i
i
i
i
i
i
i
i
216 7. Advanced Shading
A few words are in order about this equation. First, in older texts, it is
often given as Y =0.30R +0.59G +0.11B. Poynton [1028] discusses how
this form is based on older NTSC phosphors; the equation given above is
based on modern CRT and HDTV phosphors. This brings us full circle
to the photometric curve shown on page 209. This curve, representing
how a standard observer’s eye responds to light of various wavelengths, is
multiplied by the spectra of the three phosphors, and each resulting curve
is integrated. The three resulting weights are what form the luminance
equation above. The reason that a grayscale intensity value is not equal
parts red, green, and blue is because the eye has a different sensitivity to
various wavelengths of light.
The gamut affects the rendering process in a number of ways. The
gamuts and white point locations of monitors vary, both because of the
physical makeup and due to the adjustment of brightness and contrast,
meaning that how a color looks on one monitor will not be the same as
on another. The gamuts of printers differ more still from monitors, so
that there are colors that can display well on one printer, but not another.
Custom inks can be used to print outside the normal gamut, for both
artistic effect and for creating items such as banknotes. Scanners also have
a gamut of colors they can record, and so have similar mismatches and
limitations. Monitor gamuts are always triangles, by the nature of how
they produce colors. Film, print, and other media have gamuts that are
roughly triangular, but with curved edges or other irregularities, due to the
characteristics of the chemicals or inks used to capture color.
While any given spectrum can be precisely represented by an RGB
triplet, it does not follow that using RGB colors for materials and lights
is a precise basis for computations. In other words, multiplying two RGB
colors to obtain an RGB result is not the same as multiplying two spectra
together and then converting the resulting spectrum to RGB. As a simple
thought experiment, imagine a light with a spectrum going from 550 to
700 nm, and a material that has a spectral response of 400 nm to 549
nm. Ignoring red and blue, these two spectra will each convert to RGB
triplets that both have at least some positive green component. When
these two RGB values are multiplied together, the green component in
the result will also be positive. However, the spectra themselves, when
multiplied together, yield an entirely black spectrum, which would have no
green component. That said, while experiments have been done showing
differences when spectra are used instead of RGBs, in practice multiplying
RGBs together works surprisingly well [111].
The CIE XYZ system is useful for precise description, but there are
many other color spaces with different strengths and uses. For example,
CIELUV and CIELAB define color spaces that are more perceptually uni-
form [1225]. Color pairs that are perceptibly different by the same amount