Autodesk LIGHTSCAPE, Manual

Page: 260 / 362

Light and Color A
252
Lightscape
Color Matching
Color matching is the process of matching a spot of
colored light with some combination of other lights.
Researchers have found that by mixing various
amounts of three different lights, they can match
most test spots. The only requirement is that no two
lights can be mixed to produce the third.
You can match the color of a test spot by its intensity.
Some test spots cannot be matched directly.
However, all test spots can be matched if one of the
lights is mixed into the test spot. This is often
described as a
negative light . Negative lights do not
exist, but by representing the light shining on the test
spot as negative, all test spots can be described as a
mixture of the three lights.
Although spectra can have different test spots at
each wavelength, color has only three parameters.
This means that there are many more spectra than
colors. Many different spectra can give the same
perceived color. This means that you do not have to
store or transmit all the information in a spectrum
for each color. It also means that a color does not
contain enough information to reproduce the spec-
trum it came from.
Color Spaces
Choosing the three lights to mix defines a color
space. A color space is a convenient way of repre-
senting a color. Given two different sets of three
lights, it is possible to convert from one color space
to another.
Because the relationship between spectra and colors
is linear and the conversion between color spaces is
linear, most operations on color can be done in any
color space and yield identical results.
The problem with all color spaces defined by combi-
nations of three lights is that each color space has
ranges of color that can only be described by nega-
tive lights —ranges of color it cannot physically
reproduce.
Phosphors
The color from a monitor is the result of three
colored phosphors at each pixel mixing at different
intensities. The three phosphors act like the three
lights in the color-matching experiments. These
phosphors are usually described as red, green, and
blue, but each manufacturer uses different sets of
phosphors for its monitors, based on its needs. A
color defined in one color space is used as if it were
defined in another. This means that the same image
shown on two different monitors can look very
different.
If the phosphors for the monitor on which an image
is to be displayed are known, the color space of the
image can be converted to the color space of the
monitor, allowing the image to look the same on
different monitors.
There is an additional problem with monitors that
currently cannot be solved. Because every color
space based on physical lights has colors it cannot
represent (those requiring negative coefficients),
some colors will never show up correctly on a
monitor. These colors are called out-of-gamut
colors, which are generally not a serious problem.
Out-of-gamut colors are very saturated and most
real scenes contain few highly saturated colors.
Computing with Color
When you work with color or spectra, their values
are equivalent for most operations. However, they
are not equivalent when multiplying two colors or
spectra.
This is problematic because Lightscape spends
much of its processing time multiplying colors. In
theory, you can obtain arbitrarily large differences