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<H4> 7.6.7.1 Sectors and Portals </H4>
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<P>
A sector is the basic building block in Crystal Space. A world will be made out
of several sectors connected with portals (see section <A HREF="cs_170.html#SEC362">7.6.2 Portal Engine</A>).
Normally adjacent sectors can be connected with a straight portal but
portals can also warp space so that you can connect any point in space to
any other point in space. This effect can be used to create mirrors for
example.
</P><P>
This effect of portals makes them very powerful. It also means that
you can have overlapping sectors and in general other weird portal
constructs.
</P><P>
Portals starting in arbitrary space (i.e. in the middle of
a sector) are called floating portals. Some special care needs to be taken
for those because we can't rely on the Z-buffer to be useful here. So this
is a case where clipping to the 2D boundary of the portal must happen.
</P><P>
Important for this discussion is the fact that we have two important
areas where culling happens. We have in-sector visibility culling which
is all visibility testing that ignores portal boundaries and we have
portal/sector visibility which controls what you can see through a
portal. Discussing both will be the main focus of this document later.
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