Segmentation of heterogeneous document images : an ... - Tel

same location of i th zone in sub-figure (a) and (b), respectively.
where the weights are:
SP R i =
w j =
M∑
j=−M
w j P R i+j .
exp −3|j|
M+1
∑ M −3|k|
k=−M
exp
M+1
Weights are defined to decay exponentially with respect to the distance from
the zone in the middle.
Finally, the first derivative of a smoothed projection may be obtained using
a symmetric difference equation as follows:
tel-00912566, version 1 - 2 Dec 2013
2
2h∑
△SP R i (j) =
h( h 2 + 1) k. (SP R i (j + k) − SP R i (j − k)) .
k=1
where h is set to the closest odd integer value to the mean height of all
CCs. The upper and lower bounds of text lines are the local maxima and local
minima of △SP R i respectively. Likewise gap regions are identified as the areas
between consecutive minima and maxima of △SP R i .
Having the first derivatives of the smoothed projection profiles, finding the
initial text and gap regions is a matter of applying a threshold to these first
derivatives. Figure 5.3 shows the initial text and gap regions for the two image
in figures 5.1 and 5.2. Initial separators for separating text lines can be drawn
in the middle of each gap in each vertical zone. In this regard, separators are
line segments that are defined in the middle of each gap region inside every
vertical zone. They are used later to find text lines.
5.2 Refinement of initial text line separators
Initial text line separators contain two types of errors: redundant separators resulting
from misclassified text and gaps due to poor local extrema, introduced in
the smoothed derivatives and separators that cut through descenders or ascenders
of text characters. In order to correct these errors and to locate separators
more accurately, a Hidden Markov Model (HMM) [78, 91] is formulated with
parameters drawn from statistics of the initial text and gap regions. For each
document, an HMM is formulated separately on-the-fly and is applied once to
each vertical strip of text and gap regions. Viterbi decoding scheme [34] is applied
on each zone to obtain a better succession of text and gaps.
We briefly note some formal definitions of HMM. Our HMM is characterized
by the following items:
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