Graphics Reference
In-Depth Information
Figure 5.23.
Foot with bones exposed
([ScML98]). (Reprinted from Schroeder et al:
The Visualization Toolkit: An Object-Oriented
Approach to 3D Graphics, third edition, 2003,
1-930934-07-6, by permission of the publisher
Kitware Inc.).
Volume modeling is beginning to make an impact on the more conventional CAD
and CAGD. Here are some of its advantages:
(1) One can “cut away” parts of an object and look at its interior. See Figure 5.23.
(2) CSG can be implemented quite easily because at the voxel level the set oper-
ations are easy, especially if one has support for voxBlt (
voxel block transfer
)
operations that are the analog of the bitBlt operations.
(3) Rendering is viewpoint independent.
(4) It is independent of scene and object complexity.
The author has felt for many years that it was advantageous to model the whole world
and not just the objects within it. It gives one much more information. For example,
to trace a ray, one simply marches through the volume and sees what one hits along
the way, rather than having to check each object in the world for a possible intersec-
tion. Volume modeling is now making this possible.
Some disadvantages of volume modeling are:
(1) A large amount of data has to be maintained.
(2) The discretization causes loss of information.
(3) The voxelization causes aliasing effects.
Volume modeling plays an important role in the visualization of scientific data.
This is a big field in computer graphics. Although not the focus of this topic, it would
not be right to omit mentioning some examples of it:
Medical Imaging.
This was one of the first applications of volume modeling. See
[StFF91] for an overview of early work. Physicians used MRI (magnetic resonance
imaging) and CT (computed tomography) scanners to get three-dimensional data of
a person's internal organs. In tomography one gets two-dimensional slices of the
object using X-rays. One projects X-rays through the body and measures their inten-
sity with detectors on the other side of the body. The X-ray projector is rotated about
the body and measurements are taken at hundreds of locations around the patient. A
