<s>
A	O
compression	B-Algorithm
artifact	I-Algorithm
(	O
or	O
artefact	O
)	O
is	O
a	O
noticeable	O
distortion	O
of	O
media	O
(	O
including	O
images	O
,	O
audio	O
,	O
and	O
video	O
)	O
caused	O
by	O
the	O
application	O
of	O
lossy	B-Algorithm
compression	I-Algorithm
.	O
</s>
<s>
Lossy	B-Algorithm
data	I-Algorithm
compression	I-Algorithm
involves	O
discarding	O
some	O
of	O
the	O
media	O
's	O
data	O
so	O
that	O
it	O
becomes	O
small	O
enough	O
to	O
be	O
stored	O
within	O
the	O
desired	O
disk	B-General_Concept
space	I-General_Concept
or	O
transmitted	B-General_Concept
(	O
streamed	O
)	O
within	O
the	O
available	O
bandwidth	O
(	O
known	O
as	O
the	O
data	O
rate	O
or	O
bit	O
rate	O
)	O
.	O
</s>
<s>
If	O
the	O
compressor	O
cannot	O
store	O
enough	O
data	O
in	O
the	O
compressed	O
version	O
,	O
the	O
result	O
is	O
a	O
loss	O
of	O
quality	O
,	O
or	O
introduction	O
of	O
artifacts	B-Algorithm
.	O
</s>
<s>
The	O
compression	B-General_Concept
algorithm	I-General_Concept
may	O
not	O
be	O
intelligent	O
enough	O
to	O
discriminate	O
between	O
distortions	O
of	O
little	O
subjective	O
importance	O
and	O
those	O
objectionable	O
to	O
the	O
user	O
.	O
</s>
<s>
The	O
most	O
common	O
digital	O
compression	B-Algorithm
artifacts	I-Algorithm
are	O
DCT	B-General_Concept
blocks	O
,	O
caused	O
by	O
the	O
discrete	B-General_Concept
cosine	I-General_Concept
transform	I-General_Concept
(	O
DCT	B-General_Concept
)	O
compression	B-General_Concept
algorithm	I-General_Concept
used	O
in	O
many	O
digital	O
media	O
standards	O
,	O
such	O
as	O
JPEG	O
,	O
MP3	B-Application
,	O
and	O
MPEG	O
video	O
file	O
formats	O
.	O
</s>
<s>
These	O
compression	B-Algorithm
artifacts	I-Algorithm
appear	O
when	O
heavy	O
compression	O
is	O
applied	O
,	O
and	O
occur	O
often	O
in	O
common	O
digital	O
media	O
,	O
such	O
as	O
DVDs	O
,	O
common	O
computer	O
file	O
formats	O
such	O
as	O
JPEG	O
,	O
MP3	B-Application
and	O
MPEG	O
files	O
,	O
and	O
some	O
alternatives	O
to	O
the	O
compact	O
disc	O
,	O
such	O
as	O
Sony	O
's	O
MiniDisc	O
format	O
.	O
</s>
<s>
Uncompressed	B-General_Concept
media	O
(	O
such	O
as	O
on	O
Laserdiscs	O
,	O
Audio	O
CDs	O
,	O
and	O
WAV	O
files	O
)	O
or	O
losslessly	B-Algorithm
compressed	I-Algorithm
media	O
(	O
such	O
as	O
FLAC	B-Application
or	O
PNG	O
)	O
do	O
not	O
suffer	O
from	O
compression	B-Algorithm
artifacts	I-Algorithm
.	O
</s>
<s>
The	O
minimization	O
of	O
perceivable	O
artifacts	B-Algorithm
is	O
a	O
key	O
goal	O
in	O
implementing	O
a	O
lossy	B-Algorithm
compression	I-Algorithm
algorithm	O
.	O
</s>
<s>
However	O
,	O
artifacts	B-Algorithm
are	O
occasionally	O
intentionally	O
produced	O
for	O
artistic	O
purposes	O
,	O
a	O
style	O
known	O
as	O
glitch	O
art	O
or	O
datamoshing	O
.	O
</s>
<s>
Technically	O
speaking	O
,	O
a	O
compression	B-Algorithm
artifact	I-Algorithm
is	O
a	O
particular	O
class	O
of	O
data	O
error	O
that	O
is	O
usually	O
the	O
consequence	O
of	O
quantization	B-Algorithm
in	O
lossy	B-Algorithm
data	I-Algorithm
compression	I-Algorithm
.	O
</s>
<s>
Where	O
transform	B-Algorithm
coding	I-Algorithm
is	O
used	O
,	O
it	O
typically	O
assumes	O
the	O
form	O
of	O
one	O
of	O
the	O
basis	O
functions	O
of	O
the	O
coder	O
's	O
transform	O
space	O
.	O
</s>
<s>
When	O
performing	O
block-based	O
discrete	B-General_Concept
cosine	I-General_Concept
transform	I-General_Concept
(	O
DCT	B-General_Concept
)	O
coding	O
for	O
quantization	B-Algorithm
,	O
as	O
in	O
JPEG-compressed	O
images	O
,	O
several	O
types	O
of	O
artifacts	B-Algorithm
can	O
appear	O
.	O
</s>
<s>
Other	O
lossy	B-Algorithm
algorithms	O
,	O
which	O
use	O
pattern	B-Language
matching	I-Language
to	O
deduplicate	O
similar	O
symbols	O
,	O
are	O
prone	O
to	O
introducing	O
hard	O
to	O
detect	O
errors	O
in	O
printed	O
text	O
.	O
</s>
<s>
This	O
has	O
been	O
observed	O
to	O
happen	O
with	O
JBIG2	B-Algorithm
in	O
certain	O
photocopier	O
machines	O
.	O
</s>
<s>
At	O
low	O
bit	O
rates	O
,	O
any	O
lossy	B-Algorithm
block-based	O
coding	O
scheme	O
introduces	O
visible	O
artifacts	B-Algorithm
in	O
pixel	O
blocks	O
and	O
at	O
block	O
boundaries	O
.	O
</s>
<s>
These	O
boundaries	O
can	O
transform	O
block	O
boundaries	O
,	O
prediction	O
block	O
boundaries	O
,	O
or	O
both	O
,	O
and	O
may	O
coincide	O
with	O
macroblock	B-General_Concept
boundaries	O
.	O
</s>
<s>
The	O
term	O
macroblocking	O
is	O
commonly	O
used	O
regardless	O
of	O
the	O
artifact	B-Algorithm
's	O
cause	O
.	O
</s>
<s>
Block-artifacts	O
are	O
a	O
result	O
of	O
the	O
very	O
principle	O
of	O
block	O
transform	B-Algorithm
coding	I-Algorithm
.	O
</s>
<s>
The	O
transform	O
(	O
for	O
example	O
the	O
discrete	B-General_Concept
cosine	I-General_Concept
transform	I-General_Concept
)	O
is	O
applied	O
to	O
a	O
block	O
of	O
pixels	O
,	O
and	O
to	O
achieve	O
lossy	B-Algorithm
compression	I-Algorithm
,	O
the	O
transform	O
coefficients	O
of	O
each	O
block	O
are	O
quantized	B-Algorithm
.	O
</s>
<s>
The	O
lower	O
the	O
bit	O
rate	O
,	O
the	O
more	O
coarsely	O
the	O
coefficients	O
are	O
represented	O
and	O
the	O
more	O
coefficients	O
are	O
quantized	B-Algorithm
to	O
zero	O
.	O
</s>
<s>
Statistically	O
,	O
images	O
have	O
more	O
low-frequency	O
than	O
high-frequency	O
content	O
,	O
so	O
it	O
is	O
the	O
low-frequency	O
content	O
that	O
remains	O
after	O
quantization	B-Algorithm
,	O
which	O
results	O
in	O
blurry	O
,	O
low-resolution	O
blocks	O
.	O
</s>
<s>
Because	O
this	O
quantization	B-Algorithm
process	O
is	O
applied	O
individually	O
in	O
each	O
block	O
,	O
neighboring	O
blocks	O
quantize	O
coefficients	O
differently	O
.	O
</s>
<s>
Many	O
photo	O
editing	O
programs	O
,	O
for	O
instance	O
,	O
have	O
proprietary	O
JPEG	B-Algorithm
artifact	I-Algorithm
reduction	O
algorithms	O
built-in	O
.	O
</s>
<s>
Boundary	O
artifact	B-Algorithm
in	O
JPEG	O
can	O
be	O
turned	O
into	O
more	O
pleasing	O
"	O
grains	O
"	O
not	O
unlike	O
those	O
in	O
high	O
ISO	O
photographic	O
films	O
.	O
</s>
<s>
Instead	O
of	O
just	O
multiplying	O
the	O
quantized	B-Algorithm
coefficients	O
with	O
the	O
quantisation	B-Algorithm
step	O
pertaining	O
to	O
the	O
2D-frequency	O
,	O
intelligent	O
noise	O
in	O
the	O
form	O
of	O
a	O
random	O
number	O
in	O
the	O
interval	O
can	O
be	O
added	O
to	O
the	O
dequantized	O
coefficient	O
.	O
</s>
<s>
The	O
ringing	O
issue	O
can	O
be	O
reduced	O
at	O
encode	O
time	O
by	O
overshooting	O
the	O
DCT	B-General_Concept
values	O
,	O
clamping	O
the	O
rings	O
away	O
.	O
</s>
<s>
Posterization	B-Algorithm
generally	O
only	O
happens	O
at	O
low	O
quality	O
,	O
when	O
the	O
DC	O
values	O
are	O
given	O
too	O
little	O
importance	O
.	O
</s>
<s>
Tuning	O
the	O
quantization	B-Algorithm
table	O
helps	O
.	O
</s>
<s>
When	O
motion	O
prediction	O
is	O
used	O
,	O
as	O
in	O
MPEG-1	B-Algorithm
,	O
MPEG-2	B-Algorithm
or	O
MPEG-4	B-Algorithm
,	O
compression	B-Algorithm
artifacts	I-Algorithm
tend	O
to	O
remain	O
on	O
several	O
generations	O
of	O
decompressed	O
frames	O
,	O
and	O
move	O
with	O
the	O
optic	O
flow	O
of	O
the	O
image	O
,	O
leading	O
to	O
a	O
peculiar	O
effect	O
,	O
part	O
way	O
between	O
a	O
painting	O
effect	O
and	O
"	O
grime	O
"	O
that	O
moves	O
with	O
objects	O
in	O
the	O
scene	O
.	O
</s>
<s>
Data	O
errors	O
in	O
the	O
compressed	O
bit-stream	O
,	O
possibly	O
due	O
to	O
transmission	O
errors	O
,	O
can	O
lead	O
to	O
errors	O
similar	O
to	O
large	O
quantization	B-Algorithm
errors	O
,	O
or	O
can	O
disrupt	O
the	O
parsing	O
of	O
the	O
data	B-General_Concept
stream	I-General_Concept
entirely	O
for	O
a	O
short	O
time	O
,	O
leading	O
to	O
"	O
break-up	O
"	O
of	O
the	O
picture	O
.	O
</s>
<s>
In	O
MPEG	O
picture	O
coding	O
,	O
these	O
are	O
known	O
as	O
"	O
I-frames	B-General_Concept
"	O
,	O
with	O
the	O
'	O
I	O
 '	O
standing	O
for	O
"	O
intra	O
"	O
.	O
</s>
<s>
Until	O
the	O
next	O
I-frame	B-General_Concept
arrives	O
,	O
the	O
decoder	O
can	O
perform	O
error	B-Error_Name
concealment	I-Error_Name
.	O
</s>
<s>
Block	O
boundary	O
discontinuities	O
can	O
occur	O
at	O
edges	O
of	O
motion	B-Algorithm
compensation	I-Algorithm
prediction	O
blocks	O
.	O
</s>
<s>
In	O
motion	O
compensated	O
video	O
compression	O
,	O
the	O
current	O
picture	O
is	O
predicted	O
by	O
shifting	O
blocks	O
(	O
macroblocks	B-General_Concept
,	O
partitions	O
,	O
or	O
prediction	O
units	O
)	O
of	O
pixels	O
from	O
previously	O
decoded	O
frames	O
.	O
</s>
<s>
Video	B-Algorithm
compression	I-Algorithm
artifacts	I-Algorithm
include	O
cumulative	O
results	O
of	O
compression	O
of	O
the	O
comprising	O
still	O
images	O
,	O
for	O
instance	O
ringing	O
or	O
other	O
edge	O
busyness	O
in	O
successive	O
still	O
images	O
appear	O
in	O
sequence	O
as	O
a	O
shimmering	O
blur	O
of	O
dots	O
around	O
edges	O
,	O
called	O
mosquito	O
noise	O
,	O
as	O
they	O
resemble	O
mosquitoes	O
swarming	O
around	O
the	O
object	O
.	O
</s>
<s>
The	O
so-called	O
"	O
mosquito	O
noise	O
"	O
is	O
caused	O
by	O
the	O
block-based	O
discrete	B-General_Concept
cosine	I-General_Concept
transform	I-General_Concept
(	O
DCT	B-General_Concept
)	O
compression	B-General_Concept
algorithm	I-General_Concept
used	O
in	O
most	O
video	B-Algorithm
coding	I-Algorithm
standards	I-Algorithm
,	O
such	O
as	O
the	O
MPEG	O
formats	O
.	O
</s>
<s>
The	O
artifacts	B-Algorithm
at	O
block	O
boundaries	O
can	O
be	O
reduced	O
by	O
applying	O
a	O
deblocking	B-General_Concept
filter	I-General_Concept
.	O
</s>
<s>
As	O
in	O
still	O
image	O
coding	O
,	O
it	O
is	O
possible	O
to	O
apply	O
a	O
deblocking	B-General_Concept
filter	I-General_Concept
to	O
the	O
decoder	O
output	O
as	O
post-processing	O
.	O
</s>
<s>
In	O
motion-predicted	O
video	B-Algorithm
coding	I-Algorithm
with	O
a	O
closed	O
prediction	O
loop	O
,	O
the	O
encoder	O
uses	O
the	O
decoder	O
output	O
as	O
the	O
prediction	O
reference	O
from	O
which	O
future	O
frames	O
are	O
predicted	O
.	O
</s>
<s>
If	O
this	O
"	O
decoder	O
"	O
performs	O
a	O
deblocking	B-General_Concept
,	O
the	O
deblocked	O
picture	O
is	O
then	O
used	O
as	O
a	O
reference	O
picture	O
for	O
motion	B-Algorithm
compensation	I-Algorithm
,	O
which	O
improves	O
coding	O
efficiency	O
by	O
preventing	O
a	O
propagation	O
of	O
block	B-Algorithm
artifacts	I-Algorithm
across	O
frames	O
.	O
</s>
<s>
This	O
is	O
referred	O
to	O
as	O
an	O
in-loop	O
deblocking	B-General_Concept
filter	I-General_Concept
.	O
</s>
<s>
Standards	O
which	O
specify	O
an	O
in-loop	O
deblocking	B-General_Concept
filter	I-General_Concept
include	O
VC-1	B-Operating_System
,	O
H.263	B-Algorithm
Annex	O
J	O
,	O
H.264/AVC	B-Application
,	O
and	O
H.265/HEVC	B-Algorithm
.	O
</s>
<s>
Lossy	B-Algorithm
audio	O
compression	O
typically	O
works	O
with	O
a	O
psychoacoustic	O
model	O
—	O
a	O
model	O
of	O
human	O
hearing	O
perception	O
.	O
</s>
<s>
Lossy	B-Algorithm
audio	O
formats	O
typically	O
involve	O
the	O
use	O
of	O
a	O
time/frequency	O
domain	O
transform	O
,	O
such	O
as	O
a	O
modified	O
discrete	B-General_Concept
cosine	I-General_Concept
transform	I-General_Concept
.	O
</s>
<s>
A	O
lossy	B-Algorithm
compression	I-Algorithm
technique	O
might	O
identify	O
this	O
quiet	O
tone	O
and	O
attempt	O
to	O
remove	O
it	O
.	O
</s>
<s>
Also	O
,	O
quantization	B-Algorithm
noise	O
can	O
be	O
"	O
hidden	O
"	O
where	O
they	O
would	O
be	O
masked	O
by	O
more	O
prominent	O
sounds	O
.	O
</s>
<s>
When	O
the	O
psychoacoustic	O
model	O
is	O
inaccurate	O
,	O
when	O
the	O
transform	O
block	O
size	O
is	O
restrained	O
,	O
or	O
when	O
aggressive	O
compression	O
is	O
used	O
,	O
this	O
may	O
result	O
in	O
compression	B-Algorithm
artifacts	I-Algorithm
.	O
</s>
<s>
Compression	B-Algorithm
artifacts	I-Algorithm
in	O
compressed	O
audio	O
typically	O
show	O
up	O
as	O
ringing	O
,	O
pre-echo	O
,	O
"	O
birdie	O
artifacts	B-Algorithm
"	O
,	O
drop-outs	O
,	O
rattling	O
,	O
warbling	O
,	O
metallic	O
ringing	O
,	O
an	O
underwater	O
feeling	O
,	O
hissing	O
,	O
or	O
"	O
graininess	O
"	O
.	O
</s>
<s>
An	O
example	O
of	O
compression	B-Algorithm
artifacts	I-Algorithm
in	O
audio	O
is	O
applause	O
in	O
a	O
relatively	O
highly	O
compressed	O
audio	O
file	O
(	O
e.g.	O
</s>
<s>
96	O
kbit/sec	O
MP3	B-Application
)	O
.	O
</s>
<s>
A	O
highly	O
compressed	O
track	O
of	O
applause	O
may	O
have	O
"	O
metallic	O
ringing	O
"	O
and	O
other	O
compression	B-Algorithm
artifacts	I-Algorithm
.	O
</s>
<s>
Compression	B-Algorithm
artifacts	I-Algorithm
may	O
intentionally	O
be	O
used	O
as	O
a	O
visual	O
style	O
,	O
sometimes	O
known	O
as	O
glitch	O
art	O
.	O
</s>
<s>
Rosa	O
Menkman	O
's	O
glitch	O
art	O
makes	O
use	O
of	O
compression	B-Algorithm
artifacts	I-Algorithm
,	O
particularly	O
the	O
discrete	B-General_Concept
cosine	I-General_Concept
transform	I-General_Concept
blocks	O
(	O
DCT	B-General_Concept
blocks	O
)	O
found	O
in	O
most	O
digital	O
media	O
data	B-General_Concept
compression	I-General_Concept
formats	O
such	O
as	O
JPEG	O
digital	B-Algorithm
images	I-Algorithm
and	O
MP3	B-Application
digital	O
audio	O
.	O
</s>
<s>
In	O
still	O
images	O
,	O
an	O
example	O
is	O
Jpegs	O
by	O
German	O
photographer	O
Thomas	O
Ruff	O
,	O
which	O
uses	O
intentional	O
JPEG	B-Algorithm
artifacts	I-Algorithm
as	O
the	O
basis	O
of	O
the	O
picture	O
's	O
style	O
.	O
</s>
<s>
In	O
video	B-Application
art	I-Application
,	O
one	O
technique	O
used	O
is	O
datamoshing	O
,	O
where	O
two	O
videos	O
are	O
interleaved	O
so	O
intermediate	O
frames	O
are	O
interpolated	O
from	O
two	O
separate	O
sources	O
.	O
</s>
<s>
Another	O
technique	O
involves	O
simply	O
transcoding	O
from	O
one	O
lossy	B-Algorithm
video	O
format	O
to	O
another	O
,	O
which	O
exploits	O
the	O
difference	O
in	O
how	O
the	O
separate	O
video	O
codecs	O
process	O
motion	O
and	O
color	O
information	O
.	O
</s>
