<s>
In	O
3D	O
computer	O
graphics	O
,	O
a	O
smoothing	B-Data_Structure
group	I-Data_Structure
is	O
a	O
group	O
of	O
polygons	B-General_Concept
in	O
a	O
polygon	B-Algorithm
mesh	I-Algorithm
which	O
should	O
appear	O
to	O
form	O
a	O
smooth	O
surface	O
.	O
</s>
<s>
Smoothing	B-Data_Structure
groups	I-Data_Structure
are	O
useful	O
for	O
describing	O
shapes	O
where	O
some	O
polygons	B-General_Concept
are	O
connected	O
smoothly	O
to	O
their	O
neighbors	O
,	O
and	O
some	O
are	O
not	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
a	O
mesh	B-Algorithm
representing	O
a	O
cylinder	O
,	O
all	O
of	O
the	O
polygons	B-General_Concept
are	O
smoothly	O
connected	O
except	O
along	O
the	O
edges	O
of	O
the	O
end	O
caps	O
.	O
</s>
<s>
One	O
could	O
make	O
a	O
smoothing	B-Data_Structure
group	I-Data_Structure
containing	O
all	O
of	O
the	O
polygons	B-General_Concept
in	O
one	O
end	O
cap	O
,	O
another	O
containing	O
the	O
polygons	B-General_Concept
in	O
the	O
other	O
end	O
cap	O
,	O
and	O
a	O
last	O
group	O
containing	O
the	O
polygons	B-General_Concept
in	O
the	O
tube	O
shape	O
between	O
the	O
end	O
caps	O
.	O
</s>
<s>
By	O
identifying	O
the	O
polygons	B-General_Concept
in	O
a	O
mesh	B-Algorithm
that	O
should	O
appear	O
to	O
be	O
smoothly	O
connected	O
,	O
smoothing	B-Data_Structure
groups	I-Data_Structure
allow	O
3D	O
modeling	O
software	O
to	O
estimate	O
the	O
surface	O
normal	O
at	O
any	O
point	O
on	O
the	O
mesh	B-Algorithm
,	O
by	O
averaging	O
the	O
surface	O
normals	O
or	O
vertex	B-General_Concept
normals	I-General_Concept
in	O
the	O
mesh	B-Algorithm
data	O
that	O
describes	O
the	O
mesh	B-Algorithm
.	O
</s>
<s>
If	O
each	O
polygon	B-General_Concept
lies	O
in	O
a	O
plane	O
,	O
the	O
software	O
could	O
calculate	O
a	O
polygon	B-General_Concept
's	O
surface	O
normal	O
by	O
calculating	O
the	O
normal	O
of	O
the	O
polygon	B-General_Concept
's	O
plane	O
,	O
meaning	O
this	O
data	O
would	O
not	O
have	O
to	O
be	O
stored	O
in	O
the	O
mesh	B-Algorithm
.	O
</s>
<s>
Thus	O
,	O
early	O
3D	O
modeling	O
software	O
like	O
3D	O
Studio	O
Max	O
DOS	O
used	O
smoothing	B-Data_Structure
groups	I-Data_Structure
as	O
a	O
way	O
to	O
avoid	O
having	O
to	O
store	O
accurate	O
vertex	B-General_Concept
normals	I-General_Concept
for	O
each	O
vertex	O
of	O
the	O
mesh	B-Algorithm
,	O
as	O
a	O
strategy	O
for	O
computer	O
representation	O
of	O
surfaces	O
.	O
</s>
