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Specific methods differ in the definition of the starting segmentation and in the criterion for
merging. The result of region merging usually depends on the order in which regions are
merged. The simplest methods begin merging by starting the segmentation using regions of
2x2, 4x4 or 8x8 pixels. Region descriptions are then based on their statistical gray level
properties. A region description is compared with the description of an adjacent region; if
they match, they are merged into a larger region and a new region description is computed.
Otherwise regions are marked as non-matching.
Merging of adjacent regions continues between all neighbors, including newly formed ones. If a
region cannot be merged with any of its neighbors, it is marked `final' and the merging process
stops when all image regions are so marked.
Region splitting is the opposite of region merging. It begins with the whole image represented as
a single region which does not usually satisfy the condition of homogeneity. The existing image
regions are sequentially split to satisfy all the above given conditions of homogeneity.
Region splitting does not result in the same segmentation even if the same homogeneity criteria
are used. Region splitting methods generally use similar criteria of homogeneity as region
merging methods, and only differ in the direction of their application.
Splitting and merging
A combination of splitting and merging may result in a method with the advantages of both
approaches. Split-and-merge approaches work using pyramid image representations; regions are
square-shaped and correspond to elements of the appropriate pyramid level.
If any region in any pyramid level is not homogeneous (excluding the lowest level), it is split
into four sub-regions -- these are elements of higher resolution at the level below. If four regions
exist at any pyramid level with approximately the same value of homogeneity measure, they are
merged into a singl...
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This document was uploaded on 03/12/2014 for the course MECHANICAL 214 at University of Manchester.
- Spring '14