Graphics Reference
In-Depth Information
different cosmetics. Furthermore, for objects with highly peaked reflectance
spectra, the existence of peaks or valleys in the illuminant spectrum can have dras-
tic effects on the reflected light. It's perhaps better to say the following: “Objects
have reflectance spectra, and the human brain is surprisingly good at predicting,
for natural objects with not-too-peaked reflectance spectra, which are common,
how an object seen under unusual illumination (shade, 'colored' light) will look
under illumination by sunlight. This fairly consistent prediction could be called
the 'color' of the object.”
Various claims about how colors mix are commonplace. In the case of paint color
mixes, they're often misleading. For instance, painting with a blue watercolor, let-
ting it dry, and then painting a red stripe over the blue leads to one thing; doing
this in the opposite order leads to another. Mixing the colors before painting
leads to a third. So any claims about mixing of colors must include the mixing
process to be testable. In the case of colors atop others (see Figure 28.14), one
can think of light as being reflected from the top color, from the bottom color after
passing through the top, or from the underlying surface after passing through both.
If we assume that each time light passes through a color-layer, some fraction of
the energy at certain wavelengths is absorbed, this last kind of light passes twice
through each paint layer, while the first kind never passes through any paint layer.
The
Kubelka-Munk
coloring model [Kub54] carries out this analysis in detail.
This mixing problem is further compounded by the difference in the way lights
mix and pigments mix; the distinction here is purely
physical
. If I shine a red and a
green light onto a uniformly reflective piece of white paper, the reflected light will
appear yellow. By contrast, if I have a red paint or dye and apply it to a white piece
of paper, it absorbs colors outside the long-wavelength part of the spectrum so that
only light we perceive as “red” gets reflected. If I mix this with a green paint or
dye that absorbs all light except that in the green-percept part of the spectrum,
the two together will absorb almost all light. If the paints or dyes were ideal,
the result would be black paint; in practice, as noted earlier, we often get a muddy
brown, indicating that very little light is reflected. These two phenomena are given
the misleading names
additive color
and
subtractive color,
respectively; in fact,
it's spectra that are being added or filtered, and the color perception mechanism
remains unchanged, as we said in Section 28.6.1.
Figure 28.14: One color painted atop another. Light can be reflected from the top, from the
bottom after passing through the top, or from the substrate on which the bottom is painted.
Assuming some attenuation for each time the light passes through a paint layer, we get a
model of the reflected light.
