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appear blue. In fact, the appearance of some objects, such as clouds and smoke,
comes from the way light is affected as it passes through them. Such objects are
really volume objects and do not even have well-defined surface boundaries. Even
when surfaces are well defined, BRDF-based rendering is limited in that it does
not account for the diffuse transmission of light exhibited by translucent objects,
nor does it properly handle subsurface scattering. Light penetrates the surfaces of
real materials at least slightly.
In the late 1980s these issues prompted more research into volume-based ma-
terial properties, such as particle composition per volume. The term volume or
volume data is used to describe an object that has some particular internal struc-
ture or data distribution of interest. A volume may not even have a well-defined
boundary surface; a point cloud of scattered data points is an example of such a
volume. Volume rendering and volume visualization refer to the process of gen-
erating images of volumes or volume data. This chapter introduces some basic
volume-rendering techniques.
3.1.2 Two Purposes of Volume Rendering
In CG, some of the first volume-visualization techniques developed were geared
toward medical data visualization. The objective was to allow the visualization
of the internal organs before performing surgery, or to use it as an interactive
visual reference tool while performing the actual surgery. Photorealism was not
a priority. What was needed was a way to quickly visualize the structure of body
parts. One technique developed early on is to divide the data set for the body into
a regular 3D rectangular grid, each cell of which is called a voxel . The attenuation
or occlusion of light is determined in each voxel. Voxels with high density block
a larger amount of light, so their color becomes darker. The light takes a different
path through the voxel grid depending on the viewpoint and pixel positions: each
light ray crosses a different set of voxels. The color of the surface pixel in the
rendered image is computed by traversing the set of voxels for the pixel while
accumulating the net color. This is one form of volume rendering.
The volume-rendering techniques used in medical imaging were soon applied
to rendering natural phenomena such as smoke, clouds, and fog. As noted above,
these objects are really volumes of distributed particles and their appearance
comes from light scattering and absorption. A medium that attenuates or scat-
ters light is known as a participating medium , a term taken from the optics and
radiative transfer literature. The only truly transparent nonparticipating medium
is a vacuum, but pure materials such as water, clear glass, and air come close at
small scales. Earth's atmosphere becomes a participating medium at larger scales.
Light scattering is responsible for a wide variety of natural phenomena.
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