Graphics Reference
In-Depth Information
spectral flux
is the radiant power of light at wavelengths in a differential
wavelength band around
Φ
(
λ
)
.
Spectral radiance
and
spectral irradiance
are defined analogously, i.e., in terms of the spectral flux.
Wavelength dependence of light is perceived by humans as color. Color vi-
sion comes from the
cone cell
photoreceptors in the retina of which humans have
three types, each of which is sensitive to light in a different wavelength band,
corresponding roughly to the perception of red, green, and blue.
Color can be plausibly reproduced by combining varying strengths of light in
the red, green, and blue wavelength bands; most display systems work using this
principle. It is therefore common to use a
trichromatic
(three color) representation
for spectral quantities. In the RGB representation, each value has a separate red,
green, and blue component. Other trichromatic formats are described in Chap-
ter 6.
λ
divided by the width of the band
d
λ
1.2 Modeling the Behavior of Light
1.2.1 Local Illumination and Global Illumination
Modeling of light interaction for photorealistic rendering can be loosely split into
two basic problems. How the surface of an object responds to incident illumina-
tion is called
local illumination
. This includes reflection from the surface, trans-
mission through the surface, and the more complicated problem of subsurface
scattering, which describes how light bounces around below the surface before
leaving it.
In contrast,
global illumination
is concerned with the problem of how the en-
vironment is illuminated as a whole. In particular, global illumination includes
light that reaches a surface indirectly by reflection and scattering from other ob-
jects in the environment. In fact, the term has become almost synonymous with
the computation of indirect illumination. Of course, good local illumination mod-
els are essential to indirect illumination computation, as it involves the transport
(reflection and scattering) of light throughout the environment. Likewise, effec-
tive models of local illumination depend on global requirements. Local and global
illumination are thus intertwined, and there is not always a clear distinction be-
tween them.
1.2.2 Calculation of Illumination
Irradiance produced on a surface by a light source is one form of
direct illumi-
nation
. Equation (1.4) provides a formula for direct illumination from small or
