Graphics Reference
In-Depth Information
ω
→
d
L
(
x
,
)
L
L+
d
L
σ
a
(
x
)
ds
Figure 3.7
Absorption along a differential segment. (Courtesy of Pat Hanrahan.)
absorption cross section
. Note that it depends on position in the medium, much
as surface albedo depends on the surface position.
Radiance is also lost when light is diverted by collisions with particles in
the medium. This phenomenon is known as
out-scattering
. Like absorption, the
change in radiance due to out-scattering is proportional to the incident radiance:
,
ω
)=
−
σ
(
)
(
,
ω
)
,
dL
(
x
x
L
x
ds
s
(
)
where the constant
σ
x
is the
scattering coefficient
or
scattering cross-section
s
(
Figure 3.8
)
.
The sum of the absorption and scattering coefficients, the
extinction coefficient
(
extinction cross-section
)
σ
t
=
σ
a
+
σ
s
,
accounts for the total differential attenuation of light:
,
ω
)=
−
σ
t
(
,
ω
)
dL
(
x
x
)
L
(
x
ds
.
When a medium emits light internally it is said to be
emissive
or
luminous
.
Flame is an example. The light produced by a candle flame is a result of a combi-
nation of physical processes. The most significant is the yellowish glowing part,
which is caused by hot microscopic soot particles. These particles are said to be
incandescent
, meaning they produce light due to their high temperature.
The light produced by incandescent particles is usually modeled as
black-body
radiation
.A
black body
absorbs all radiation incident upon it, and the absorbed
d
L
ω
→
(
x
,
)
L
L+
d
L
σ
s
(
x
)
ds
Figure 3.8
Out-scattering from a differential segment. (Courtesy of Pat Hanrahan.)
