Illumination and Shading - Computer Graphics and Geometric Modeling - page 317

9.4 Global Illumination Models

The problem with the simple models discussed in Section 9.2 is that they are only
local illumination models in the sense that they totally ignore the contribution of
other objects in the world. We have assumed that the light comes to a point directly
from a single source (with no shadows) and have dealt only with reflections from a
single surface when in reality light often reflects from several surfaces before reach-
ing the eye. Transparency properties have also been ignored. This section looks at
some global aspects of illumination and how to deal with them.

9.4.1 Shadows

In trying to produce realistic pictures, one will quickly discover that they do not look
that great if we omit shadows. A good discussion of shadow algorithms can be found
in [WoPF90] and [WatW92]. The algorithms are classified into the following
approaches:

(1) as part of early scan line visible surface algorithms,
(2) as part of the Weiler-Atherton visible surface algorithm ([AtWG78]),
(3) using “shadow volumes” ([Crow77b]),
(4) using “shadow z-buffers” ([Will78]).
(5) as part of ray tracing, and
(6) as part of radiosity methods.

We shall not go into the first two of these approaches. Approach (2) amounted to
running the basic Weiler-Atherton algorithm twice and involved lots of clipping of
polygons.
In the shadow volumes approach one extends each edge of an object that is an
outline. See Figure 9.9. The volume between the tetrahedron ABCD and its projec-
tion A ¢ B ¢ C ¢ D ¢ from the light source on some fixed distant plane is called the shadow
volume generated by the object ABCD . The faces obtained in this way bound a volume
in which light has been obscured. For several light sources we get several such which
are tagged by the light source. Real polygons along with these shadow ones are passed
to the visible surface determination algorithm. Potentially many shadow polygons , that
is, faces of shadow volumes, will lie between the viewpoint and a surface. One uses a

Figure 9.9. Shadow volumes.