<s>
Global	B-Algorithm
motion	I-Algorithm
compensation	I-Algorithm
(	O
GMC	O
)	O
is	O
a	O
motion	B-Algorithm
compensation	I-Algorithm
technique	O
used	O
in	O
video	O
compression	O
to	O
reduce	O
the	O
bitrate	O
required	O
to	O
encode	O
video	O
.	O
</s>
<s>
It	O
is	O
most	O
commonly	O
used	O
in	O
MPEG-4	B-Application
ASP	I-Application
,	O
such	O
as	O
with	O
the	O
DivX	B-Application
and	O
Xvid	B-Algorithm
codecs	I-Algorithm
.	O
</s>
<s>
Global	B-Algorithm
motion	I-Algorithm
compensation	I-Algorithm
describes	O
the	O
motion	O
in	O
a	O
scene	O
based	O
on	O
a	O
single	O
affine	B-Algorithm
transform	I-Algorithm
instruction	O
.	O
</s>
<s>
Each	O
macroblock	B-General_Concept
in	O
such	O
a	O
frame	O
can	O
be	O
compensated	O
using	O
global	O
motion	O
(	O
no	O
further	O
motion	O
information	O
is	O
then	O
signalled	O
)	O
or	O
,	O
alternatively	O
,	O
local	O
motion	O
(	O
as	O
if	O
GMC	O
were	O
off	O
)	O
.	O
</s>
<s>
This	O
choice	O
,	O
while	O
costing	O
an	O
additional	O
bit	O
per	O
macroblock	B-General_Concept
,	O
can	O
improve	O
prediction	O
quality	O
and	O
therefore	O
reduce	O
residual	B-General_Concept
.	O
</s>
<s>
Because	O
the	O
transforms	O
used	O
in	O
global	B-Algorithm
motion	I-Algorithm
compensation	I-Algorithm
are	O
only	O
added	O
to	O
the	O
encoding	O
stream	O
when	O
used	O
,	O
they	O
do	O
not	O
have	O
a	O
constant	O
bitrate	O
overhead	O
.	O
</s>
<s>
A	O
predicted	O
frame	O
which	O
uses	O
GMC	O
is	O
called	O
an	O
S-frame	O
(	O
sprite	O
frame	O
)	O
while	O
a	O
predicted	O
frame	O
encoded	O
without	O
GMC	O
is	O
called	O
either	O
a	O
P-frame	O
,	O
if	O
it	O
was	O
predicted	O
purely	O
by	O
previous	O
(	O
past	O
)	O
frames	O
,	O
or	O
a	O
B-frame	O
if	O
it	O
was	O
predicted	O
jointly	O
with	O
past	O
and	O
future	O
frames	O
(	O
an	O
unpredicted	O
frame	O
encoded	O
as	O
a	O
whole	O
image	O
is	O
referred	O
to	O
as	O
an	O
I-frame	B-General_Concept
)	O
.	O
</s>
<s>
Offers	O
1	O
warp-point	O
GMC	O
encoding	O
:	O
This	O
enables	O
easier	O
hardware	O
support	O
in	O
DivX	B-Application
certified	O
and	O
non-certified	O
devices	O
.	O
</s>
<s>
GMC	O
failed	O
to	O
meet	O
expectations	O
of	O
dramatic	O
improvements	O
in	O
motion	B-Algorithm
compensation	I-Algorithm
,	O
and	O
as	O
a	O
result	O
it	O
was	O
omitted	O
from	O
the	O
H.264/MPEG	B-Application
-4	I-Application
AVC	I-Application
specification	O
-	O
designed	O
as	O
a	O
successor	O
to	O
MPEG-4	B-Application
ASP	I-Application
.	O
</s>
<s>
Due	O
to	O
the	O
extra	O
decoding	O
CPU	O
cost	O
of	O
global	B-Algorithm
motion	I-Algorithm
compensation	I-Algorithm
,	O
most	O
hardware	O
players	O
do	O
not	O
support	O
global	B-Algorithm
motion	I-Algorithm
compensation	I-Algorithm
.	O
</s>
