<s>
Automatic	B-Application
image	I-Application
annotation	I-Application
(	O
also	O
known	O
as	O
automatic	O
image	O
tagging	O
or	O
linguistic	O
indexing	O
)	O
is	O
the	O
process	O
by	O
which	O
a	O
computer	O
system	O
automatically	O
assigns	O
metadata	O
in	O
the	O
form	O
of	O
captioning	O
or	O
keywords	O
to	O
a	O
digital	B-Algorithm
image	I-Algorithm
.	O
</s>
<s>
This	O
application	O
of	O
computer	B-Application
vision	I-Application
techniques	O
is	O
used	O
in	O
image	B-General_Concept
retrieval	I-General_Concept
systems	O
to	O
organize	O
and	O
locate	O
images	O
of	O
interest	O
from	O
a	O
database	O
.	O
</s>
<s>
This	O
method	O
can	O
be	O
regarded	O
as	O
a	O
type	O
of	O
multi-class	B-General_Concept
image	O
classification	O
with	O
a	O
very	O
large	O
number	O
of	O
classes	O
-	O
as	O
large	O
as	O
the	O
vocabulary	O
size	O
.	O
</s>
<s>
Typically	O
,	O
image	B-General_Concept
analysis	I-General_Concept
in	O
the	O
form	O
of	O
extracted	O
feature	B-Algorithm
vectors	I-Algorithm
and	O
the	O
training	O
annotation	O
words	O
are	O
used	O
by	O
machine	O
learning	O
techniques	O
to	O
attempt	O
to	O
automatically	O
apply	O
annotations	O
to	O
new	O
images	O
.	O
</s>
<s>
The	O
first	O
methods	O
learned	O
the	O
correlations	O
between	O
image	B-Algorithm
features	I-Algorithm
and	O
training	O
annotations	O
,	O
then	O
techniques	O
were	O
developed	O
using	O
machine	B-Application
translation	I-Application
to	O
try	O
to	O
translate	O
the	O
textual	O
vocabulary	O
with	O
the	O
'	O
visual	O
vocabulary	O
 '	O
,	O
or	O
clustered	O
regions	O
known	O
as	O
blobs	O
.	O
</s>
<s>
The	O
advantages	O
of	O
automatic	B-Application
image	I-Application
annotation	I-Application
versus	O
content-based	B-Application
image	I-Application
retrieval	I-Application
(	O
CBIR	B-Application
)	O
are	O
that	O
queries	O
can	O
be	O
more	O
naturally	O
specified	O
by	O
the	O
user	O
.	O
</s>
<s>
CBIR	B-Application
generally	O
(	O
at	O
present	O
)	O
requires	O
users	O
to	O
search	O
by	O
image	O
concepts	O
such	O
as	O
color	O
and	O
texture	O
,	O
or	O
finding	O
example	O
queries	O
.	O
</s>
<s>
Certain	O
image	B-Algorithm
features	I-Algorithm
in	O
example	O
images	O
may	O
override	O
the	O
concept	O
that	O
the	O
user	O
is	O
really	O
focusing	O
on	O
.	O
</s>
<s>
The	O
traditional	O
methods	O
of	O
image	B-General_Concept
retrieval	I-General_Concept
such	O
as	O
those	O
used	O
by	O
libraries	O
have	O
relied	O
on	O
manually	O
annotated	O
images	O
,	O
which	O
is	O
expensive	O
and	O
time-consuming	O
,	O
especially	O
given	O
the	O
large	O
and	O
constantly	O
growing	O
image	O
databases	O
in	O
existence	O
.	O
</s>
