<s>
In	O
computing	O
,	O
hardware	B-Architecture
overlay	I-Architecture
,	O
a	O
type	O
of	O
video	O
overlay	O
,	O
provides	O
a	O
method	O
of	O
rendering	O
an	O
image	O
to	O
a	O
display	O
screen	O
with	O
a	O
dedicated	O
memory	O
buffer	O
inside	O
computer	O
video	O
hardware	O
.	O
</s>
<s>
The	O
technique	O
aims	O
to	O
improve	O
the	O
display	O
of	O
a	O
fast-moving	O
video	O
image	O
—	O
such	O
as	O
a	O
computer	O
game	O
,	O
a	O
DVD	O
,	O
or	O
the	O
signal	O
from	O
a	O
TV	B-Device
card	I-Device
.	O
</s>
<s>
Most	O
video	B-Device
cards	I-Device
manufactured	O
since	O
about	O
1998	O
and	O
most	O
media	O
players	O
support	O
hardware	B-Architecture
overlay	I-Architecture
.	O
</s>
<s>
The	O
overlay	O
is	O
a	O
dedicated	O
buffer	O
into	O
which	O
one	O
app	O
can	O
render	O
(	O
typically	O
video	O
)	O
,	O
without	O
incurring	O
the	O
significant	O
performance	O
cost	O
of	O
checking	O
for	O
clipping	B-Algorithm
and	O
overlapping	O
rendering	O
by	O
other	O
apps	O
.	O
</s>
<s>
The	O
framebuffer	B-Algorithm
has	O
hardware	O
support	O
for	O
importing	O
and	O
rendering	O
the	O
buffer	O
contents	O
without	O
going	O
through	O
the	O
GPU	B-Architecture
.	O
</s>
<s>
The	O
use	O
of	O
a	O
hardware	B-Architecture
overlay	I-Architecture
is	O
important	O
for	O
several	O
reasons	O
:	O
</s>
<s>
In	O
a	O
graphical	B-Application
user	I-Application
interface	I-Application
(	O
GUI	B-Application
)	O
operating	B-General_Concept
system	I-General_Concept
such	O
as	O
Windows	B-Application
,	O
one	O
display-device	O
can	O
typically	O
display	O
multiple	O
applications	O
simultaneously	O
.	O
</s>
<s>
Consider	O
how	O
a	O
display	O
works	O
without	O
a	O
hardware	B-Architecture
overlay	I-Architecture
.	O
</s>
<s>
When	O
each	O
application	O
draws	O
to	O
the	O
screen	O
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
's	O
graphical	O
subsystem	O
must	O
constantly	O
check	O
to	O
ensure	O
that	O
the	O
objects	O
being	O
drawn	O
appear	O
on	O
the	O
appropriate	O
location	O
on	O
the	O
screen	O
,	O
and	O
that	O
they	O
do	O
n't	O
collide	O
with	O
overlapping	O
and	O
neighboring	O
windows	B-Application
.	O
</s>
<s>
The	O
graphical	O
subsystem	O
must	O
clip	B-Algorithm
objects	O
while	O
they	O
are	O
being	O
drawn	O
when	O
a	O
collision	O
occurs	O
.	O
</s>
<s>
This	O
constant	O
checking	O
and	O
clipping	B-Algorithm
ensures	O
that	O
different	O
applications	O
can	O
cooperate	O
with	O
one	O
another	O
in	O
sharing	O
a	O
display	O
,	O
but	O
also	O
consumes	O
a	O
significant	O
proportion	O
of	O
computing	O
power	O
.	O
</s>
<s>
Without	O
any	O
hardware	B-Architecture
overlays	I-Architecture
,	O
only	O
one	O
chunk	O
of	O
video	O
memory	O
exists	O
which	O
all	O
applications	O
must	O
share	O
-	O
and	O
the	O
location	O
of	O
a	O
given	O
application	O
's	O
video	O
memory	O
moves	O
whenever	O
the	O
user	O
changes	O
the	O
position	O
of	O
the	O
application	O
's	O
window	O
.	O
</s>
<s>
When	O
running	O
a	O
high-bandwidth	O
video	O
application	O
such	O
as	O
a	O
movie	O
player	O
or	O
some	O
games	O
,	O
the	O
computing	O
power	O
and	O
complexity	O
needed	O
to	O
perform	O
constant	O
clipping	B-Algorithm
and	O
checking	O
negatively	O
impacts	O
performance	O
and	O
compatibility	O
.	O
</s>
<s>
A	O
hardware	B-Architecture
overlay	I-Architecture
escapes	O
these	O
limitations	O
.	O
</s>
<s>
In	O
addition	O
,	O
the	O
graphics	B-Architecture
processing	I-Architecture
unit	I-Architecture
(	O
GPU	B-Architecture
)	O
provides	O
an	O
efficient	O
way	O
to	O
scale	O
the	O
video	O
in	O
size	O
and	O
often	O
performs	O
color-format	O
conversions	O
(	O
such	O
as	O
MPEG-2	O
'	O
s	O
YCbCr	O
into	O
RGB	O
)	O
.	O
</s>
<s>
An	O
application	O
using	O
a	O
hardware	B-Architecture
overlay	I-Architecture
gets	O
a	O
completely	O
separate	O
section	O
of	O
video	O
memory	O
that	O
belongs	O
only	O
to	O
that	O
application	O
.	O
</s>
<s>
To	O
get	O
the	O
image	O
from	O
the	O
separate	O
video	O
memory	O
to	O
display	O
in	O
tandem	O
with	O
the	O
remaining	O
shared	O
elements	O
on	O
the	O
display	O
,	O
the	O
graphical	O
subsystem	O
associates	O
a	O
certain	O
attribute	O
(	O
for	O
example	O
,	O
a	O
particular	O
color	O
)	O
as	O
a	O
"	O
mask	O
"	O
for	O
that	O
overlay	O
,	O
which	O
the	O
graphics	B-Device
card	I-Device
understands	O
to	O
mean	O
that	O
it	O
is	O
to	O
draw	O
from	O
the	O
separate	O
overlay	O
buffer	O
onto	O
the	O
screen	O
.	O
</s>
<s>
The	O
graphics	B-Device
card	I-Device
will	O
render	O
the	O
DVD	O
playback	O
only	O
inside	O
the	O
purple	O
area	O
.	O
</s>
<s>
If	O
another	O
window	O
moves	O
over	O
top	O
of	O
the	O
purple	O
area	O
and	O
obscures	O
a	O
part	O
of	O
it	O
,	O
then	O
the	O
graphics	O
hardware	O
performs	O
the	O
clipping	B-Algorithm
by	O
itself	O
.	O
</s>
<s>
As	O
a	O
consequence	O
of	O
hardware	B-Architecture
overlay	I-Architecture
use	O
,	O
a	O
screenshot	B-Application
program	O
(	O
for	O
example	O
,	O
the	O
one	O
automatically	O
built	O
into	O
Windows	B-Application
that	O
activates	O
when	O
a	O
user	O
presses	O
the	O
PrtSc	O
key	O
)	O
often	O
does	O
not	O
capture	O
the	O
content	O
appearing	O
in	O
the	O
hardware	B-Architecture
overlay	I-Architecture
window	O
.	O
</s>
<s>
Many	O
graphics	B-Device
cards	I-Device
can	O
support	O
more	O
than	O
one	O
monitor	O
and/or	O
a	O
TV	O
screen	O
as	O
output	O
devices	O
.	O
</s>
<s>
Typically	O
one	O
of	O
these	O
output	O
devices	O
has	O
to	O
be	O
declared	O
the	O
"	O
primary	O
"	O
device	O
,	O
and	O
only	O
the	O
primary	O
device	O
can	O
display	O
hardware	B-Architecture
overlays	I-Architecture
.	O
</s>
<s>
There	O
are	O
exceptions	O
:	O
Intel	O
writes	O
in	O
the	O
FAQ	O
for	O
their	O
Embedded	O
Graphics	O
Drivers	O
that	O
the	O
overlay	O
can	O
be	O
attached	O
to	O
either	O
one	O
of	O
the	O
displays	O
but	O
not	O
to	O
both	O
(	O
note	O
:	O
the	O
Intel	O
945	O
,	O
G33-Q965	O
chipsets	O
have	O
dual	O
hardware	B-Architecture
overlay	I-Architecture
,	O
and	O
are	O
capable	O
of	O
good	O
quality	O
mpeg2	O
on	O
secondary	O
monitors	O
when	O
appropriate	O
software	O
is	O
installed	O
)	O
,	O
and	O
some	O
newer	O
Matrox	O
graphics	B-Device
card	I-Device
support	O
overlay	O
on	O
both	O
displays	O
(	O
for	O
example	O
,	O
the	O
Parhelia	O
Series	O
)	O
.	O
</s>
<s>
Secondary	O
displays	O
require	O
both	O
hardware	O
and	O
driver	O
support	O
;	O
some	O
graphics	B-Device
cards	I-Device
may	O
support	O
overlay	O
on	O
the	O
second	O
display	O
while	O
their	O
drivers	O
may	O
not	O
yet	O
support	O
it	O
(	O
note	O
:	O
graphics	O
chipset	O
driver	O
bugs	O
can	O
cause	O
most	O
video	O
formats	O
apart	O
from	O
mpeg2	O
to	O
work	O
on	O
both	O
monitors	O
,	O
and	O
mpeg2	O
only	O
on	O
the	O
primary	O
with	O
most	O
players	O
)	O
.	O
</s>
<s>
Sometimes	O
,	O
the	O
use	O
of	O
hardware	B-Architecture
overlays	I-Architecture
may	O
have	O
to	O
be	O
disabled	O
in	O
the	O
media	O
player	O
.	O
</s>
<s>
However	O
,	O
some	O
graphics	B-Device
cards	I-Device
have	O
the	O
option	O
to	O
completely	O
redirect	O
hardware	B-Architecture
overlay	I-Architecture
to	O
the	O
TV	O
screen	O
.	O
</s>
<s>
Windows	B-Application
Vista	I-Application
's	O
enhanced	O
graphics	O
capabilities	O
replace	O
the	O
basic	O
concept	O
of	O
hardware	B-Architecture
overlays	I-Architecture
with	O
full	O
hardware	O
compositing	O
for	O
every	O
application	O
window	O
running	O
on	O
the	O
system	O
,	O
not	O
just	O
movie	O
players	O
or	O
games	O
,	O
through	O
the	O
Desktop	O
Window	O
Manager	O
.	O
</s>
<s>
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
has	O
used	O
hardware	O
compositing	O
since	O
the	O
introduction	O
of	O
Quartz	O
Extreme	O
into	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
10.2	I-Operating_System
.	O
</s>
<s>
(	O
In	O
Windows	B-Application
Vista	I-Application
,	O
each	O
hardware	B-Architecture
overlay	I-Architecture
is	O
more	O
correctly	O
known	O
as	O
a	O
Direct3D	O
surface	O
)	O
.	O
</s>
<s>
Then	O
the	O
system	O
's	O
GPU	B-Architecture
assembles	O
each	O
of	O
the	O
windows	B-Application
into	O
a	O
single	O
display	O
screen	O
in	O
real	O
time	O
.	O
</s>
<s>
With	O
modern	O
GPUs	B-Architecture
capable	O
of	O
advanced	O
3D	O
graphics	O
(	O
as	O
a	O
consequence	O
of	O
the	O
video	O
game	O
industry	O
)	O
,	O
operating	B-General_Concept
systems	I-General_Concept
can	O
apply	O
computationally	O
intensive	O
motion	O
,	O
scaling	O
,	O
and	O
lighting	O
effects	O
to	O
normal	O
2D	O
windows	B-Application
.	O
</s>
<s>
Due	O
to	O
a	O
need	O
to	O
reduce	O
power	O
consumption	O
,	O
Windows	B-Device
7	I-Device
added	O
back	O
limited	O
support	O
for	O
overlays	O
,	O
and	O
Windows	B-Device
8.1	I-Device
added	O
support	O
for	O
multi-plane	O
overlays	O
.	O
</s>
<s>
In	O
the	O
X	B-Operating_System
Window	I-Operating_System
System	I-Operating_System
,	O
the	O
windowing	O
system	O
of	O
most	O
Unix	B-Application
operating	I-Application
systems	I-Application
,	O
the	O
XVideo	B-Protocol
extension	O
can	O
allow	O
applications	O
to	O
employ	O
hardware	B-Architecture
overlays	I-Architecture
.	O
</s>
<s>
Compositing	O
is	O
also	O
used	O
,	O
with	O
compiz	B-Language
and	O
Beryl	B-Language
compositing	B-Operating_System
window	I-Operating_System
managers	I-Operating_System
being	O
the	O
most	O
prominent	O
examples	O
since	O
2006	O
.	O
</s>
<s>
They	O
are	O
able	O
to	O
take	O
advantage	O
of	O
OpenGL	B-Application
(	O
through	O
glx	O
extension	O
)	O
for	O
3D	O
and	O
2D	O
overlay	O
visual	O
effects	O
.	O
</s>
<s>
Other	O
implementations	O
like	O
Metacity	B-Language
and	O
xfwm	B-Device
have	O
been	O
available	O
since	O
2004	O
.	O
</s>
<s>
Some	O
dedicated	O
hardware	B-Architecture
overlay	I-Architecture
devices	O
use	O
embedded	O
Linux	B-Application
as	O
an	O
operating	B-General_Concept
system	I-General_Concept
,	O
for	O
example	O
the	O
video	O
logger	O
by	O
Racelogic	O
uses	O
a	O
Texas	O
Instruments	O
DM355	O
micro-controller	O
to	O
blend	O
a	O
frame	B-Algorithm
buffer	I-Algorithm
containing	O
graphics	O
onto	O
a	O
live	O
video	O
feed	O
,	O
and	O
then	O
store	O
the	O
result	O
in	O
MPEG-4	B-Algorithm
format	O
on	O
a	O
flash	O
card	O
.	O
</s>
<s>
Overlay	O
support	O
aka	O
Picture	O
In	O
Picture	O
(	O
PIP	O
)	O
was	O
introduced	O
to	O
AmigaOS	B-Application
with	O
the	O
introduction	O
of	O
the	O
PC	O
graphic	B-Device
cards	I-Device
,	O
Picasso96	O
and	O
CyberGraphX	B-Library
graphic	O
driver	O
systems	O
aka	O
ReTargetable	B-Application
Graphics	I-Application
,	O
for	O
Picasso	O
IV	O
and	O
Voodoo3	B-Device
cards	O
,	O
picasso’96	O
driver	O
system	O
became	O
standard	O
in	O
AmigaOS	B-Application
3.5	O
.	O
</s>
<s>
Moovid	O
was	O
one	O
of	O
the	O
first	O
video	O
players	O
that	O
supported	O
hardware	B-Architecture
overlay	I-Architecture
(	O
PIP	O
)	O
on	O
AmigaOS	B-Application
.	O
</s>
