Similarly, an interior pixel is one in which all of its ¿-neighbors
belong to the property set. The pixel connectivity map for an inte
rior pixel may be tested for all d-bits being equal to 1.
B7
B6
B5
B4
B3
B2
B1
BO
--
1
--
1
--
1
--
1
Furthermore, arc endpoints or contour folds can be recognized as
contour pixels with only one neighbor and similarly, collapsing arcs
are contour pixels with no interior pixels as neighbors (Figure 1,
c and e)
Cederberg (1979) uses a 3 x 3 neighborhood template to identify local
maxima and minima pixels of a connected component in the generation
of his raster scan chain code. A complete set of templates for
detecting maxima and minima points in contours, arcs, and holes is
presented in Table 1 (O'Connor, 1982). The templates for detecting
a local minima are obtained by a 180° rotation of the maxima tem
plates, which can be achieved by a four place logical shift operation
of the 8-bit template.
Component junctions or intersections can be detected by scanning the
pixel connectivity maps for hole maximum and hole minimum pixels
whose distance is not greater than the width of the object they
represent. The junction point lies between these pixels.
12 3 4
1
HOLE MINIMUM ^ 2
3
4
Each of these characteristics is further illustrated in Figure 1.
ENHANCED EDIT SEQUENCE
As a means of illustrating the application of the previously described
techniques, a sample edit sequence is presented in Figure 2a-e. This
figure presents cased roads which contain a line gap and line over
shoot to be edited (Figure 2a). The cartographic editing operations
referenced in this sequence are defined as follows (Kolassa, 1982):
Join. Connects and relabels components.
Divide. Separates a labeled connected component into two
separately labeled components without altering them spatially.
Delete. Eliminates all members of a similarly labeled connected
component.
477