Image Processing Reference
In-Depth Information
Fig. 7.15.
Surface rendering and volume rendering applied to the same object. Left:
Surface rendering with smooth shading. Center: Polygon model used in generation
of the left image. Right: Volume rendering.
(b) Surface rendering
Surface rendering, on the other hand, has the following properties that are in
strong contrast to those mentioned above.
(1) We can designate each object or obtain quantitative measurements of
features interactively on a rendered 2D image, since a border surface is
determined strictly beforehand.
(2) Computation time tends to be shorter because the accumulation of density
values along a ray is not necessary.
(3) The detection and extraction of border surfaces satisfying geometrical
constraints is possible, if a border extraction algorithm works well. The
description of a 3D image structure is also possible by using those results.
Results may be affected strongly by the failure of those algorithms. Border
detection or segmentation is required before rendering.
(4) Algorithms to extract border surfaces of each object have to be prepared
before rendering.
(5) After rendering a surface of one object, other objects behind them become
invisible unless some particular methods are employed.
Examples of applications of both methods on the same object are presented
in Fig. 7.15.
Remark 7.9.
Before application of the volume rendering, the density values
of a 3D image to be visualized are replaced by parameter values
{
α
0
}
and
{
7.16. Such parameter values are stored beforehand
in a 3D array or the replacement of density values by
b
i
}
giveninEqs.7.11
∼
is performed in
the procedure of rendering. In the first case an extra 3D array is needed. It is
desirable that these values are calculated at sample points selected with equal
intervals on a line representing a ray. They are calculated by suitably interpo-
lating voxel values of the 3D image to be visualized. It is worth noticing that
this digitization along a ray might cause errors in feature measurements on a
displayed image or on unexpected artifacts. Additionally, the following should
{
α
i
}
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