pigmented colorants: dependence on media and time - Cornell ...

Figure 4.1: Common phase functions for particles whose diameters are
approximately the size of the wavelength of light. For each, the incident
light ray is from the left; each scattering event is at the intersection of
the axes. Left: The hazy Mie function; middle: the murky Mie function;
right: the Henyey-Greenstein function. Adapted from [G95b]
The results include accurate prediction of overall paint reflectance, absorption, and
covering power as compared to measurements of actual samples.
Approaches similar to this, while capable of simulating all of the effects of sub-
surface scattering, are still computationally very expensive. This is due to the fact
that techniques based on path tracing are particularly inefficient for highly scat-
tering materials (such as milk, skin or paint), in which the light scatters multiple
(often several hundred) times before exiting the material. Moreover, the sampling
nature of Monte Carlo often results in noise, which is not satisfactory for the sub-
tleties in spatially varying <strong>pigmented</strong> materials. Ultimately, while this approach
will eventually converge to the desired solution, this level of detail is somewhat too
complex for calculating information about <strong>pigmented</strong> solutions.
4.2.3 Diffusion approximation
Another approach to handling the appearance of <strong>pigmented</strong> materials is to use
an analytic technique. Jensen notes that light distribution in highly scattering
media tends to be isotropic since each scattering event blurs the light [JMLH01].
Hence, single scattering (where incident light only scatters once before exiting)
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