Volume visualization - The Visualization of Spatial Social Structure - page 276

9.6 Volume rendering

It is certainly feasible, and may prove useful, to offer a 'biod's eye'
view of the dataset as viewed by one of the biods, using stereoscopic
viewing and other 'virtual reality' techniques as they develop.
(Kerlick, 1990, p. 127; 'biod' is 'bird' and 'icon' combined)
So far this chapter has concentrated on rendering volume visualization, not
looking into the volume. That is because what most of the facilities available
in quantitative social science can at best achieve is only to show surface views
of three-dimensional structures. 13 That is why only this chapter of this topic has
been devoted to the subject which, in the late 1980s and early 1990s, was the
concern, above all others, of most interest in computer science visualization.
What was then new software was being written to look inside the surfaces, to
create images on the screen that we could not see in a picture on paper. Volume
rendering defines what can be done with this kind of software, which can only
be described in these pages; it cannot be shown.
The key theme is translucence. Surfaces can be peeled off a volume, like
two-dimensional contours, but really to see the structure you must be able to
see all the contours at once. To do that objects must emit and transmit light, so
that they can be seen through, but also not be transparent, in order to still be
seen themselves. There is obviously a limit to how many layers can be pierced;
each obscures a little more than the last. The combination of translucence with
perspective, animation and lighting allows us almost to see inside the volume as
we move around and through it.
Imagine the economic spacetime of Britain with the unemployed areas shown
like dark storm clouds through which it is possible to see better times ahead
(in time) or to the side (in space). The whole structure would be held in the
holographic image where no pocket of prosperity or despair could remain hidden.
What would the spacetime continuum of childhood leukaemia incidence look like
seen through that translucent space? When these words were originally written,
translucency was cutting edge. Now, for much basic everyday software, it is
the common format in the most widely used microcomputer windows operating
system used by billions worldwide.
In translucent space no case could ever quite eclipse another. 14 More impor-
tantly, when two or more cases fall in almost exactly the same time and place

13 The old approach was to show three-dimensional structures through two-dimensional surfaces:
'The second, newer approach to volume visualization is called direct volume rendering, volume
imaging, direct voxel rendering, or just volume rendering. This approach maintains an explicit con-
nection between the volume data set and the volume visualization. The algorithms use no intermediate
geometric representation. The resulting voxel clouds, perhaps more visually ambiguous, permit users
to explore directly the contents of their data. The scientist can slice-and-dice the visualization to
explore arbitrary cross-sections of the original volume data set. Viewing is not limited to surfaces,
although surfaces are sometimes portrayed' (Herr, 1990, pp. 201 - 202).
14 Great claims were made for the future of interactive computer graphics in the 1980s, especially
for volume visualization where the volume changed shape (called here 4D), but also for looking at