<s>
The	O
keystone	B-General_Concept
effect	I-General_Concept
is	O
the	O
apparent	O
distortion	O
of	O
an	O
image	O
caused	O
by	O
projecting	O
it	O
onto	O
an	O
angled	O
surface	O
.	O
</s>
<s>
In	O
stereoscopy	B-Algorithm
,	O
two	O
lenses	O
are	O
used	O
to	O
view	O
the	O
same	O
subject	O
image	O
,	O
each	O
from	O
a	O
slightly	O
different	O
perspective	O
,	O
allowing	O
a	O
three-dimensional	O
view	O
of	O
the	O
subject	O
.	O
</s>
<s>
If	O
the	O
two	O
images	O
are	O
not	O
exactly	O
parallel	O
,	O
this	O
causes	O
a	O
keystone	B-General_Concept
effect	I-General_Concept
.	O
</s>
<s>
This	O
is	O
particularly	O
noticeable	O
when	O
the	O
lenses	O
are	O
close	O
to	O
the	O
subject	O
,	O
as	O
with	O
a	O
stereo	O
microscope	O
,	O
but	O
is	O
also	O
a	O
common	O
problem	O
with	O
many	O
3D	B-Algorithm
stereo	I-Algorithm
camera	O
lenses	O
.	O
</s>
<s>
The	O
problem	O
arises	O
for	O
screen	B-Device
projectors	I-Device
that	O
do	O
n't	O
have	O
the	O
depth	O
of	O
focus	O
necessary	O
to	O
keep	O
all	O
lines	O
(	O
from	O
top	O
to	O
bottom	O
)	O
focused	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
Keystone	B-General_Concept
correction	I-General_Concept
,	O
colloquially	O
also	O
called	O
keystoning	B-General_Concept
,	O
is	O
a	O
function	O
that	O
allows	O
multimedia	O
projectors	O
that	O
are	O
not	O
placed	O
perpendicular	O
to	O
the	O
horizontal	O
centerline	O
of	O
the	O
screen	O
(	O
too	O
high	O
or	O
too	O
low	O
)	O
to	O
skew	O
the	O
output	O
image	O
,	O
thereby	O
making	O
it	O
rectangular	O
.	O
</s>
<s>
Keystone	B-General_Concept
correction	I-General_Concept
is	O
a	O
feature	O
included	O
with	O
many	O
projectors	O
that	O
provides	O
the	O
ability	O
to	O
intentionally	O
"	O
distort	O
"	O
the	O
output	O
image	O
to	O
recreate	O
the	O
original	O
rectangular	O
image	O
provided	O
by	O
the	O
video	O
or	O
computer	O
source	O
,	O
thus	O
eliminating	O
the	O
skewed	O
output	O
that	O
would	O
otherwise	O
result	O
due	O
to	O
angled	O
projection	O
.	O
</s>
<s>
In	O
modern	O
projectors	O
keystone	B-General_Concept
correction	I-General_Concept
technology	O
is	O
performed	O
digitally	O
(	O
rather	O
than	O
optically	O
)	O
via	O
the	O
internal	O
(	O
LCD	B-Device
)	O
panels	O
or	O
(	O
DLP	B-General_Concept
)	O
mirrors	O
of	O
the	O
projector	O
,	O
depending	O
on	O
the	O
technology	O
used	O
.	O
</s>
<s>
Thus	O
,	O
when	O
applying	O
keystone	B-General_Concept
correction	I-General_Concept
to	O
an	O
image	O
,	O
the	O
number	O
of	O
individual	O
pixels	O
used	O
is	O
reduced	O
,	O
lowering	O
the	O
resolution	O
and	O
thus	O
degrading	O
the	O
quality	O
of	O
the	O
image	O
projected	O
.	O
</s>
<s>
Home	O
theater	O
enthusiasts	O
would	O
argue	O
that	O
keystoning	B-General_Concept
should	O
not	O
be	O
used	O
because	O
of	O
the	O
impact	O
it	O
has	O
on	O
image	O
quality	O
.	O
</s>
