<s>
In	O
3D	O
computer	O
graphics	O
,	O
hidden-surface	B-Algorithm
determination	I-Algorithm
(	O
also	O
known	O
as	O
shown-surface	O
determination	O
,	O
hidden-surface	O
removal	O
(	O
HSR	O
)	O
,	O
occlusion	O
culling	B-Algorithm
(	O
OC	O
)	O
or	O
visible-surface	O
determination	O
(	O
VSD	O
)	O
)	O
is	O
the	O
process	O
of	O
identifying	O
what	O
surfaces	O
and	O
parts	O
of	O
surfaces	O
can	O
be	O
seen	O
from	O
a	O
particular	O
viewing	O
angle	O
.	O
</s>
<s>
A	O
hidden-surface	B-Algorithm
determination	I-Algorithm
algorithm	O
is	O
a	O
solution	O
to	O
the	O
visibility	O
problem	O
,	O
which	O
was	O
one	O
of	O
the	O
first	O
major	O
problems	O
in	O
the	O
field	O
of	O
3D	O
computer	O
graphics	O
.	O
</s>
<s>
The	O
process	O
of	O
hidden-surface	B-Algorithm
determination	I-Algorithm
is	O
sometimes	O
called	O
hiding	O
,	O
and	O
such	O
an	O
algorithm	O
is	O
sometimes	O
called	O
a	O
hider	B-Algorithm
.	O
</s>
<s>
When	O
referring	O
to	O
line	O
rendering	O
it	O
is	O
known	O
as	O
hidden-line	B-Algorithm
removal	I-Algorithm
.	O
</s>
<s>
Hidden-surface	B-Algorithm
determination	I-Algorithm
is	O
necessary	O
to	O
render	O
a	O
scene	O
correctly	O
,	O
so	O
that	O
one	O
may	O
not	O
view	O
features	O
hidden	O
behind	O
the	O
model	O
itself	O
,	O
allowing	O
only	O
the	O
naturally	O
viewable	O
portion	O
of	O
the	O
graphic	O
to	O
be	O
visible	O
.	O
</s>
<s>
Hidden-surface	B-Algorithm
determination	I-Algorithm
is	O
a	O
process	O
by	O
which	O
surfaces	O
that	O
should	O
not	O
be	O
visible	O
to	O
the	O
user	O
(	O
for	O
example	O
,	O
because	O
they	O
lie	O
behind	O
opaque	O
objects	O
such	O
as	O
walls	O
)	O
are	O
prevented	O
from	O
being	O
rendered	O
.	O
</s>
<s>
There	O
are	O
many	O
techniques	O
for	O
hidden-surface	B-Algorithm
determination	I-Algorithm
.	O
</s>
<s>
They	O
are	O
fundamentally	O
an	O
exercise	O
in	O
sorting	B-Algorithm
and	O
usually	O
vary	O
in	O
the	O
order	O
in	O
which	O
the	O
sort	O
is	O
performed	O
and	O
how	O
the	O
problem	O
is	O
subdivided	O
.	O
</s>
<s>
Sorting	B-Algorithm
large	O
quantities	O
of	O
graphics	O
primitives	O
is	O
usually	O
done	O
by	O
divide	B-Algorithm
and	I-Algorithm
conquer	I-Algorithm
.	O
</s>
<s>
Considering	O
the	O
rendering	B-General_Concept
pipeline	I-General_Concept
,	O
the	O
projection	B-Algorithm
,	O
the	O
clipping	B-Algorithm
,	O
and	O
the	O
rasterization	O
steps	O
are	O
handled	O
differently	O
by	O
the	O
following	O
algorithms	O
:	O
</s>
<s>
Coverage	O
buffers	O
(	O
)	O
and	O
surface	O
buffer	O
(	O
S-buffer	B-Algorithm
)	O
Faster	O
than	O
Z-buffers	O
and	O
commonly	O
used	O
in	O
games	O
in	O
the	O
Quake	B-Application
I	I-Application
era	O
.	O
</s>
<s>
An	O
S-buffer	B-Algorithm
can	O
display	O
unsorted	O
polygons	O
,	O
while	O
a	O
C-buffer	O
requires	O
polygons	O
to	O
be	O
displayed	O
from	O
the	O
nearest	O
to	O
the	O
furthest	O
.	O
</s>
<s>
This	O
was	O
commonly	O
used	O
with	O
binary	O
space	O
partitioning	O
(	O
BSP	O
)	O
trees	O
,	O
which	O
would	O
provide	O
sorting	B-Algorithm
for	O
the	O
polygons	O
.	O
</s>
<s>
Sorted	O
active	O
edge	O
list	O
Used	O
in	O
Quake	B-Application
1	I-Application
,	O
this	O
was	O
storing	O
a	O
list	O
of	O
the	O
edges	O
of	O
already	O
displayed	O
polygons	O
(	O
see	O
scanline	B-Algorithm
rendering	I-Algorithm
)	O
.	O
</s>
<s>
Painter	B-Algorithm
's	I-Algorithm
algorithm	I-Algorithm
Sorts	O
polygons	O
by	O
their	O
barycenter	O
and	O
draws	O
them	O
back	O
to	O
front	O
.	O
</s>
<s>
This	O
produces	O
few	O
artifacts	O
when	O
applied	O
to	O
scenes	O
with	O
polygons	O
of	O
similar	O
size	O
forming	O
smooth	O
meshes	O
and	O
back-face	B-Algorithm
culling	I-Algorithm
turned	O
on	O
.	O
</s>
<s>
The	O
cost	O
here	O
is	O
the	O
sorting	B-Algorithm
step	O
and	O
the	O
fact	O
that	O
visual	O
artifacts	O
can	O
occur	O
.	O
</s>
<s>
Ray	B-Algorithm
tracing	I-Algorithm
Attempts	O
to	O
model	O
the	O
path	O
of	O
light	O
rays	O
to	O
a	O
viewpoint	O
by	O
tracing	O
rays	O
from	O
the	O
viewpoint	O
into	O
the	O
scene	O
.	O
</s>
<s>
Effectively	O
this	O
is	O
equivalent	O
to	O
sorting	B-Algorithm
all	O
the	O
geometry	O
on	O
a	O
per-pixel	O
basis	O
.	O
</s>
<s>
The	O
Warnock	B-Algorithm
algorithm	I-Algorithm
Divides	O
the	O
screen	O
into	O
smaller	O
areas	O
and	O
sorts	O
triangles	O
within	O
these	O
.	O
</s>
<s>
A	O
related	O
area	O
to	O
visible-surface	O
determination	O
(	O
VSD	O
)	O
is	O
culling	B-Algorithm
,	O
which	O
usually	O
happens	O
before	O
VSD	O
in	O
a	O
rendering	B-General_Concept
pipeline	I-General_Concept
.	O
</s>
<s>
The	O
advantage	O
of	O
culling	B-Algorithm
early	O
on	O
in	O
the	O
pipeline	O
is	O
that	O
entire	O
objects	O
that	O
are	O
invisible	O
do	O
not	O
have	O
to	O
be	O
fetched	O
,	O
transformed	O
,	O
rasterized	O
,	O
or	O
shaded	O
.	O
</s>
<s>
Here	O
are	O
some	O
types	O
of	O
culling	B-Algorithm
algorithms	O
:	O
</s>
<s>
The	O
viewing	B-General_Concept
frustum	I-General_Concept
is	O
a	O
geometric	O
representation	O
of	O
the	O
volume	O
visible	O
to	O
the	O
virtual	O
camera	O
.	O
</s>
<s>
Often	O
,	O
objects	O
lie	O
on	O
the	O
boundary	O
of	O
the	O
viewing	B-General_Concept
frustum	I-General_Concept
.	O
</s>
<s>
These	O
objects	O
are	O
cut	O
into	O
pieces	O
along	O
this	O
boundary	O
in	O
a	O
process	O
called	O
clipping	B-Algorithm
,	O
and	O
the	O
pieces	O
that	O
lie	O
outside	O
the	O
frustum	O
are	O
discarded	O
as	O
there	O
is	O
no	O
place	O
to	O
draw	O
them	O
.	O
</s>
<s>
They	O
are	O
determined	O
by	O
the	O
vertex	O
winding	O
order	O
:	O
if	O
the	O
triangle	O
drawn	O
has	O
its	O
vertices	O
in	O
clockwise	O
order	O
on	O
the	O
projection	B-Algorithm
plane	O
when	O
facing	O
the	O
camera	O
,	O
they	O
switch	O
into	O
counter-clockwise	O
order	O
when	O
the	O
surface	O
turns	O
away	O
from	O
the	O
camera	O
.	O
</s>
<s>
no	O
back-face	B-Algorithm
culling	I-Algorithm
is	O
done	O
)	O
or	O
have	O
separate	O
inside	O
surfaces	O
.	O
</s>
<s>
These	O
objects	O
are	O
thrown	O
away	O
if	O
their	O
screen	O
projection	B-Algorithm
is	O
too	O
small	O
.	O
</s>
<s>
See	O
Clipping	B-Algorithm
plane	O
.	O
</s>
<s>
There	O
are	O
several	O
types	O
of	O
occlusion	O
culling	B-Algorithm
approaches	O
:	O
</s>
<s>
Portal	B-General_Concept
rendering	I-General_Concept
divides	O
a	O
scene	O
into	O
cells/sectors	O
(	O
rooms	O
)	O
and	O
portals	B-General_Concept
(	O
doors	O
)	O
,	O
and	O
computes	O
which	O
sectors	O
are	O
visible	O
by	O
clipping	B-Algorithm
them	O
against	O
portals	B-General_Concept
.	O
</s>
<s>
Hansong	O
Zhang	O
's	O
dissertation	O
"	O
Effective	O
Occlusion	O
Culling	B-Algorithm
for	O
the	O
Interactive	O
Display	O
of	O
Arbitrary	O
Models	O
"	O
describes	O
an	O
occlusion	O
culling	B-Algorithm
approach	O
.	O
</s>
<s>
A	O
popular	O
theme	O
in	O
the	O
VSD	O
literature	O
is	O
divide	B-Algorithm
and	I-Algorithm
conquer	I-Algorithm
.	O
</s>
<s>
The	O
Warnock	B-Algorithm
algorithm	I-Algorithm
pioneered	O
dividing	O
the	O
screen	O
.	O
</s>
<s>
Beam	B-Algorithm
tracing	I-Algorithm
is	O
a	O
ray-tracing	B-Algorithm
approach	O
that	O
divides	O
the	O
visible	O
volumes	O
into	O
beams	O
.	O
</s>
<s>
tiling	O
,	O
or	O
screen-space	O
BSP	O
clipping	B-Algorithm
.	O
</s>
<s>
Z-buffer	O
hardware	O
may	O
typically	O
include	O
a	O
coarse	O
"	O
hi-Z	O
"	O
,	O
against	O
which	O
primitives	O
can	O
be	O
rejected	O
early	O
without	O
rasterization	O
,	O
this	O
is	O
a	O
form	O
of	O
occlusion	O
culling	B-Algorithm
.	O
</s>
<s>
Bounding	O
volume	O
hierarchies	O
(	O
BVHs	O
)	O
are	O
often	O
used	O
to	O
subdivide	O
the	O
scene	O
's	O
space	O
(	O
examples	O
are	O
the	O
BSP	O
tree	O
,	O
the	O
octree	B-Data_Structure
and	O
the	O
kd-tree	B-Data_Structure
)	O
.	O
</s>
