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

Chapter 6
Paragraph detection
tel-00912566, version 1 - 2 Dec 2013
Having specified all text lines, in this stage of the method, we focus
on regrouping these lines to produce paragraphs. At this point it is
expected that text lines from different columns or marginal notes are
segmented properly and they belong to separate text regions. However
the text region detection method described in chapter 4 has its focus on
separating text regions that are located far from one another or belong to different
columns. Several factors that are overlooked are the size of the text lines
and the paragraph model that includes them. For example, a text line that has
a considerably larger text components compared to its adjacent text lines is expected
to be a title, header or sub-header. These text lines should be separated
from the rest of the text lines in a region. Moreover there are several paragraph
models that exist on either handwritten or printed historical documents. They
can be immediately recognized from documents based on the geometric location
of text lines and their indentations. The simplest paragraph model is the model
for articles, magazines and technical journals that the first line of the paragraph
has a larger left indentation than the rest of the text lines. More complicated
paragraph models can be found in historical documents. One such paragraph
model is that except for the first text lines, all remaining text lines of the paragraph
have a large left indentation. Another paragraph model which can be
seen mostly in poems, has text lines that are center justified. The ending of
text lines may or may not be aligned. The paragraph detection module that we
describe in this section, should have the capability to handle all these cases.
Our paragraph model should be applies to each text region independently.
An overview of different stages for paragraph detection is as follows:
• A minimum spanning tree (MST) is applied to connect all text lines in a
way that only one link exists between each pair of lines. The weights of
the MST is specified in a way that the natural reading order of the text
lines are preserved.
• The MST is converted to a binary partition tree of text lines. The leaves
of the tree represent individual text lines. The remaining nodes represent
group of text lines that are obtained by merging text lines represented by
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