<s>
In	O
computer-generated	O
imagery	O
and	O
real-time	O
3D	O
computer	O
graphics	O
,	O
portal	B-General_Concept
rendering	I-General_Concept
is	O
an	O
algorithm	O
for	O
visibility	B-Algorithm
determination	I-Algorithm
.	O
</s>
<s>
For	O
example	O
,	O
consider	O
a	O
3D	O
computer	O
game	O
environment	O
,	O
which	O
may	O
contain	O
many	O
polygons	B-General_Concept
,	O
only	O
a	O
few	O
of	O
which	O
may	O
be	O
visible	O
on	O
screen	O
at	O
a	O
given	O
time	O
.	O
</s>
<s>
By	O
determining	O
which	O
polygons	B-General_Concept
are	O
currently	O
not	O
visible	O
,	O
and	O
not	O
rendering	O
those	O
objects	O
,	O
significant	O
performance	O
improvements	O
can	O
be	O
achieved	O
.	O
</s>
<s>
A	O
portal	B-General_Concept
system	O
is	O
based	O
on	O
using	O
the	O
partitioning	B-Algorithm
of	I-Algorithm
space	I-Algorithm
to	O
form	O
generalizations	O
about	O
the	O
visibility	O
of	O
objects	O
within	O
those	O
spaces	O
.	O
</s>
<s>
Adjacent	O
zones	O
are	O
linked	O
to	O
one	O
another	O
via	O
shared	O
dividing	O
polygons	B-General_Concept
termed	O
portals	B-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
in	O
a	O
computer	O
game	O
such	O
as	O
Descent	B-Application
,	O
the	O
game	O
area	O
might	O
be	O
divided	O
into	O
several	O
zones	O
.	O
</s>
<s>
These	O
openings	O
are	O
referred	O
to	O
as	O
portals	B-General_Concept
.	O
</s>
<s>
When	O
the	O
zone	O
behind	O
a	O
portal	B-General_Concept
needs	O
to	O
be	O
drawn	O
,	O
the	O
only	O
parts	O
that	O
are	O
visible	O
are	O
the	O
parts	O
that	O
can	O
be	O
seen	O
through	O
the	O
portal	B-General_Concept
.	O
</s>
<s>
Therefore	O
,	O
the	O
zone	O
can	O
be	O
clipped	B-Algorithm
against	O
the	O
portal	B-General_Concept
boundaries	O
to	O
remove	O
overdraw	O
.	O
</s>
<s>
The	O
use	O
of	O
portals	B-General_Concept
simplifies	O
the	O
game	O
engine	O
's	O
task	O
of	O
determining	O
visible	O
areas	O
and	O
objects	O
from	O
any	O
given	O
point	O
of	O
view	O
of	O
the	O
level	O
,	O
and	O
simplifies	O
rendering	O
by	O
allowing	O
it	O
to	O
use	O
each	O
portal	B-General_Concept
as	O
a	O
viewing	B-General_Concept
frustum	I-General_Concept
for	O
the	O
area	O
it	O
leads	O
to	O
.	O
</s>
<s>
Ideally	O
,	O
portals	B-General_Concept
are	O
formed	O
of	O
confined	O
areas	O
(	O
like	O
doors	O
or	O
tunnels	O
)	O
,	O
connecting	O
two	O
complex	O
areas	O
of	O
the	O
scene	O
,	O
where	O
each	O
of	O
these	O
areas	O
would	O
be	O
enclosed	O
in	O
such	O
a	O
polygonal	O
body	O
.	O
</s>
<s>
Portals	B-General_Concept
are	O
best	O
suited	O
for	O
indoor	O
scenes	O
such	O
as	O
mazes	O
.	O
</s>
