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```					Chapter 10, Part II
   An approach incorporating
◦ The previous three approaches:
detection of discontinuities,
thresholding, and region growing.
◦ knowledge-based constraints.
       Viewing images as 3-D topography
◦     Two spatial coordinates plus the gray-level.

       Three types of points are considered:
1. regional minimum
2. catchment basin or watershed.
3. divide lines or watershed lines.

       The objective:
◦     According to the concepts and find the
watershed lines.
   Find the regional minima in the image.
   Flood the entire topography from the
regional minima at an uniform rate.
   When the rising water in distinct
watershed is close to merge, a dam is built
to prevent the merging.
   The flooding eventually stops when only
the tops of the dams are visible above the
water line.
   These dam boundaries correspond to the
divide lines of the watersheds.
Illustration of Morphological
Watershed
Illustration of Morphological Watershed
       The watershed lines form a connected path,
thus giving continuous boundaries between
regions.
       Regions characterized by small variations in
gray levels have small gradient values.
◦     In practice, we often see watershed segmentation
applied to the gradient of an image.
   Let M1 and M2 denote the set of coordinates
of points in two regional minima.
   Cn-1(M1) and Cn-1(M2): the set of coordinates
of points in the catchment basin associated
with M1 and M2 at stage n-1.
   Let the union of the two sets be C[n-1].
   The two components merge at the flooding
step n and let the connected component be
denoted as q.
       Dilate each region by the structure element,
consider the following two conditions:
1) The dilation has to be constrained to q.
2) The dilation cannot be performed on points that
would cause sets being dilated to merge.
       In Fig. 10.45, during the second dilation,
some points satisfy Condition (2) but do not
satisfy (1).
◦     resulting in broken perimeter.
       The only points in q that satisfy the two
conditions under consideration describe the
one-pixel-thick connected path shown by
cross-hatched points.
◦     The path constitutes the desired separation dam
at stage n of flooding.
Illustration of Dam Construction
   Definitions:
◦   Let M1, M2 ….MR be sets denoting the coordinates
of the points in the regional minima of an image
g(x,y).

◦   Let C(Mi) be a set denoting the coordinates of the
points in the catchment basin associated with
regional minimum Mi .
◦ Let T[n] represent the set of coordinates (s, t) for
which g(s,t)<n. i.e., T[n]={(g,s)|g(s,t)<n}

◦ The topology will be flooded in integer flood
increments from n=min+1 to n=max +1 where min
and max are the minimum and maximum value of
g(x,y).
◦   Let Cn(Mi) denote the set of coordinates of
points in the catchment basin associated with Mi
that are flooded at stage n.

◦   Cn(Mi) can be viewed as a binary image given by
Cn(Mi) = C(Mi)ÇT[n]
   Cn(Mi)=1 at location (x,y) if (x,y)Î C(Mi) AND (x,y)Î
T[n], otherwise Cn(Mi)=0

◦   Let C[n] denote the union of the flooded
catchment basins portion at stage n as:
◦   Then C[max+1] is the union of all catchment
basins as:

◦   C[n-1] is subset of C[n].
◦   C[n] is a subset of T[n].
◦   The algorithm for finding the watershed lines is
initialized with C[min+1]=T[min+1].
◦   Let Q[n] denote the set of connected components
in T[n].
   Obtain C[n] from C[n-1] as follows:
◦   For each q Î Q[n], consider p = qÇC[n-1]. If
(a) p is empty : a new minimum is encountered.
Add q into C[n-1] to form C[n].

(b) p contains one connected component: q lies
within the catchment basin of some regional
minimum. Add q into C[n-1] to form C[n]

(c) p contains more than one component:
Flooding would cause water level in these
catchment basin to merge; therefore, a dam
must built within q by dilating of p.
Example 10.18

The segmentation boundaries have the important property of
being connected paths.
   The problem of watershed segmentation:
Oversegmentation
◦   Due to noise and other irregularities of the
   Solutions:
◦   Pre-procesing.
◦   Various merging algorithms can be seen in
other references.
◦   Markers: determined according to predefined
criteria. For example,
   A region surrounded by points of higher “altitude”
   Points in the regions form a connected component.
   Points in the connected region with the same gray-
level value.
Illustration of Oversegmentation
Watershed Algorithm Using The Markers
   Segmentation using internal and external
markers.
   Simplify the problem as partition each
region into a single object and its
background.
   Marker selection can be based on gray-
level value and connectivity, and more
complex description involving size, shape,
location, texture content, and so on.

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 views: 0 posted: 7/28/2013 language: English pages: 21