<s>
Coding	B-Algorithm
tree	I-Algorithm
unit	I-Algorithm
(	O
CTU	O
)	O
is	O
the	O
basic	O
processing	O
unit	O
of	O
the	O
High	B-Algorithm
Efficiency	I-Algorithm
Video	I-Algorithm
Coding	I-Algorithm
(	O
HEVC	B-Algorithm
)	O
video	O
standard	O
and	O
conceptually	O
corresponds	O
in	O
structure	O
to	O
macroblock	B-General_Concept
units	O
that	O
were	O
used	O
in	O
several	O
previous	O
video	O
standards	O
.	O
</s>
<s>
CTU	O
is	O
also	O
referred	O
to	O
as	O
largest	B-Algorithm
coding	I-Algorithm
unit	I-Algorithm
(	O
LCU	O
)	O
.	O
</s>
<s>
The	O
first	O
video	O
standard	O
that	O
uses	O
CTUs	O
is	O
HEVC/H.265	O
which	O
became	O
an	O
ITU-T	O
standard	O
on	O
April	O
13	O
,	O
2013	O
.	O
</s>
<s>
Macroblock	B-General_Concept
encoding	O
methods	O
have	O
been	O
used	O
in	O
digital	O
video	O
coding	O
standards	O
since	O
H.261	B-Algorithm
which	O
was	O
first	O
released	O
in	O
1988	O
.	O
</s>
<s>
However	O
,	O
for	O
error	B-Error_Name
correction	I-Error_Name
and	O
signal-to-noise	O
ratio	O
the	O
standard	O
16x16	O
macroblock	B-General_Concept
size	O
is	O
not	O
capable	O
of	O
getting	O
the	O
kind	O
of	O
bit	O
reductions	O
that	O
information	O
theory	O
and	O
coding	B-Error_Name
theory	I-Error_Name
suggest	O
are	O
theoretically	O
and	O
practically	O
possible	O
.	O
</s>
<s>
HEVC	B-Algorithm
replaces	O
macroblocks	B-General_Concept
,	O
which	O
were	O
used	O
with	O
previous	O
video	O
standards	O
,	O
with	O
CTUs	O
which	O
can	O
use	O
larger	O
block	O
structures	O
of	O
up	O
to	O
64×64	O
pixels	O
and	O
can	O
better	O
sub-partition	O
the	O
picture	O
into	O
variable	O
sized	O
structures	O
.	O
</s>
<s>
HEVC	B-Algorithm
initially	O
divides	O
the	O
picture	O
into	O
CTUs	O
which	O
are	O
then	O
divided	O
for	O
each	O
luma/chroma	O
component	O
into	O
coding	B-Algorithm
tree	I-Algorithm
blocks	I-Algorithm
(	O
CTBs	O
)	O
.	O
</s>
<s>
CTBs	O
are	O
then	O
divided	O
into	O
one	O
or	O
more	O
coding	O
units	O
(	O
CUs	O
)	O
,	O
so	O
that	O
the	O
CTU	O
size	O
is	O
also	O
the	O
largest	B-Algorithm
coding	I-Algorithm
unit	I-Algorithm
size	O
.	O
</s>
<s>
The	O
arrangement	O
of	O
CUs	O
in	O
a	O
CTB	O
is	O
known	O
as	O
a	O
quadtree	B-Data_Structure
since	O
a	O
subdivision	O
results	O
in	O
four	O
smaller	O
regions	O
.	O
</s>
<s>
To	O
code	O
the	O
prediction	O
residual	O
,	O
a	O
CU	O
is	O
divided	O
into	O
a	O
quadtree	B-Data_Structure
of	O
DCT	B-General_Concept
transform	O
units	O
(	O
TUs	O
)	O
.	O
</s>
<s>
At	O
the	O
July	O
2012	O
HEVC	B-Algorithm
meeting	O
it	O
was	O
decided	O
,	O
based	O
on	O
proposal	O
JCTVC-J0334	O
,	O
that	O
HEVC	B-Algorithm
level	O
5	O
and	O
higher	O
would	O
be	O
required	O
to	O
use	O
CTB	O
sizes	O
of	O
either	O
32×32	O
or	O
64×64	O
.	O
</s>
<s>
This	O
was	O
added	O
to	O
HEVC	B-Algorithm
in	O
the	O
Draft	O
International	O
Standard	O
as	O
a	O
level	O
limit	O
for	O
the	O
Log2MaxCtbSize	O
variable	O
.	O
</s>
<s>
Log2MaxCtbSize	O
was	O
renamed	O
CtbSizeY	O
in	O
the	O
October	O
2012	O
HEVC	B-Algorithm
draft	O
and	O
then	O
renamed	O
CtbLog2SizeY	O
in	O
the	O
January	O
2013	O
HEVC	B-Algorithm
draft	O
.	O
</s>
<s>
HEVC	B-Algorithm
benefits	O
from	O
the	O
use	O
of	O
larger	O
CTB	O
sizes	O
.	O
</s>
<s>
This	O
has	O
been	O
shown	O
in	O
PSNR	O
tests	O
with	O
a	O
HM-8.0	O
HEVC	B-Algorithm
encoder	O
where	O
it	O
was	O
forced	O
to	O
use	O
progressively	O
smaller	O
CTU	O
sizes	O
.	O
</s>
<s>
For	O
all	O
test	O
sequences	O
when	O
compared	O
to	O
a	O
64×64	O
CTU	O
size	O
it	O
was	O
shown	O
that	O
the	O
HEVC	B-Algorithm
bit	O
rate	O
increased	O
by	O
2.2	O
%	O
when	O
forced	O
to	O
use	O
a	O
32×32	O
CTU	O
size	O
and	O
increased	O
by	O
11.0	O
%	O
when	O
forced	O
to	O
use	O
a	O
16×16	O
CTU	O
size	O
.	O
</s>
<s>
In	O
the	O
Class	O
A	O
test	O
sequences	O
,	O
where	O
the	O
resolution	O
of	O
the	O
video	O
was	O
2560×1600	O
,	O
when	O
compared	O
to	O
a	O
64×64	O
CTU	O
size	O
it	O
was	O
shown	O
that	O
the	O
HEVC	B-Algorithm
bit	O
rate	O
increased	O
by	O
5.7	O
%	O
when	O
forced	O
to	O
use	O
a	O
32×32	O
CTU	O
size	O
and	O
increased	O
by	O
28.2	O
%	O
when	O
forced	O
to	O
use	O
a	O
16×16	O
CTU	O
size	O
.	O
</s>
<s>
The	O
tests	O
also	O
showed	O
that	O
it	O
took	O
60%	O
longer	O
to	O
decode	O
HEVC	B-Algorithm
video	O
encoded	O
at	O
16×16	O
CTU	O
size	O
than	O
at	O
64×64	O
CTU	O
size	O
.	O
</s>
<s>
The	O
tests	O
were	O
conducted	O
with	O
the	O
Main	O
profile	O
of	O
HEVC	B-Algorithm
based	O
on	O
equal	O
peak	O
signal-to-noise	O
ratio	O
(	O
PSNR	O
)	O
.	O
</s>
