<s>
In	O
computer	B-Application
vision	I-Application
,	O
visual	B-General_Concept
descriptors	I-General_Concept
or	O
image	B-General_Concept
descriptors	I-General_Concept
are	O
descriptions	O
of	O
the	O
visual	B-Algorithm
features	I-Algorithm
of	O
the	O
contents	O
in	O
images	O
,	O
videos	O
,	O
or	O
algorithms	O
or	O
applications	O
that	O
produce	O
such	O
descriptions	O
.	O
</s>
<s>
This	O
system	O
can	O
be	O
compared	O
to	O
the	O
search	B-Application
engines	I-Application
for	O
textual	O
contents	O
.	O
</s>
<s>
Descriptors	O
are	O
the	O
first	O
step	O
to	O
find	O
out	O
the	O
connection	O
between	O
pixels	B-Algorithm
contained	O
in	O
a	O
digital	B-Algorithm
image	I-Algorithm
and	O
what	O
humans	O
recall	O
after	O
having	O
observed	O
an	O
image	O
or	O
a	O
group	O
of	O
images	O
after	O
some	O
minutes	O
.	O
</s>
<s>
Visual	B-General_Concept
descriptors	I-General_Concept
are	O
divided	O
in	O
two	O
main	O
groups	O
:	O
</s>
<s>
They	O
observe	O
the	O
region	O
homogeneity	O
and	O
the	O
histograms	B-Algorithm
of	O
these	O
region	O
borders	O
.	O
</s>
<s>
However	O
,	O
this	O
information	O
can	O
only	O
be	O
extracted	O
by	O
means	O
of	O
a	O
segmentation	B-Algorithm
similar	O
to	O
the	O
one	O
that	O
the	O
human	O
visual	O
system	O
implements	O
.	O
</s>
<s>
Nowadays	O
,	O
such	O
a	O
segmentation	B-Algorithm
system	O
is	O
not	O
available	O
yet	O
,	O
however	O
there	O
exists	O
a	O
serial	O
of	O
algorithms	O
which	O
are	O
considered	O
to	O
be	O
a	O
good	O
approximation	O
.	O
</s>
<s>
These	O
descriptors	O
describe	O
regions	O
,	O
contours	O
and	O
shapes	O
for	O
2D	B-Algorithm
images	O
and	O
for	O
3D	O
volumes	O
.	O
</s>
<s>
This	O
last	O
information	O
is	O
provided	O
by	O
the	O
capture	O
device	O
,	O
whereas	O
the	O
rest	O
is	O
implemented	O
by	O
means	O
of	O
image	B-Algorithm
processing	I-Algorithm
.	O
</s>
<s>
Multimedia	O
documents	O
search	B-Application
engines	I-Application
and	O
classifiers	O
.	O
</s>
<s>
Digital	O
library	O
:	O
visual	B-General_Concept
descriptors	I-General_Concept
allow	O
a	O
very	O
detailed	O
and	O
concrete	O
search	O
of	O
any	O
video	O
or	O
image	O
by	O
means	O
of	O
different	O
search	O
parameters	O
.	O
</s>
