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probabilistic classifier on histograms of data values. However, information about
the approximate densities and their relative distribution of the constituent mate-
rials is needed to make this work. Because of this, the method detailed in the
“Volume Visualization” paper requires a certain amount of a priori knowledge of
the materials being imaged.
However, CT scans of the human body are not the only use of 3D imaging by
X-ray absorption, and in some cases, very different CT scans of the human body
are not the only form of 3D imaging by X-ray absorption. Very different sub-
stances can have similar absorption densities. For example, if plastic explosives
and cheese have nearly the same X-ray absorption, an airport security scanner
could not distinguish between a bomb and a block of vintage cheddar using only
the method described in the paper. On the other hand, the usefulness of the method
is enhanced by a good understanding of the materials being visualized. Known
conditions such as “fat is never contained in the bone” and “muscle does not ex-
ist in the skull” can be applied to better distinguish features when visualizing the
human body. Although it was not clearly described in the paper, basing the ren-
dering on the density offers a lot of flexibility in visualization. This is because a
lookup table can be added to convert “true” densities of the various materials into
virtual densities and opacities for the purposes of visualizing particular aspects of
the data. For example, the skeletal structure alone can be displayed by setting the
virtual density of fat and flesh to zero. This can be generalized: manually varying
the percentage and the material histogram assignments allows the user to control
many aspects of the visualization, although creating an ideal rendering can require
a lot of trial and error.
The “Volume Visualization” paper described in this section is part theoretical,
but at the same time it presents a practical and flexible volume rendering method.
In the latter sense it is closer to the recent trend in SIGGRAPH presentations,
which tend to emphasize practical rendering methods. This paper continues to be
referenced in the context of rendering humans, especially in the motion picture
industry.
3.3 Participating Media
3.3.1 Participating Media Visualization
As described previously, a spatial region is said to contain (or be) a
participating
medium
if the effects of absorption and scattering of light is significant. Participat-
ing media are so named because they “participate” in light transport. In contrast
to surface rendering, which considers only reflection and scattering at object sur-
