<s>
Photometric	B-Algorithm
stereo	I-Algorithm
is	O
a	O
technique	O
in	O
computer	B-Application
vision	I-Application
for	O
estimating	O
the	O
surface	O
normals	O
of	O
objects	O
by	O
observing	O
that	O
object	O
under	O
different	O
lighting	O
conditions	O
.	O
</s>
<s>
The	O
special	O
case	O
where	O
the	O
data	O
is	O
a	O
single	O
image	O
is	O
known	O
as	O
shape	B-Algorithm
from	I-Algorithm
shading	I-Algorithm
,	O
and	O
was	O
analyzed	O
by	O
B	O
.	O
K	O
.	O
P	O
.	O
Horn	O
in	O
1989	O
.	O
</s>
<s>
Photometric	B-Algorithm
stereo	I-Algorithm
has	O
since	O
been	O
generalized	O
to	O
many	O
other	O
situations	O
,	O
including	O
extended	O
light	O
sources	O
and	O
non-Lambertian	O
surface	O
finishes	O
.	O
</s>
<s>
The	O
classical	O
photometric	B-Algorithm
stereo	I-Algorithm
problem	O
concerns	O
itself	O
only	O
with	O
Lambertian	O
surfaces	O
,	O
with	O
perfectly	O
diffuse	O
reflection	O
.	O
</s>
<s>
Computer	B-Application
vision	I-Application
followed	O
a	O
similar	O
course	O
with	O
photometric	B-Algorithm
stereo	I-Algorithm
.	O
</s>
<s>
This	O
puts	O
constraints	O
on	O
the	O
possible	O
normal	O
vectors	O
the	O
surface	O
may	O
have	O
,	O
and	O
reduces	O
the	O
photometric	B-Algorithm
stereo	I-Algorithm
problem	O
to	O
an	O
interpolation	O
between	O
measurements	O
.	O
</s>
<s>
However	O
,	O
such	O
methods	O
are	O
still	O
fairly	O
restrictive	O
in	O
photometric	B-Algorithm
stereo	I-Algorithm
.	O
</s>
<s>
Better	O
results	O
have	O
been	O
achieved	O
with	O
structured	B-General_Concept
light	I-General_Concept
.	O
</s>
