Graphics Reference
In-Depth Information
700 nm
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Wavelength (nm)
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AM radio
Microwave
Ultraviolet
Gamma rays
FM radio, TV
Infrared
X-rays
Figure 14.9: The visible spectrum is part of the full electromagnetic spectrum. The color
of light that we perceive from an electromagnetic wave is determined by its frequency.
The relationship between frequency and wavelength is determined by the medium through
which the wave is propagating. (Courtesy of Leonard McMillan)
individual photons. For example, a mixture of “red” and “green” photons appears
yellow, and is mostly indistinguishable from pure “yellow” photons. This
alias-
ing
(i.e., the substitutability of one item for another) is fortunate. It allows dis-
plays to create the appearance of many colors using only three relatively narrow
frequency bands. Digital cameras also rely on this principle—because the image
will be displayed using three frequencies, they only need to measure three.
2
Most
significantly for our purposes, almost all 3D rendering treats photons as belong-
ing to three distinct frequencies (or bands of frequencies), corresponding to red,
green, and blue. This includes film and games; some niche predictive rendering
does simulate more spectral samples. We'll informally refer to rendering with
three “frequencies,” when what we really mean is “rendering with three frequency
bands,” Using only three frequencies in simulation minimizes both the space and
time cost of rendering algorithms. It creates two limitations. The first is that certain
phenomena are impossible to simulate with only three frequencies. For example,
the colors of clothing often appear different under fluorescent light and sunlight,
even though these light sources may themselves appear fairly similar. This is partly
because fluorescent bulbs produce white light by mixing a set of narrow frequency
bands, while photons from the sun span the entire visible spectrum. The second
limitation of using only three frequencies is that renderers, cameras, and displays
rarely use the
same
three frequencies. Each system is able to create the perception
of a slightly different space of colors, called a
gamut.
Some colors may simply be
outside the gamut of a particular device and lost during capture or display. This
also means that the input and output image data for a renderer must be adjusted
based on the color profile of the device. Today most devices automatically convert
to and from a standard color profile, called sRGB, so color shifts are minimized
on such devices but gamut remains a problem.
2. This is not strictly true; Chapter 28 explains why.
