up in the past as well, when using planes.
We don't usually run into such problems with typical media-filled objects, or
even CSG constructs that have the proper 'master' hollow keyword.

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