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Mie particles:
H
R
= 7994
m
,
H
M
= 1200
m
[Nishita et al. 93]. Both scattering
and absorption are proportional to the particle density, thus scattering/absorp-
tion coecient at altitude
h
is given by scaling the appropriate coecient at sea
level with the factor
e
−h/H
.
2.4.2 Scattering Integral and Aerial Perspective
In our derivation of the airlight integral, we will follow a single scattering model
that assumes that sunlight can only be scattered once before it reaches the cam-
era. This is a reasonable approximation for day time. During twilight, multiple
scattering becomes more important and should be considered in the production of
realistic images [Haber et al. 05]. Still, a single scattering model produces reason-
ably convincing results. As we understand it, the only real-time method that ap-
proximates multiple scattering was proposed by Bruneton and Neyret [Bruneton
and Neyret 08]. It requires a 4D lookup table with nonlinear parameterization.
Performing multiple lookups into the table at runtime is quite expensive.
Consider some point
P
on the view ray starting at camera location
C
and
terminating at point
O
(Figure 2.2). If the ray does not hit Earth or the camera
is located outside the atmosphere, then either
O
or
C
is assumed to be the
corresponding intersection of the ray with the top of the atmosphere. The amount
of light that reaches
P
after attenuation by air molecules and aerosols can be
expressed as
L
Sun
·
e
−T
(
A→P
)
,where
L
Sun
is the sunlight radiance before entering
the atmosphere, and
A
is the point on the top of the atmosphere through which
the light reached
P
.The
T
(
A
B
) term is called optical depth along the path
from point
A
to point
B
. It is essentially the integral of the total extinction
→
L
Sun
A
φ
θ
C
P
O
h
R
Earth
C
Earth
Figure 2.2.
Scattering in the atmosphere.
