文档介绍：5
Sources, Shadows,
and Shading
Surfaces are bright or dark for two main reasons: their albedo and the amount of light they are re-
ceiving. A model of how the brightness of a surface is obtained is usually called a shading model.
Shading models are important because with an appropriate shading model we can interpret pixel
values. If the right shading model applies, it is possible to reconstruct objects and their albedos
using just a few images. Furthermore, we can interpret shadows and explain their puzzling and
seldom-noticed absence in most indoor scenes.
QUALITATIVE RADIOMETRY
We should like to know how “bright” surfaces are going to be under various lighting conditions,
and how this “brightness” depends on local surface properties, on surface shape, and on illumi-
nation. As we saw in chapter 4, foreshortening means that different sources can have the same
effect on a surface. The most powerful tool for analyzing this problem is to think about what a
source looks like from the radiometry is one of these tricks that looks
unsophisticated—no hard math—but is extremely powerful. In some cases, this technique gives
qualitative descriptions of “brightness” without even knowing what the term means.
Recall from Section and Figure that a surface patch sees the world through a
hemisphere of directions at that patch. The radiation arriving at the surface along a particular
direction passes through a point on the hemisphere. If two surface patches have equivalent in-
coming hemispheres, they must have the same ing radiation, whatever the outside world
looks like. This means that any difference in “brightness” between patches with the same -
ing hemisphere is a result of different surface properties. In particular, if two surface patches with
the same BRDF see the same ing hemisphere, then the radiation they output must be the
same.
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Sec. Sources and their Effects 71
Overcast