<s>
H.263	B-Algorithm
is	O
a	O
video	B-Algorithm
compression	I-Algorithm
standard	I-Algorithm
originally	O
designed	O
as	O
a	O
low-bit-rate	O
compressed	O
format	O
for	O
videotelephony	B-Application
.	O
</s>
<s>
It	O
was	O
standardized	O
by	O
the	O
ITU-T	B-Application
Video	I-Application
Coding	I-Application
Experts	I-Application
Group	I-Application
(	O
VCEG	B-Application
)	O
in	O
a	O
project	O
ending	O
in	O
1995/1996	O
.	O
</s>
<s>
It	O
is	O
a	O
member	O
of	O
the	O
H.26x	B-Application
family	O
of	O
video	B-Algorithm
coding	I-Algorithm
standards	I-Algorithm
in	O
the	O
domain	O
of	O
the	O
ITU-T	O
.	O
</s>
<s>
Like	O
the	O
previous	O
H.26x	B-Application
standards	O
,	O
H.263	B-Algorithm
is	O
a	O
block-based	O
hybrid	O
video	B-Algorithm
coding	I-Algorithm
scheme	O
using	O
16×16	O
macroblocks	B-General_Concept
of	O
YCbCr	O
color	O
sample	O
arrays	O
,	O
motion-compensated	B-Algorithm
prediction	I-Algorithm
,	O
an	O
8×8	O
discrete	B-General_Concept
cosine	I-General_Concept
transform	I-General_Concept
for	O
prediction	O
differences	O
,	O
zig-zag	O
scanning	O
of	O
transform	O
coefficients	O
,	O
scalar	B-Algorithm
quantization	I-Algorithm
,	O
run-length	B-Algorithm
transform	I-Algorithm
coefficient	I-Algorithm
symbols	I-Algorithm
,	O
and	O
variable-length	B-Algorithm
coding	I-Algorithm
(	O
basically	O
like	O
Huffman	B-General_Concept
coding	I-General_Concept
but	O
with	O
structured	O
coding	O
tables	O
)	O
.	O
</s>
<s>
The	O
first	O
(	O
1995	O
)	O
version	O
of	O
H.263	B-Algorithm
included	O
some	O
optional	O
features	O
including	O
overlapped	O
block	O
motion	B-Algorithm
compensation	I-Algorithm
and	O
variable	O
block-size	O
motion	B-Algorithm
compensation	I-Algorithm
,	O
and	O
the	O
spec	O
was	O
later	O
extended	O
to	O
add	O
various	O
additional	O
enhanced	O
features	O
in	O
1998	O
and	O
2000	O
.	O
</s>
<s>
The	O
H.263	B-Algorithm
standard	O
was	O
first	O
designed	O
to	O
be	O
utilized	O
in	O
H.324	O
based	O
systems	O
(	O
PSTN	O
and	O
other	O
circuit-switched	B-Protocol
network	I-Protocol
videoconferencing	B-Application
and	O
videotelephony	B-Application
)	O
,	O
but	O
it	O
also	O
found	O
use	O
in	O
H.323	B-Protocol
(	O
RTP/IP	O
-based	O
videoconferencing	B-Application
)	O
,	O
H.320	B-Application
(	O
ISDN-based	O
videoconferencing	B-Application
,	O
where	O
it	O
became	O
the	O
most	O
widely	O
used	O
video	B-Algorithm
compression	I-Algorithm
standard	I-Algorithm
)	O
,	O
RTSP	B-Protocol
(	O
streaming	O
media	O
)	O
and	O
SIP	B-Application
(	O
IP-based	O
videoconferencing	B-Application
)	O
solutions	O
.	O
</s>
<s>
H.263	B-Algorithm
is	O
a	O
required	O
video	B-Algorithm
coding	I-Algorithm
format	I-Algorithm
in	O
ETSI	O
3GPP	O
technical	O
specifications	O
for	O
IP	B-Protocol
Multimedia	I-Protocol
Subsystem	I-Protocol
(	O
IMS	O
)	O
,	O
Multimedia	O
Messaging	O
Service	O
(	O
MMS	O
)	O
and	O
Transparent	O
end-to-end	O
Packet-switched	O
Streaming	O
Service	O
(	O
PSS	O
)	O
.	O
</s>
<s>
In	O
3GPP	O
specifications	O
,	O
H.263	B-Algorithm
video	O
is	O
usually	O
used	O
in	O
3GP	O
container	O
format	O
.	O
</s>
<s>
H.263	B-Algorithm
also	O
found	O
many	O
applications	O
on	O
the	O
internet	O
:	O
much	O
Flash	B-Operating_System
Video	I-Operating_System
content	O
(	O
as	O
used	O
on	O
sites	O
such	O
as	O
YouTube	B-General_Concept
,	O
Google	O
Video	O
,	O
and	O
MySpace	B-Application
)	O
used	O
to	O
be	O
encoded	O
in	O
Sorenson	O
Spark	O
format	O
(	O
an	O
incomplete	O
implementation	O
of	O
H.263	B-Algorithm
)	O
.	O
</s>
<s>
The	O
original	O
version	O
of	O
the	O
RealVideo	B-Algorithm
codec	O
was	O
based	O
on	O
H.263	B-Algorithm
until	O
the	O
release	O
of	O
RealVideo	B-Algorithm
8	O
.	O
</s>
<s>
H.263	B-Algorithm
was	O
developed	O
as	O
an	O
evolutionary	O
improvement	O
based	O
on	O
experience	O
from	O
H.261	B-Algorithm
and	O
H.262	B-Algorithm
(	O
aka	O
MPEG-2	B-Algorithm
Video	I-Algorithm
)	O
,	O
the	O
previous	O
ITU-T	O
standards	O
for	O
video	O
compression	O
,	O
and	O
the	O
MPEG-1	B-Algorithm
standard	O
developed	O
in	O
ISO/IEC	O
.	O
</s>
<s>
The	O
first	O
version	O
of	O
H.263	B-Algorithm
was	O
completed	O
in	O
1995	O
and	O
provided	O
a	O
suitable	O
replacement	O
for	O
H.261	B-Algorithm
at	O
all	O
bit	O
rates	O
.	O
</s>
<s>
It	O
was	O
further	O
enhanced	O
in	O
projects	O
known	O
as	O
H.263v2	B-Algorithm
(	O
also	O
known	O
as	O
H.263	B-Algorithm
+	O
or	O
H.263	B-Algorithm
1998	O
)	O
and	O
H.263v3	O
(	O
also	O
known	O
as	O
H.263	B-Algorithm
++	O
or	O
H.263	B-Algorithm
2000	O
)	O
.	O
</s>
<s>
It	O
was	O
also	O
used	O
as	O
the	O
basis	O
for	O
the	O
development	O
of	O
MPEG-4	B-Application
Part	I-Application
2	I-Application
.	O
</s>
<s>
MPEG-4	B-Application
Part	I-Application
2	I-Application
is	O
H.263	B-Algorithm
compatible	O
in	O
the	O
sense	O
that	O
basic	O
"	O
baseline	O
"	O
H.263	B-Algorithm
bitstreams	O
are	O
correctly	O
decoded	O
by	O
an	O
MPEG-4	B-Algorithm
Video	O
decoder	O
.	O
</s>
<s>
The	O
next	O
enhanced	O
format	O
developed	O
by	O
ITU-T	O
VCEG	B-Application
(	O
in	O
partnership	O
with	O
MPEG	O
)	O
after	O
H.263	B-Algorithm
was	O
the	O
H.264	B-Application
standard	O
,	O
also	O
known	O
as	O
AVC	O
and	O
MPEG-4	B-Application
part	I-Application
10	I-Application
.	O
</s>
<s>
As	O
H.264	B-Application
provides	O
a	O
significant	O
improvement	O
in	O
capability	O
beyond	O
H.263	B-Algorithm
,	O
the	O
H.263	B-Algorithm
standard	O
is	O
now	O
considered	O
a	O
legacy	O
design	O
that	O
is	O
only	O
used	O
for	O
compatibility	O
with	O
older	O
products	O
.	O
</s>
<s>
Newer	O
videoconferencing	B-Application
products	O
now	O
include	O
H.264	B-Application
as	O
well	O
as	O
,	O
or	O
instead	O
of	O
,	O
H.263	B-Algorithm
and	O
H.261	B-Algorithm
capabilities	O
.	O
</s>
<s>
Even	O
newer	O
standard	O
formats	O
,	O
HEVC	B-Algorithm
and	O
VVC	B-Algorithm
,	O
have	O
also	O
been	O
developed	O
by	O
VCEG	B-Application
and	O
MPEG	O
,	O
and	O
have	O
begun	O
to	O
replace	O
H.264	B-Application
in	O
some	O
applications	O
.	O
</s>
<s>
Since	O
the	O
original	O
ratification	O
of	O
H.263	B-Algorithm
in	O
March	O
1996	O
(	O
approving	O
a	O
document	O
that	O
was	O
produced	O
in	O
November	O
1995	O
)	O
,	O
there	O
have	O
been	O
two	O
subsequent	O
additions	O
which	O
improved	O
on	O
the	O
original	O
standard	O
by	O
additional	O
optional	O
extensions	O
(	O
for	O
example	O
,	O
the	O
H.263v2	B-Algorithm
project	O
added	O
a	O
deblocking	B-General_Concept
filter	I-General_Concept
in	O
its	O
Annex	O
J	O
)	O
.	O
</s>
<s>
The	O
original	O
version	O
of	O
H.263	B-Algorithm
specified	O
the	O
following	O
annexes	O
:	O
</s>
<s>
The	O
first	O
version	O
of	O
H.263	B-Algorithm
supported	O
a	O
limited	O
set	O
of	O
picture	O
sizes	O
:	O
</s>
<s>
H.263v2	B-Algorithm
(	O
also	O
known	O
as	O
H.263	B-Algorithm
+	O
,	O
or	O
as	O
the	O
1998	O
version	O
of	O
H.263	B-Algorithm
)	O
is	O
the	O
informal	O
name	O
of	O
the	O
second	O
edition	O
of	O
the	O
ITU-T	O
H.263	B-Algorithm
international	O
video	B-Algorithm
coding	I-Algorithm
standard	I-Algorithm
.	O
</s>
<s>
It	O
retained	O
the	O
entire	O
technical	O
content	O
of	O
the	O
original	O
version	O
of	O
the	O
standard	O
,	O
but	O
enhanced	O
H.263	B-Algorithm
capabilities	O
by	O
adding	O
several	O
annexes	O
which	O
can	O
substantially	O
improve	O
encoding	O
efficiency	O
and	O
provide	O
other	O
capabilities	O
(	O
such	O
as	O
enhanced	O
robustness	O
against	O
data	O
loss	O
in	O
the	O
transmission	O
channel	O
)	O
.	O
</s>
<s>
The	O
H.263	B-Algorithm
+	O
project	O
was	O
ratified	O
by	O
the	O
ITU	O
in	O
February	O
1998	O
.	O
</s>
<s>
H.263v2	B-Algorithm
also	O
added	O
support	O
for	O
flexible	O
customized	O
picture	O
formats	O
and	O
custom	O
picture	O
clock	O
frequencies	O
.	O
</s>
<s>
As	O
noted	O
above	O
,	O
the	O
only	O
picture	O
formats	O
previously	O
supported	O
in	O
H.263	B-Algorithm
had	O
been	O
Sub-QCIF	O
,	O
QCIF	B-Application
,	O
CIF	B-Application
,	O
4CIF	B-Application
,	O
and	O
16CIF	B-Application
,	O
and	O
the	O
only	O
picture	O
clock	O
frequency	O
had	O
been	O
30000/1001	O
(	O
approximately	O
29.97	O
)	O
clock	O
ticks	O
per	O
second	O
.	O
</s>
<s>
H.263v2	B-Algorithm
specified	O
a	O
set	O
of	O
recommended	O
modes	O
in	O
an	O
informative	O
appendix	O
(	O
Appendix	O
II	O
,	O
since	O
deprecated	O
)	O
:	O
</s>
<s>
The	O
definition	O
of	O
H.263v3	O
(	O
also	O
known	O
as	O
H.263	B-Algorithm
++	O
or	O
as	O
the	O
2000	O
version	O
of	O
H.263	B-Algorithm
)	O
added	O
three	O
annexes	O
.	O
</s>
<s>
In	O
June	O
2001	O
,	O
another	O
informative	O
appendix	O
(	O
Appendix	O
III	O
,	O
Examples	O
for	O
H.263	B-Algorithm
encoder/decoder	O
implementations	O
)	O
was	O
approved	O
.	O
</s>
<s>
In	O
January	O
2005	O
,	O
a	O
unified	O
H.263	B-Algorithm
specification	O
document	O
was	O
produced	O
(	O
with	O
the	O
exception	O
of	O
Appendix	O
III	O
,	O
which	O
remains	O
as	O
a	O
separately-published	O
document	O
)	O
.	O
</s>
<s>
H.263	B-Algorithm
was	O
developed	O
under	O
the	O
"	O
reasonable	O
and	O
non-discriminatory	O
"	O
patent	O
licensing	O
policy	O
of	O
ITU-T	O
,	O
although	O
in	O
practice	O
,	O
the	O
licensing	O
for	O
patent	O
rights	O
on	O
the	O
standard	O
did	O
not	O
become	O
the	O
subject	O
of	O
substantial	O
litigation	O
.	O
</s>
<s>
Open-source	O
implementations	O
include	O
the	O
LGPL-licensed	O
libavcodec	B-Language
library	O
(	O
part	O
of	O
the	O
FFmpeg	B-Library
project	O
)	O
which	O
is	O
used	O
by	O
programs	O
such	O
as	O
ffdshow	B-Language
,	O
VLC	B-Application
media	I-Application
player	I-Application
and	O
MPlayer	B-Application
.	O
</s>
