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Unformatted text preview: and 8connectivity. (algorithm 5.8) •
• The above Algorithm works for all regions larger than one pixel.
This algorithm is able to find region borders but does not find borders of region holes. To
search for hole borders as well, the border must be traced starting in each region or hole
border element if this element has never been a member of any border previously traced. • Note that if objects are of unit width, more conditions must be added. • The inner border is always part of a region but the outer border never is. Therefore, if two
regions are adjacent, they never have a common border, which causes difficulties in
higher processing levels with region description, region merging, etc. The inter-pixel
boundary extracted, for instance, from crack edges is common to adjacent borders,
nevertheless, its position cannot be specified in pixel co-ordinates. • Boundary properties better than those of outer borders may be found in extended
o single common border between adjacent regions
o may be specified using standard pixel co-ordinates
o the boundary shape is exactly equal to the inter-pixel shape but is shifted one halfpixel down and one half-pixel right The existence of a common border between regions makes it possible to incorporate into the
boundary tracing a boundary description process. Faculty of Engineering Robotics Technology MECH 4041 15
B.Eng (Hons.) Mechatronics S. Venkannah Mechanical and Production Engineering Department A more sophisticated method for extended boundary tracing is based on detecting common
boundary segments between adjacent regions and vertex points in boundary segment
connections. The detection process is based on a look-up table, which defines all possible
situations of the local configuration of 2*2 pixel windows, depending on the previous detected
direction of boundary, and on the status of window pixels which can be inside or outside a
region. Note that no hole-border tracing is included in the algorithm. The holes are considered
separate regions and therefore the borders between the region and its hole are traced as a...
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- Spring '14