<s>
Image	B-Algorithm
differencing	I-Algorithm
is	O
an	O
image	B-Algorithm
processing	I-Algorithm
technique	O
used	O
to	O
determine	O
changes	O
between	O
images	O
.	O
</s>
<s>
For	O
this	O
technique	O
to	O
work	O
,	O
the	O
two	O
images	O
must	O
first	O
be	O
aligned	O
so	O
that	O
corresponding	O
points	O
coincide	O
,	O
and	O
their	O
photometric	O
values	O
must	O
be	O
made	O
compatible	O
,	O
either	O
by	O
careful	O
calibration	O
,	O
or	O
by	O
post-processing	O
(	O
using	O
color	B-Algorithm
mapping	I-Algorithm
)	O
.	O
</s>
<s>
Image	B-Algorithm
differencing	I-Algorithm
techniques	O
are	O
commonly	O
used	O
in	O
astronomy	O
to	O
locate	O
objects	O
that	O
fluctuate	O
in	O
brightness	O
or	O
move	O
against	O
the	O
star	O
field	O
.	O
</s>
<s>
The	O
Hutchinson	O
metric	O
can	O
be	O
used	O
to	O
"	O
measure	O
of	O
the	O
discrepancy	O
between	O
two	O
images	O
for	O
use	O
in	O
fractal	B-Application
image	B-Algorithm
processing	I-Algorithm
"	O
.	O
</s>
