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634 24. Global Illumination
where E(x
) is the radiant exitance (power per unit area) at point x
,andn is the
Phong exponent. You get a diffuse light for n =1. If the light is non-uniform
across its area, e.g., as a television set is, then E will not be a constant.
Frequently Asked Questions
• My pixel values are no longer in some sensible zero-to-one range. What
should I display?
You should use one of the tone reproduction techniques described in Chapter 23.
• What global illumination techniques are used in practice?
For batch rendering of complex scenes, path tracing with one level of reflection
is often used. Path tracing is often augmented with a particle tracing prepro-
cess as described in Jensen’s book in the chapter notes. For walkthrough games,
some form of world-space preprocess is often used, such as the particle tracing
described in this chapter. For scenes with very complicated specular transport, an
elegant but involved method, Metropolis Light Transport (Veach & Guibas, 1997)
may be the best choice.
• How does the ambient component relate to global illumination?
For diffuse scenes, the radiance of a surface is proportional to the product of the
irradiance at the surface and the reflectance of the surface. The ambient com-
ponent is just an approximation to the irradiance scaled by the inverse of π.So
although it is a crude approximation, there can be some methodology to guessing
it (M. F. Cohen et al., 1988), and it is probably more accurate than doing nothing,
i.e., using zero for the ambient term. Because the indirect irradiance can vary
widely within a scene, using a different constant for each surface can be used for
better results rather than using a global ambient term.
• Why do most algorithms compute direct lighting using traditional ray
tracing?
Although global illumination algorithms automatically compute direct lighting,
and it is in fact slightly more complicated to make them compute only indi-
rect lighting, it is usually faster to compute direct lighting separately. There are
three reasons for this. First, indirect lighting tends to be smooth compared to