Image Processing Reference
In-Depth Information
Fig. 7.16.
Example of rendering by the MIP method (chest X-ray CT image of the
human body).
Remark 7.13.
The maximum value of densities on a ray may be projected
instead of accumulation along a ray. This is called the
maximum intensity
projection
(
MIP
) and is used frequently in the visualization of CT images
(Fig. 7.16).
7.8.5 Gradient shading
The methods of rendering explained in Sections 7.5 and 7.6, and in particular
the surface rendering method, are necessary for the normal vector of a surface
to be rendered. However, voxel data obtained by binarizing a gray-tone CT
image, e.g., a binary image in which a value
1
corresponds to the skull, do
not have enough information to uniquely determine the normal vector of the
object surface. One method to obtain the normal vector in such a case is to
fit a plane to each voxel and its neighborhood and to use the normal plane.
An alternative method is to utilize the gradient vector of the density value
distribution in the neighborhood of a surface and to regard the gradient as
the surface normal [Hoehne86].
Let us discuss this briefly. Consider the coordinate system as in Fig. 7.17.
We consider an image plane
L
and a pixel P
=(
i
,j
)on
L
. The directions
i
and
j
are assumed to be coincident to the row and the column directions
(
i
and
j
) of the 3D space including a point P
and a 3D image (or a 3D
object) to be visualized. The surface including a point (
i, j, k
) to be rendered
is assumed at the distance
k
from a point P
on the image plane in the direction
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