Graphics Reference
In-Depth Information
σ
s
[
mm
−
1
σ
a
[
mm
−
1
Material
]
]
η
Refractive
RGB R
G
B
index
Marble
2.19
2.62
3.00
0.0021
0.0041
0.0071
1.5
Skimmed milk
0.70
1.22
1.90
0.0014
0.0025
0.0142
1.3
Whole milk
2.55
3.21
3.77
0.0011
0.0024
0.014
1.3
Skin 1
0.74
0.88
1.01
0.032
0.17
0.48
1.3
Skin 2
1.09
1.59
1.79
0.013
0.070
0.145
1.3
Tabl e 4. 1
Estimated parameters for the dipole-based BSSRDF obtained by fitting measured data.
would be useful. However, they were also aware of its limitations: in addition to
the diffusion approximation, it assumes a infinitely deep homogeneous material
and a perfectly smooth surface.
Rendering with dipoles.
Whether the rendering method employs ray tracing
or uses a scanline approach, the outgoing radiance has to be computed at points
on the surface of each object being rendered. For ordinary surface rendering, this
comes from integrating the BRDF over the set of directions of the light source, or,
if indirect illumination is considered, over all directions above the surface. In the
case of subsurface scattering the BSSRDF has to be integrated over all light source
directions
and over the entire surface
(although in practice the integration can be
restricted to a region near the exit point). The integration is done using a Monte
Carlo sampling of points near the point of incident illumination (
Figure 4.9(a)
)
.
Another advantage of separating the BSSRDF into single and multiple scattering
Negative point light source ( - )
Top view
1
'
−
4
+
3
−
'
t
σ
σ
Rendering point
(exit point)
x
o
r
x
i
Sample points (incident points)
−
Rendering point
(exitant point)
'
t
σ
Positive point
light source ( + )
x
o
Sample point
(incident point)
x
i
(a)
(b)
Figure 4.9
Rendering using the dipole-based BSSRDF. At a “rendering point”
x
o
where the outgoing
radiance is needed, the BSSRDF is integrated by sampling points
x
i
over the surface where
light is incident. (a) A dipole is constructed at each sample point. (b) The sample points
are concentrated near
x
o
to improve efficiency.
