Graphics Reference
In-Depth Information
Z
v
,
−
1
Z
r
,
−
1
Z
v
,
0
R
(
r
)
→
A
2
A
3
z
b
=
P
i
P
o
σ
t
1
'
t
r
σ
z
→
Z
r
,
0
d
r
2
A
3
P
o
z
b
=
σ
t
B
Z
v
,
1
T
(
r
)
Z
r
,
1
Figure 4.17
The multipole model for a homogeneous slab. Infinitely many positive and negative
sources are needed to properly satisfy the boundary conditions, but a few suffice in practice.
with distance, so a finite number of sources provides a sufficient approximation.
(The boundary conditions of Equations (4.15) and (4.16) are themselves approx-
imations to the true diffuse boundary condition.) In practice, ten pairs of vir-
tual point light sources usually gives a sufficient approximation, although thinner
regions generally require more sources. The diffuse reflectance computed from
these sources is known as a
multipole approximation
to diffuse multiple scattering
in a thin slab.
The diffuse reflectance profile
R
, the ratio of the radiant exitance at the top
surface to the incident flux, is computed by summing the contributions of each
(
r
)
