<s>
Image	B-General_Concept
compression	I-General_Concept
is	O
a	O
type	O
of	O
data	B-General_Concept
compression	I-General_Concept
applied	O
to	O
digital	B-Algorithm
images	I-Algorithm
,	O
to	O
reduce	O
their	O
cost	O
for	O
storage	B-General_Concept
or	O
transmission	O
.	O
</s>
<s>
Algorithms	O
may	O
take	O
advantage	O
of	O
visual	O
perception	O
and	O
the	O
statistical	O
properties	O
of	O
image	O
data	O
to	O
provide	O
superior	O
results	O
compared	O
with	O
generic	O
data	B-General_Concept
compression	I-General_Concept
methods	O
which	O
are	O
used	O
for	O
other	O
digital	O
data	O
.	O
</s>
<s>
Image	B-General_Concept
compression	I-General_Concept
may	O
be	O
lossy	B-Algorithm
or	O
lossless	B-Algorithm
.	O
</s>
<s>
Lossless	B-Algorithm
compression	I-Algorithm
is	O
preferred	O
for	O
archival	O
purposes	O
and	O
often	O
for	O
medical	O
imaging	O
,	O
technical	O
drawings	O
,	O
clip	O
art	O
,	O
or	O
comics	O
.	O
</s>
<s>
Lossy	B-Algorithm
compression	I-Algorithm
methods	O
,	O
especially	O
when	O
used	O
at	O
low	O
bit	O
rates	O
,	O
introduce	O
compression	B-Algorithm
artifacts	I-Algorithm
.	O
</s>
<s>
Lossy	B-Algorithm
methods	O
are	O
especially	O
suitable	O
for	O
natural	O
images	O
such	O
as	O
photographs	O
in	O
applications	O
where	O
minor	O
(	O
sometimes	O
imperceptible	O
)	O
loss	O
of	O
fidelity	O
is	O
acceptable	O
to	O
achieve	O
a	O
substantial	O
reduction	O
in	O
bit	O
rate	O
.	O
</s>
<s>
Lossy	B-Algorithm
compression	I-Algorithm
that	O
produces	O
negligible	O
differences	O
may	O
be	O
called	O
visually	O
lossless	B-Algorithm
.	O
</s>
<s>
Methods	O
for	O
lossy	B-Algorithm
compression	I-Algorithm
:	O
</s>
<s>
Transform	B-Algorithm
coding	I-Algorithm
–	O
This	O
is	O
the	O
most	O
commonly	O
used	O
method	O
.	O
</s>
<s>
Discrete	B-General_Concept
Cosine	I-General_Concept
Transform	I-General_Concept
(	O
DCT	B-General_Concept
)	O
–	O
The	O
most	O
widely	O
used	O
form	O
of	O
lossy	B-Algorithm
compression	I-Algorithm
.	O
</s>
<s>
It	O
is	O
a	O
type	O
of	O
Fourier-related	B-Algorithm
transform	I-Algorithm
,	O
and	O
was	O
originally	O
developed	O
by	O
Nasir	O
Ahmed	O
,	O
T	O
.	O
Natarajan	O
and	O
K	O
.	O
R	O
.	O
Rao	O
in	O
1974	O
.	O
</s>
<s>
The	O
DCT	B-General_Concept
is	O
sometimes	O
referred	O
to	O
as	O
"	O
DCT-II	O
"	O
in	O
the	O
context	O
of	O
a	O
family	O
of	O
discrete	B-General_Concept
cosine	I-General_Concept
transforms	I-General_Concept
(	O
see	O
discrete	B-General_Concept
cosine	I-General_Concept
transform	I-General_Concept
)	O
.	O
</s>
<s>
It	O
is	O
generally	O
the	O
most	O
efficient	O
form	O
of	O
image	B-General_Concept
compression	I-General_Concept
.	O
</s>
<s>
DCT	B-General_Concept
is	O
used	O
in	O
JPEG	O
,	O
the	O
most	O
popular	O
lossy	B-Algorithm
format	O
,	O
and	O
the	O
more	O
recent	O
HEIF	O
.	O
</s>
<s>
The	O
more	O
recently	O
developed	O
wavelet	B-Algorithm
transform	I-Algorithm
is	O
also	O
used	O
extensively	O
,	O
followed	O
by	O
quantization	B-Algorithm
and	O
entropy	B-Algorithm
coding	I-Algorithm
.	O
</s>
<s>
Color	B-Algorithm
quantization	I-Algorithm
-	O
Reducing	O
the	O
color	B-Algorithm
space	I-Algorithm
to	O
a	O
few	O
"	O
representative	O
"	O
colors	O
in	O
the	O
image	O
.	O
</s>
<s>
The	O
selected	O
colors	O
are	O
specified	O
in	O
the	O
colour	O
palette	B-Data_Structure
in	O
the	O
header	O
of	O
the	O
compressed	B-General_Concept
image	I-General_Concept
.	O
</s>
<s>
Each	O
pixel	O
just	O
references	O
the	O
index	O
of	O
a	O
color	O
in	O
the	O
color	O
palette	B-Data_Structure
.	O
</s>
<s>
This	O
method	O
can	O
be	O
combined	O
with	O
dithering	O
to	O
avoid	O
posterization	B-Algorithm
.	O
</s>
<s>
Whole-image	O
palette	B-Data_Structure
,	O
typically	O
256	O
colors	O
,	O
used	O
in	O
GIF	O
and	O
PNG	O
file	O
formats	O
.	O
</s>
<s>
block	O
palette	B-Data_Structure
,	O
typically	O
2	O
or	O
4	O
colors	O
for	O
each	O
block	O
of	O
4x4	O
pixels	O
,	O
used	O
in	O
BTC	O
,	O
CCC	O
,	O
S2TC	B-Algorithm
,	O
and	O
S3TC	O
.	O
</s>
<s>
Fractal	B-Algorithm
compression	I-Algorithm
.	O
</s>
<s>
More	O
recently	O
,	O
methods	O
based	O
on	O
Machine	O
Learning	O
were	O
applied	O
,	O
using	O
Multilayer	B-Algorithm
perceptrons	I-Algorithm
,	O
Convolutional	B-Architecture
neural	I-Architecture
networks	I-Architecture
and	O
Generative	B-Algorithm
adversarial	I-Algorithm
networks	I-Algorithm
.	O
</s>
<s>
Implementations	O
are	O
available	O
in	O
OpenCV	B-Language
,	O
TensorFlow	B-Language
,	O
MATLAB	B-Language
's	O
Image	O
Processing	O
Toolbox	O
(	O
IPT	O
)	O
,	O
and	O
the	O
High-Fidelity	O
Generative	O
Image	B-General_Concept
Compression	I-General_Concept
(	O
HiFiC	O
)	O
open	O
source	O
project	O
.	O
</s>
<s>
Methods	O
for	O
lossless	B-Algorithm
compression	I-Algorithm
:	O
</s>
<s>
The	O
best	O
image	O
quality	O
at	O
a	O
given	O
compression	O
rate	O
(	O
or	O
bit	O
rate	O
)	O
is	O
the	O
main	O
goal	O
of	O
image	B-General_Concept
compression	I-General_Concept
,	O
however	O
,	O
there	O
are	O
other	O
important	O
properties	O
of	O
image	B-General_Concept
compression	I-General_Concept
schemes	O
:	O
</s>
<s>
Despite	O
its	O
contrary	O
nature	O
,	O
scalability	O
also	O
may	O
be	O
found	O
in	O
lossless	B-Algorithm
codecs	I-Algorithm
,	O
usually	O
in	O
form	O
of	O
coarse-to-fine	O
pixel	O
scans	O
.	O
</s>
<s>
Compressed	B-General_Concept
data	I-General_Concept
may	O
contain	O
information	O
about	O
the	O
image	O
which	O
may	O
be	O
used	O
to	O
categorize	O
,	O
search	O
,	O
or	O
browse	O
images	O
.	O
</s>
<s>
Such	O
information	O
may	O
include	O
color	O
and	O
texture	O
statistics	O
,	O
small	O
preview	B-Application
images	O
,	O
and	O
author	O
or	O
copyright	O
information	O
.	O
</s>
<s>
Compression	B-General_Concept
algorithms	I-General_Concept
require	O
different	O
amounts	O
of	O
processing	O
power	O
to	O
encode	O
and	O
decode	O
.	O
</s>
<s>
Some	O
high	O
compression	B-General_Concept
algorithms	I-General_Concept
require	O
high	O
processing	O
power	O
.	O
</s>
<s>
It	O
measures	O
the	O
amount	O
of	O
noise	O
introduced	O
through	O
a	O
lossy	B-Algorithm
compression	I-Algorithm
of	O
the	O
image	O
,	O
however	O
,	O
the	O
subjective	O
judgment	O
of	O
the	O
viewer	O
also	O
is	O
regarded	O
as	O
an	O
important	O
measure	O
,	O
perhaps	O
,	O
being	O
the	O
most	O
important	O
measure	O
.	O
</s>
<s>
Entropy	B-Algorithm
coding	I-Algorithm
started	O
in	O
the	O
1940s	O
with	O
the	O
introduction	O
of	O
Shannon	B-Algorithm
–	I-Algorithm
Fano	I-Algorithm
coding	I-Algorithm
,	O
the	O
basis	O
for	O
Huffman	B-General_Concept
coding	I-General_Concept
which	O
was	O
developed	O
in	O
1950	O
.	O
</s>
<s>
Transform	B-Algorithm
coding	I-Algorithm
dates	O
back	O
to	O
the	O
late	O
1960s	O
,	O
with	O
the	O
introduction	O
of	O
fast	O
Fourier	O
transform	O
(	O
FFT	O
)	O
coding	O
in	O
1968	O
and	O
the	O
Hadamard	B-Algorithm
transform	I-Algorithm
in	O
1969	O
.	O
</s>
<s>
An	O
important	O
development	O
in	O
image	O
data	B-General_Concept
compression	I-General_Concept
was	O
the	O
discrete	B-General_Concept
cosine	I-General_Concept
transform	I-General_Concept
(	O
DCT	B-General_Concept
)	O
,	O
a	O
lossy	B-Algorithm
compression	I-Algorithm
technique	O
first	O
proposed	O
by	O
Nasir	O
Ahmed	O
,	O
T	O
.	O
Natarajan	O
and	O
K	O
.	O
R	O
.	O
Rao	O
in	O
1973	O
.	O
</s>
<s>
JPEG	O
was	O
largely	O
responsible	O
for	O
the	O
wide	O
proliferation	O
of	O
digital	B-Algorithm
images	I-Algorithm
and	O
digital	B-Application
photos	I-Application
,	O
with	O
several	O
billion	O
JPEG	O
images	O
produced	O
every	O
day	O
as	O
of	O
2015	O
.	O
</s>
<s>
Lempel	B-Algorithm
–	I-Algorithm
Ziv	I-Algorithm
–	I-Algorithm
Welch	I-Algorithm
(	O
LZW	B-Algorithm
)	O
is	O
a	O
lossless	B-Algorithm
compression	I-Algorithm
algorithm	O
developed	O
by	O
Abraham	O
Lempel	O
,	O
Jacob	O
Ziv	O
and	O
Terry	O
Welch	O
in	O
1984	O
.	O
</s>
<s>
DEFLATE	B-Algorithm
,	O
a	O
lossless	B-Algorithm
compression	I-Algorithm
algorithm	O
developed	O
by	O
Phil	O
Katz	O
and	O
specified	O
in	O
1996	O
,	O
is	O
used	O
in	O
the	O
Portable	O
Network	O
Graphics	O
(	O
PNG	O
)	O
format	O
.	O
</s>
<s>
In	O
contrast	O
to	O
the	O
DCT	B-General_Concept
algorithm	O
used	O
by	O
the	O
original	O
JPEG	O
format	O
,	O
JPEG	O
2000	O
instead	O
uses	O
discrete	O
wavelet	B-Algorithm
transform	I-Algorithm
(	O
DWT	O
)	O
algorithms	O
.	O
</s>
<s>
It	O
uses	O
the	O
CDF	O
9/7	O
wavelet	B-Algorithm
transform	I-Algorithm
(	O
developed	O
by	O
Ingrid	O
Daubechies	O
in	O
1992	O
)	O
for	O
its	O
lossy	B-Algorithm
compression	I-Algorithm
algorithm	O
,	O
and	O
the	O
Le	O
Gall	O
–	O
Tabatabai	O
(	O
LGT	O
)	O
5/3	O
wavelet	B-Algorithm
transform	I-Algorithm
(	O
developed	O
by	O
Didier	O
Le	O
Gall	O
and	O
Ali	O
J	O
.	O
Tabatabai	O
in	O
1988	O
)	O
for	O
its	O
lossless	B-Algorithm
compression	I-Algorithm
algorithm	O
.	O
</s>
<s>
JPEG	O
2000	O
technology	O
,	O
which	O
includes	O
the	O
Motion	O
JPEG	O
2000	O
extension	O
,	O
was	O
selected	O
as	O
the	O
video	B-Algorithm
coding	I-Algorithm
standard	I-Algorithm
for	O
digital	O
cinema	O
in	O
2004	O
.	O
</s>
