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

Johnson and Fairchild point out that under some lighting conditions, any trichro-
matic color space as the RGB or ACC will give incorrect results [JF99]. Such a
problem exists when one tries to color match under one lighting condition, only to
appear differently in another lighting situation due to metamerism. As a result,
there is no way for a three-component color representation to capture this effect.
Johnson and Fairchild argue that for color-sensitive applications, full-spectral color
representations are necessary. Also, none of the previous implementations were
truly real-time, interactive systems.
Recently, William Baxter presented a model (denoted as IMPaSTo) as an at-
tempt to address previous Kubelka-Munk issues [BWL04]. Accurate compositing
and rendering of <strong>pigmented</strong> mixtures is done similarly to previous work in graphics
with K-M theory.
The absorption and scattering coefficients were determined from many real-
world oil paints. Multiple tints of different concentrations were made for each of
the 11 pigments, for a total of 71 measurements (including samples only including
the pure pigments). 101 reflectance values were obtained from each paint sample
over the visible spectrum, and K-M absorption and scattering coefficients were
calculated at each wavelength.
The following equation was used to relate the reflectance of a mixture, R∞,i, to
the absorption Kj and scattering Sj of each constituent pigment and their relative
concentrations, cij.
K
=
S mixture,i
j Kjcij
j Sjcij
= (1 − R∞,i) 2
2R∞,i
For pigments not involved with a particular mixture, cij =0.
111
(4.10)
While Johnson and Fairchild showed that full spectral data provides better