<s>
In	O
the	O
geometry	O
of	O
computer	O
graphics	O
,	O
a	O
vertex	B-General_Concept
normal	I-General_Concept
at	O
a	O
vertex	O
of	O
a	O
polyhedron	O
is	O
a	O
directional	O
vector	O
associated	O
with	O
a	O
vertex	O
,	O
intended	O
as	O
a	O
replacement	O
to	O
the	O
true	O
geometric	O
normal	O
of	O
the	O
surface	O
.	O
</s>
<s>
Vertex	B-General_Concept
normals	I-General_Concept
can	O
also	O
be	O
computed	O
for	O
polygonal	O
approximations	O
to	O
surfaces	O
such	O
as	O
NURBS	B-Algorithm
,	O
or	O
specified	O
explicitly	O
for	O
artistic	O
purposes	O
.	O
</s>
<s>
Vertex	B-General_Concept
normals	I-General_Concept
are	O
used	O
in	O
Gouraud	B-Algorithm
shading	I-Algorithm
,	O
Phong	O
shading	B-General_Concept
and	O
other	O
lighting	B-General_Concept
models	I-General_Concept
.	O
</s>
<s>
Using	O
vertex	B-General_Concept
normals	I-General_Concept
,	O
much	O
smoother	O
shading	B-General_Concept
than	O
flat	O
shading	B-General_Concept
can	O
be	O
achieved	O
;	O
however	O
,	O
without	O
some	O
modifications	O
,	O
it	O
cannot	O
produce	O
a	O
sharp	O
edge	O
.	O
</s>
