Information Technology Reference
In-Depth Information
Figure 6.7.
A polyhedral object and its labels
E
F
+
D
+
K
+
J
C
-
+
B
I
+
G
+
A
H
But consider the five horizontal lines that appear in the image. Part of making sense
of this image is realizing that these lines do not all mean the same thing:
The top horizontal line is a boundary of the object. The viewer understands that
just below the line is a face of the object, and just above the line is empty space
(unless the object is up against a wall).
The bottom horizontal line is also a boundary of the object. But this time the
space just above the line is part of the object, and just below the line is empty
space (unless the object is on top of another).
The middle horizontal line represents the intersection of two faces of the object.
Moreover, the intersection is
concave
(or hinged inward, like the letter V as seen
from above).
The two remaining horizontal lines also represent the intersection of two faces of
the object. In these cases, however, the intersection is
convex
(or hinged outward,
like the letter V as seen from below).
Interpreting an image of a polyhedral object involves labeling each edge in this way.
The image at the right in figure 6.7 shows the vertices named and the edges labeled:
+
for a convex edge,
-
for a concave edge, and an arrow for a boundary edge with a
surface to the right relative to its direction. The edge AH is labeled
<
meaning that
in going from A to H the surface is to the left. Of course, there is nothing inherently
directional in the edges of an image. So the reverse edge HA should be understood
