<s>
Visual	B-General_Concept
words	I-General_Concept
,	O
as	O
used	O
in	O
image	B-General_Concept
retrieval	I-General_Concept
systems	O
,	O
refer	O
to	O
small	O
parts	O
of	O
an	O
image	O
which	O
carry	O
some	O
kind	O
of	O
information	O
related	O
to	O
the	O
features	O
(	O
such	O
as	O
the	O
color	O
,	O
shape	O
or	O
texture	O
)	O
,	O
or	O
changes	O
occurring	O
in	O
the	O
pixels	B-Algorithm
such	O
as	O
the	O
filtering	O
,	O
low-level	O
feature	O
descriptors	O
(	O
SIFT	B-Algorithm
,	O
SURF	B-Algorithm
,	O
etc	O
...	O
.	O
</s>
<s>
The	O
approaches	O
of	O
text	B-Algorithm
retrieval	I-Algorithm
system	O
(	O
or	O
information	B-Library
retrieval	I-Library
IR	O
system	O
)	O
,	O
which	O
developed	O
over	O
40	O
years	O
,	O
are	O
based	O
on	O
keywords	O
or	O
Term	O
.	O
</s>
<s>
Text-search	B-Application
engines	I-Application
are	O
able	O
quickly	O
to	O
find	O
documents	O
from	O
hundreds	O
or	O
millions	O
(	O
by	O
using	O
vector	O
space	O
model	O
)	O
.	O
</s>
<s>
In	O
the	O
same	O
time	O
of	O
that	O
,	O
text	B-Algorithm
retrieval	I-Algorithm
systems	O
have	O
a	O
huge	O
success	O
,	O
the	O
standard	O
image	B-General_Concept
retrieval	I-General_Concept
systems	O
(	O
like	O
simple	O
search	O
by	O
colors	O
,	O
shapes	O
...	O
etc	O
.	O
)	O
</s>
<s>
Consequently	O
,	O
researchers	O
try	O
to	O
take	O
advantage	O
from	O
text	B-Algorithm
retrieval	I-Algorithm
techniques	O
to	O
apply	O
them	O
to	O
image	B-General_Concept
retrieval	I-General_Concept
.	O
</s>
<s>
That	O
can	O
be	O
by	O
a	O
new	O
kind	O
of	O
vision	O
to	O
understand	O
images	O
as	O
textual	B-General_Concept
documents	I-General_Concept
,	O
which	O
is	O
visual	B-General_Concept
words	I-General_Concept
approach	O
.	O
</s>
<s>
Let	O
's	O
consider	O
that	O
the	O
pixels	B-Algorithm
of	O
an	O
image	O
,	O
which	O
are	O
the	O
smallest	O
parts	O
in	O
a	O
digital	B-Algorithm
images	I-Algorithm
(	O
can	O
not	O
be	O
divided	O
into	O
smaller	O
ones	O
)	O
,	O
are	O
like	O
the	O
letters	O
of	O
an	O
alphabetical	O
language	O
.	O
</s>
<s>
Then	O
,	O
a	O
set	O
of	O
pixels	B-Algorithm
in	O
an	O
image	O
(	O
patches	O
or	O
arrays	O
of	O
pixels	B-Algorithm
)	O
is	O
a	O
word	O
.	O
</s>
<s>
By	O
this	O
view	O
,	O
researchers	O
can	O
take	O
advantage	O
from	O
text	B-Algorithm
retrieval	I-Algorithm
techniques	O
to	O
apply	O
them	O
to	O
image	B-General_Concept
retrieval	I-General_Concept
system	O
.	O
</s>
<s>
The	O
idea	O
is	O
to	O
try	O
to	O
understand	O
the	O
images	O
like	O
a	O
collection	O
of	O
"	O
visual	B-General_Concept
words	I-General_Concept
"	O
.	O
</s>
<s>
Definition	O
1	O
:	O
Visual	B-General_Concept
word	I-General_Concept
:	O
it	O
is	O
a	O
small	O
patch	O
on	O
the	O
image	O
(	O
array	O
of	O
pixels	B-Algorithm
)	O
which	O
can	O
carry	O
any	O
kind	O
of	O
interesting	O
information	O
in	O
any	O
feature	O
space	O
(	O
color	O
changes	O
,	O
texture	O
changes	O
etc	O
...	O
.	O
</s>
<s>
In	O
general	O
visual	B-General_Concept
words	I-General_Concept
(	O
VWs	O
)	O
exist	O
in	O
their	O
feature	O
space	O
of	O
continuous	O
values	O
implying	O
huge	O
number	O
of	O
words	O
and	O
therefore	O
a	O
huge	O
language	O
.	O
</s>
<s>
Since	O
image	B-General_Concept
retrieval	I-General_Concept
systems	O
need	O
to	O
use	O
text	B-Algorithm
retrieval	I-Algorithm
techniques	O
which	O
are	O
dependent	O
on	O
natural	O
languages	O
and	O
this	O
ones	O
have	O
a	O
limit	O
to	O
the	O
number	O
of	O
terms	O
and	O
words	O
,	O
there	O
are	O
important	O
needs	O
to	O
reduce	O
the	O
number	O
of	O
visual	B-General_Concept
words	I-General_Concept
.	O
</s>
<s>
Definition	O
3	O
:	O
Visual	O
language	O
:	O
it	O
's	O
a	O
set	O
of	O
visual	B-General_Concept
words	I-General_Concept
&	O
visual	O
terms	O
,	O
(	O
we	O
can	O
consider	O
the	O
visual	O
terms	O
alone	O
is	O
the	O
“	O
Visual	O
Vocabulary	O
”	O
which	O
will	O
be	O
the	O
reference	O
and	O
the	O
retrieval	O
system	O
will	O
depend	O
on	O
it	O
for	O
retrieve	O
images	O
)	O
.	O
</s>
<s>
Definition	O
4	O
:	O
Bag	B-General_Concept
of	I-General_Concept
visual	I-General_Concept
words	I-General_Concept
:	O
it	O
's	O
a	O
collection	O
of	O
visual	B-General_Concept
words	I-General_Concept
which	O
together	O
can	O
give	O
information	O
about	O
the	O
meaning	O
of	O
the	O
image	O
at	O
all	O
(	O
or	O
parts	O
of	O
it	O
)	O
.	O
</s>
<s>
Based	O
on	O
this	O
kind	O
of	O
image	O
representation	O
,	O
we	O
can	O
then	O
use	O
text	B-Algorithm
retrieval	I-Algorithm
techniques	O
to	O
design	O
an	O
image	B-General_Concept
retrieval	I-General_Concept
system	O
.	O
</s>
<s>
However	O
,	O
since	O
all	O
text	B-Algorithm
retrieval	I-Algorithm
systems	O
depend	O
on	O
terms	O
,	O
the	O
user	O
's	O
query	B-Library
images	O
must	O
be	O
converted	O
into	O
a	O
set	O
of	O
visual	O
terms	O
in	O
the	O
system	O
.	O
</s>
