<s>
Global	B-Algorithm
illumination	I-Algorithm
(	O
GI	O
)	O
,	O
or	O
indirect	B-Algorithm
illumination	I-Algorithm
,	O
is	O
a	O
group	O
of	O
algorithms	O
used	O
in	O
3D	O
computer	O
graphics	O
that	O
are	O
meant	O
to	O
add	O
more	O
realistic	O
lighting	O
to	O
3D	O
scenes	O
.	O
</s>
<s>
Such	O
algorithms	O
take	O
into	O
account	O
not	O
only	O
the	O
light	O
that	O
comes	O
directly	O
from	O
a	O
light	O
source	O
(	O
direct	O
illumination	O
)	O
,	O
but	O
also	O
subsequent	O
cases	O
in	O
which	O
light	O
rays	O
from	O
the	O
same	O
source	O
are	O
reflected	O
by	O
other	O
surfaces	O
in	O
the	O
scene	O
,	O
whether	O
reflective	O
or	O
not	O
(	O
indirect	B-Algorithm
illumination	I-Algorithm
)	O
.	O
</s>
<s>
Theoretically	O
,	O
reflections	O
,	O
refractions	O
,	O
and	O
shadows	O
are	O
all	O
examples	O
of	O
global	B-Algorithm
illumination	I-Algorithm
,	O
because	O
when	O
simulating	O
them	O
,	O
one	O
object	O
affects	O
the	O
rendering	O
of	O
another	O
(	O
as	O
opposed	O
to	O
an	O
object	O
being	O
affected	O
only	O
by	O
a	O
direct	O
source	O
of	O
light	O
)	O
.	O
</s>
<s>
In	O
practice	O
,	O
however	O
,	O
only	O
the	O
simulation	O
of	O
diffuse	O
inter-reflection	O
or	O
caustics	O
is	O
called	O
global	B-Algorithm
illumination	I-Algorithm
.	O
</s>
<s>
Images	O
rendered	O
using	O
global	B-Algorithm
illumination	I-Algorithm
algorithms	I-Algorithm
often	O
appear	O
more	O
photorealistic	O
than	O
those	O
using	O
only	O
direct	O
illumination	O
algorithms	O
.	O
</s>
<s>
One	O
common	O
approach	O
is	O
to	O
compute	O
the	O
global	B-Algorithm
illumination	I-Algorithm
of	O
a	O
scene	O
and	O
store	O
that	O
information	O
with	O
the	O
geometry	O
(	O
e.g.	O
,	O
radiosity	O
)	O
.	O
</s>
<s>
Radiosity	O
,	O
ray	B-Algorithm
tracing	I-Algorithm
,	O
beam	B-Algorithm
tracing	I-Algorithm
,	O
cone	B-Algorithm
tracing	I-Algorithm
,	O
path	B-Algorithm
tracing	I-Algorithm
,	O
volumetric	B-Algorithm
path	I-Algorithm
tracing	I-Algorithm
,	O
Metropolis	B-Algorithm
light	I-Algorithm
transport	I-Algorithm
,	O
ambient	O
occlusion	O
,	O
photon	B-Application
mapping	I-Application
,	O
signed	O
distance	O
field	O
and	O
image-based	B-Algorithm
lighting	I-Algorithm
are	O
all	O
examples	O
of	O
algorithms	O
used	O
in	O
global	B-Algorithm
illumination	I-Algorithm
,	O
some	O
of	O
which	O
may	O
be	O
used	O
together	O
to	O
yield	O
results	O
that	O
are	O
not	O
fast	O
,	O
but	O
accurate	O
.	O
</s>
<s>
These	O
algorithms	O
model	O
diffuse	O
inter-reflection	O
which	O
is	O
a	O
very	O
important	O
part	O
of	O
global	B-Algorithm
illumination	I-Algorithm
;	O
however	O
most	O
of	O
these	O
(	O
excluding	O
radiosity	O
)	O
also	O
model	O
specular	O
reflection	O
,	O
which	O
makes	O
them	O
more	O
accurate	O
algorithms	O
to	O
solve	O
the	O
lighting	O
equation	O
and	O
provide	O
a	O
more	O
realistically	O
illuminated	O
scene	O
.	O
</s>
<s>
The	O
algorithms	O
used	O
to	O
calculate	O
the	O
distribution	O
of	O
light	O
energy	O
between	O
surfaces	O
of	O
a	O
scene	O
are	O
closely	O
related	O
to	O
heat	O
transfer	O
simulations	O
performed	O
using	O
finite-element	B-Application
methods	I-Application
in	O
engineering	O
design	O
.	O
</s>
<s>
Achieving	O
accurate	O
computation	O
of	O
global	B-Algorithm
illumination	I-Algorithm
in	O
real-time	O
remains	O
difficult	O
.	O
</s>
<s>
In	O
real-time	O
3D	O
graphics	O
,	O
the	O
diffuse	O
inter-reflection	O
component	O
of	O
global	B-Algorithm
illumination	I-Algorithm
is	O
sometimes	O
approximated	O
by	O
an	O
"	O
ambient	O
"	O
term	O
in	O
the	O
lighting	O
equation	O
,	O
which	O
is	O
also	O
called	O
"	O
ambient	O
lighting	O
"	O
or	O
"	O
ambient	O
color	O
"	O
in	O
3D	O
software	O
packages	O
.	O
</s>
<s>
Though	O
this	O
method	O
of	O
approximation	O
(	O
also	O
known	O
as	O
a	O
"	O
cheat	O
"	O
because	O
it	O
's	O
not	O
really	O
a	O
global	B-Algorithm
illumination	I-Algorithm
method	O
)	O
is	O
easy	O
to	O
perform	O
computationally	O
,	O
when	O
used	O
alone	O
it	O
does	O
not	O
provide	O
an	O
adequately	O
realistic	O
effect	O
.	O
</s>
<s>
More	O
and	O
more	O
specialized	O
algorithms	O
are	O
used	O
in	O
3D	O
programs	O
that	O
can	O
effectively	O
simulate	O
the	O
global	B-Algorithm
illumination	I-Algorithm
.	O
</s>
<s>
Well	O
known	O
algorithms	O
for	O
computing	O
global	B-Algorithm
illumination	I-Algorithm
include	O
path	B-Algorithm
tracing	I-Algorithm
,	O
photon	B-Application
mapping	I-Application
and	O
radiosity	O
.	O
</s>
<s>
Another	O
way	O
to	O
simulate	O
real	O
global	B-Algorithm
illumination	I-Algorithm
is	O
the	O
use	O
of	O
high-dynamic-range	O
images	O
(	O
HDRIs	O
)	O
,	O
also	O
known	O
as	O
environment	O
maps	O
,	O
which	O
encircle	O
and	O
illuminate	O
the	O
scene	O
.	O
</s>
<s>
This	O
process	O
is	O
known	O
as	O
image-based	B-Algorithm
lighting	I-Algorithm
.	O
</s>
<s>
Method	O
Description/Notes	O
Ray	B-Algorithm
tracing	I-Algorithm
Several	O
enhanced	O
variants	O
exist	O
for	O
solving	O
problems	O
related	O
to	O
sampling	O
,	O
aliasing	O
,	O
and	O
soft	O
shadows	O
:	O
Distributed	B-Algorithm
ray	I-Algorithm
tracing	I-Algorithm
,	O
cone	B-Algorithm
tracing	I-Algorithm
,	O
and	O
beam	B-Algorithm
tracing	I-Algorithm
.	O
</s>
<s>
Path	B-Algorithm
tracing	I-Algorithm
Unbiased	O
,	O
variant	O
:	O
Bi-directional	O
path	B-Algorithm
tracing	I-Algorithm
and	O
energy	O
redistribution	O
path	B-Algorithm
tracing	I-Algorithm
Photon	B-Application
mapping	I-Application
Consistent	O
,	O
biased	O
;	O
enhanced	O
variants	O
:	O
Progressive	O
photon	B-Application
mapping	I-Application
,	O
stochastic	O
progressive	O
photon	B-Application
mapping	I-Application
(	O
)	O
Lightcuts	O
Enhanced	O
variants	O
:	O
Multidimensional	O
lightcuts	O
and	O
bidirectional	O
lightcuts	O
Point	O
based	O
global	B-Algorithm
illumination	I-Algorithm
Extensively	O
used	O
in	O
movie	O
animations	O
Radiosity	O
Finite	B-Application
element	I-Application
method	I-Application
,	O
very	O
good	O
for	O
precomputations	O
.	O
</s>
<s>
Improved	O
versions	O
are	O
instant	O
radiosity	O
and	O
bidirectional	O
instant	O
radiosity	O
Metropolis	B-Algorithm
light	I-Algorithm
transport	I-Algorithm
Builds	O
upon	O
bi-directional	O
path	B-Algorithm
tracing	I-Algorithm
,	O
unbiased	O
,	O
and	O
multiplexed	O
Spherical	B-General_Concept
harmonic	I-General_Concept
lighting	I-General_Concept
Encodes	O
global	B-Algorithm
illumination	I-Algorithm
results	O
for	O
real-time	B-General_Concept
rendering	I-General_Concept
of	O
static	O
scenes	O
Ambient	O
occlusion	O
Not	O
a	O
physically	O
correct	O
method	O
,	O
but	O
it	O
gives	O
good	O
results	O
in	O
general	O
.	O
</s>
<s>
Voxel-based	O
global	B-Algorithm
illumination	I-Algorithm
Several	O
variants	O
exist	O
,	O
including	O
voxel	O
cone	B-Algorithm
tracing	I-Algorithm
global	B-Algorithm
illumination	I-Algorithm
,	O
sparse	O
voxel	O
octree	O
global	B-Algorithm
illumination	I-Algorithm
,	O
and	O
voxel	O
global	B-Algorithm
illumination	I-Algorithm
(	O
VXGI	B-Algorithm
)	O
Light	O
propagation	O
volumes	O
global	B-Algorithm
illumination	I-Algorithm
Light	O
propagation	O
volumes	O
is	O
a	O
technique	O
to	O
approximately	O
achieve	O
global	B-Algorithm
illumination	I-Algorithm
(	O
GI	O
)	O
in	O
real-time	O
.	O
</s>
